OLED Display with Protection Circuit

Abstract

A display includes a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein the control circuitry of the silicon-based backplane includes at least one driving transistor, wherein a first terminal of the driving transistor is electrically connected to an external power supply V DD , and a second terminal of the driving transistor is electrically connected to a bottom electrode of the OLED stack; wherein a gate of the driving transistor is controlled by a data signal provided by a scanning transistor that is controlled by a signal from select line selection 1; and the control circuitry further includes a protection circuit that includes a bipolar junction transistor. For microdisplay applications, a switching transistor may be present between the scanning transistor and the gate of the driving transistor. The OLED stack may include two or more OLED light-emitting units.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of priority of U.S. Provisional Application No. 62 / 966,757, filed on January 28, 2020, entitled "Stacked OLED Microdisplays with Low - Voltage Silicon Backplanes" under Attorney Docket No. OLWK - 0021 - USP, and U.S. Provisional Application No. 63 / 054,387, filed on July 21, 2020, entitled "Stacked OLED Microdisplays with Low - Voltage Silicon Backplanes" under Attorney Docket No. OLWK - 0021 - USP2. Background of the Invention

[0003] OLED displays, especially OLED microdisplays, are very interesting and have practical applications. Generally, a microdisplay is an ultra - small display size with a diagonal less than two inches (about 5 cm) and even less than 0.25 inches. In most cases, microdisplays have a high resolution, and the pixel pitch is typically 5 to 15 micrometers. They were first commercially available in the late 1990s and are commonly used in rear - projection TVs, head - mounted displays, and digital camera viewfinders. In recent years, devices such as smartwatches have utilized the high resolution and low power consumption of these displays. It is expected that microdisplays will achieve a global market growth at a compound annual growth rate of 20% in the next few years. One of the trends driving this growth will be the increasing adoption of near - eye displays, augmented reality devices, and virtual reality devices, such as head - mounted displays (HMDs), head - up displays (HUDs), and electronic viewfinders (EVFs).

[0004] There are two main categories of microdisplays. The first category is projection microdisplays, which involve highly magnified images projected onto a surface. Types of projection microdisplays include rear - projection TVs and compact data projectors. The second category is near - eye displays (NEDs), which consist of highly magnified virtual images viewed through an eyepiece, such as a virtual reality headset or a portable camera viewfinder. These displays are increasingly being used in HMDs and HUDs, especially in the military and medical industries.

[0005] Both types of microdisplays have significant advantages over conventional direct - view displays, such as flat - panel LCDs. The advantages of microdisplays include: the ability to produce a large image from a very small, lightweight source display unit, making it easy to integrate into space - constrained technologies, such as wearable devices; a large pixel capacity, resulting in high resolution and clarity; and greater power efficiency compared to other display types. The higher the resolution and brightness and the lower the power consumption, the better the quality of the microdisplay. However, for microdisplay manufacturers, the challenges are relatively high production costs, as well as the need for high brightness, contrast, and long service life.

[0006] Microdisplays can be made with a variety of display technologies, including liquid crystal on silicon (LCoS), liquid crystal display (LCD), digital micromirror device (DMD), digital light processing (DLP), and more recently, micro light-emitting diodes (MicroLED) (light-emitting diodes) and organic light-emitting diodes (OLED).

[0007] In recent years, LCDs have dominated the microdisplay market. LCD technology offers high brightness, relatively low cost, and a relatively simple manufacturing process. By using LCDs, device manufacturers have been able to reduce the size of microdisplay components over time. LCD displays are currently used in some head-mounted displays (HMDs), head-up displays (HUDs), electronic viewfinders (EVFs), thermal imaging glasses, and wearable devices. However, LCD microdisplays require a light source or backlight to form an image with the liquid crystal array to modulate light. This technology has limitations such as polarization, color space, maximum brightness limitation, LC temperature sensitivity, viewing angle, LCD transmittance and extinction ratio, system limited size, etc., which may not provide all the desired performance characteristics.

[0008] Microdisplays based on micro light-emitting diode (microLED) technology can offer advantages over LCD microdisplays, such as self-emission, a larger color gamut, wide viewing angles, better contrast, faster refresh rates, lower power consumption (depending on the image), and a wide operating temperature range. Currently, microLED microdisplays are based on standard gallium nitride (GaN) wafers adopted from standard LEDs. This approach has the potential to provide high-brightness display devices without lifetime issues at a relatively low cost. Generally, standard GaN wafers are patterned into an array of microLEDs. Then, a microLED display is produced by integrating the microLED array and transistors. However, this approach has several manufacturing issues, including: monolithic formation of microLEDs on transistors, pixel pitch, color generation, and spatial uniformity due to color and brightness variations between individual microLEDs.

[0009] OLED technology has many attractive features of microLED technology for microdisplays. It is self-emissive, has excellent image quality, is very efficient compared to LCDs or LCoS, and has ultra-high color reproduction and a wide color space. Self-emissive OLED devices have an important advantage over backlit devices such as LCDs because each pixel only generates the intensity required for the image, while backlit pixels generate maximum intensity and then absorb unwanted light. Additionally, since the OLED layer can be vacuum deposited or directly coated on the transistor backplane, it is much easier and less costly to form an OLED on a transistor compared to forming a microLED. On the other hand, OLEDs can have limited brightness and lifetime.

[0010] For control circuits in OLED microdisplays (sampling hold displays), it is also important to solve the motion blur problem (see https: / / www.blurbusters.com / faq / oled-motion-blur / dated December 28, 2018; "Why Do Some OLEDs Have Motion Blur?") and https: / / www.soundandvision.com / content / motion-resolution-issue-oled-tvs dated January 15, 2015, "Is Motion Resolution an Issue with OLED TVs?").

[0011] The only way to reduce motion blur caused by sampling hold is to shorten the amount of time the frame is displayed. This can be achieved by using additional refreshes (higher Hz) or via a black period (blinking) between refreshes. For OLED microdisplays, the best solution is to "turn off" the displayed image, which is done by turning off the entire active area simultaneously or by a "rolling" technique (turning off only part of the displayed image at a time in a sequential manner). The "rolling" technique is preferred. The time the pixels are turned off is very short and well below the threshold detectable by the human eye to avoid perceptible blinking. In the control circuit, this is achieved by including a shutter transistor that prevents current from flowing through the OLED when activated by a select line and "breaks" the emission of the OLED pixels for the desired period of time. In other words, the shutter transistor is a switching transistor as it only turns the pixels "on" or "off" and does not regulate voltage or current. However, since the average brightness on the frame perceptible to the eye when the OLED is in the "on" state, this solution (turning off the pixels for part of the time the image is displayed, typically called the frame time) only increases the demand for increased brightness through the OLED when the OLED is in the "on" state. The shutter for reducing motion blur can be applied to any method of powering the OLED stack; for example, current control or PWM.

[0012] From a cost and manufacturability perspective, OLED displays and microdisplays utilizing silicon backplanes are very attractive. See, e.g., Ali et al., "Recent advances in small molecule OLED-on-Silicon microdisplays", Proc. of SPIE, Vol. 7415 74150Q-1, 2006; Ying, W., "Silicon Backplane Design for OLED-on-Silicon Microdisplay", MSE thesis, Nanyang Technological University, 2011; Jang et al., J. Information Display, 20(1), 1-8 (2019); Fujii et al., "4032ppi High-Resolution OLED Microdisplay", SID 2018 Digest, p. 613; US2019 / 0259337; Prache, Display, 22(2), 49 (2001); Vogel et al., 48th European Solid-State Device Research Conference 2018 (2018 48 th European Solid-State Device Research Conference), p. 90, September 2018; and Wartenberg et al., "High Frame-Rate 1"WUXGA OLED Microdisplay and Advanced Free-Form Optics for Ultra-Compact VR Headsets", SID Symposium Digest, 49 (1), Paper 40-5, 514 (2018).

[0013] Displays incorporating microdisplays may require very high brightness in order to be useful under all environmental conditions (such as outdoors in bright sunlight). For example, microdisplays require very high brightness even under controlled environmental conditions (such as in VR Google) in order to create an immersive visual experience. For microdisplays, very high brightness would allow the use of less efficient optical devices that are smaller, lighter, and less expensive, resulting in a more competitive headset.

[0014] To achieve high brightness from an OLED display, the power supplied to the OLED is typically close to or at the maximum limit of the OLED. A particular problem with microdisplays is that they also need to have extremely high resolution, which requires that the size of each pixel must be as small as possible, and the active (light-emitting) area of the microdisplay contains as many pixels as possible. This requires the transistors in the control circuit of the backplane to be small, but large enough to handle the maximum voltage and current required without causing permanent damage or current leakage.

[0015] Typically, as transistors become smaller, they have lower rated voltages because leakage currents and other failure mechanisms cannot handle higher power. Smaller low-voltage transistors have a thinner insulating layer at the gate, so they also have more static current leakage. For this reason, larger transistors that can handle higher voltages are typically used in OLED devices that require higher voltages. WO2008 / 057372 discusses the problems and prior art related to reducing the size of pixel circuits in microdisplays. See also, for example, O. Prache, Journal of the Society for Information Display, 10(2), 133 (2002); O. Prache's "Active Matrix Molecular OLED Microdisplays", Displays, 22, 49-56 (2001); and Howard et al., "Microdisplays based upon organic light emitting diodes", IBM J. of Res. & Dev, 45(1), 15 (2001) discuss the need for microdisplays on silicon backplanes that provide high brightness with a large contrast ratio at low voltages.

[0016] In addition, when using MOSFET p-channel transistors to supply a constant current from a power supply at V DD to an OLED with a cathode voltage of V CATHODE , the total voltage difference must be large enough to power the transistor and "turn on" the OLED to high brightness. However, at these high voltages, if the current leakage through the transistor is large enough when attempting to turn off the OLED to form a black pixel, because (V 阳极 - V 阴极)If it remains greater than the OLED threshold voltage, the OLED will continue to emit light. In an OLED display, current leakage through the driving transistor will reduce the contrast because the OLED pixels will continue to emit light when they should remain dark. Contrast is the difference in light emission when a pixel should be "off", "black", or non-emitting (usually the image signal code value (CV) = 0) and when a pixel should be fully "on", "white", or at maximum emission (usually, the image signal CV = 255). This effect will cause pure black (no desired emission) to turn gray (some emission occurs) and reduce the size of the tonal range between pure black and pure white. This is not desired.

[0017] OLED-based microdisplays typically include a protection circuit in the MOSFET-based control circuit of the backplane to limit the amount of power flowing through the transistor to prevent damage. It is desirable to include a protection circuit in the control circuit of the backplane of a microdisplay with an OLED because the power required to cause this device to emit light is relatively high. The protection circuit should at least maintain or "limit" the voltage at the bottom electrode of the OLED such that it does not fall below the required voltage level when the OLED is not emitting light. Such a protection circuit may also be referred to as a "voltage maintenance" circuit.

[0018] To protect the low-voltage transistors present in the control circuit and keep them within the specified operating range of the transistors as set by the foundry, it will be desirable for the protection circuit to maintain the black level current (CV = 0 or pixel "off") at the bottom electrode of the stacked OLED for pixels below 4 μA / cm2 (or more desirably 2 μA / cm2 or less for a 3-cell stacked OLED with a threshold voltage Vth of approximately 7.5 V). For a 4-cell stacked OLED device, a similar black level current is required, and the typical Vth is approximately 10 V.

[0019] There is a need to improve the performance of OLED displays, particularly microdisplays, on a silicon backplane by utilizing OLED stacks that can provide high brightness. However, the control circuit on the silicon backplane must be able to handle higher voltage and current demands without significantly increasing in size in order to maintain the resolution and pixel pitch within the active area of the OLED. Specifically, the control circuit should maintain the contrast by preventing or minimizing current leakage through the transistors and damage to the transistors due to increased power demands.

[0020] In the semiconductor foundry industry for manufacturing backplanes, analog transistors with an operating range of 5V or lower are typically considered standard "low voltage" (LV) transistors. There is usually a 10% safety margin in the voltage rating to allow reliable operation up to 5.5V without reducing the lifespan of the "5V transistor"; 5.5V is high enough to allow for a degree of overvoltage in the OLED dynamic voltage range and the drive circuit overhead voltage. Although the voltage limit generally applies between any pair of contacts of the transistor (gate, source, drain, body (also known as the substrate or well)), this voltage limit specifically applies to the maximum gate-drain voltage such that the performance of the transistor remains within the specified range during operation, typically for 43,000 hours. Sometimes, depending on the design of the transistor, the voltage limit for other pairs of contacts may be higher (e.g., 7V), but the transistor is still referred to as an LV or 5V transistor. 5V analog transistors are widely available throughout the industry due to their compatibility with the traditional TTL logic voltage levels used for communication between integrated circuit (IC) chips. With the trend of decreasing voltages for input-output communication (e.g., 3.3V and 1.8V standards), these 5V transistors are sometimes also referred to as medium voltage (MV) transistors, thus changing the LV label to the new "lower voltage" analog transistors. Although relative labels such as LV and MV may change over time, in this patent application, the term LV or "low voltage" refers to transistors rated 5V or lower, and the term MV or "medium voltage" refers to transistors with a rated voltage above 5V. Higher voltage analog transistors are also common, but in the entire integrated circuit (IC) manufacturing industry, the exact voltage has not been as standardized as that of 5V transistors. For example, industries such as automotive typically require higher voltage transistors.

[0021] Currently, silicon backplanes with low voltage 5V drive transistors for emitting light using OLED stacks in series (two light-emitting OLED units separated by a CGL) are available. See, for example, Cho et al., Information Display Journal, 20(4), 249 - 255, 2019; https: / / www.ravepubs.com / oled-silicon-come-new-joint-venture / , published in 2018; Xiao, "Recent Developments in Tandem White Organic Light-Emitting Diodes", Molecules, 24, 151 (2019). Such examples are not sufficient to meet the technical requirements in terms of brightness.

[0022] Currently, the state-of-the-art OLED microdisplays do not provide as much brightness as required. For example, a press release from a manufacturer of a tandem OLED microdisplay describes a full-color product that can provide up to 2.5k nits, but acknowledges that 5k nits would be a more desirable goal (see https: / / www.kopin.com / kopin-to-showcase-latest-advances-in-its-lightning-oled-microdisplay-line-up-at-ces-2020 / , dated January 7, 2020). Some manufacturers suggest that the goal should be 10,000 nits or higher (see https: / / hdguru.com / calibration-expert-is-10000-nits-of-brightness-enough / , dated July 26, 2018). A recent press release on June 20, 2020 (https: / / www.businesswire.com / news / home / 20200630005205 / en / Kopin-Announces-Breakthrough-ColorMax%E2%84%A2-Technology-Unparalleled-Color) describes its tandem (2-stack) OLED display that emits >1000 nits. It also announced: "By optimizing the OLED deposition conditions, it is expected to further increase the brightness (>2000 nits) and color fidelity. By incorporating structures that improve the output coupling efficiency, the brightness of the OLED microdisplay can be increased to >5000 nits within a few years".

[0023] One possible solution for increasing the total amount of light emitted from an OLED device is to stack multiple OLED cells on top of each other, so that the total light emitted from the stack is the sum of the light emitted by each individual cell. However, while the total light emitted from such an OLED stack is added based on the total number of individual OLED light-emitting cells, the voltage required to drive the OLED stack is also added based on the voltage required to drive each individual OLED cell. For example, if a light-emitting OLED cell requires 3V to produce 250 nits at a given current, then a stack of two such cells will require 6V to provide 500 nits at the same current, so a stack of 3 cells will require 9V to provide 750 nits, and so on.

[0024] OLED stacks are well-known; for example, US7273663, US9379346, US9741957; US 9281487 and US2020 / 0013978 all describe stacked OLED stacks with multiple light-emitting OLED units, each stack separated by an intermediate connection layer or a charge generation layer. Springer et al., Optics Express, 24(24), 28131 (2016) reported OLED stacks with 2 and 3 light-emitting units, where each unit has a different color. OLED stacks with up to six light-emitting units have been reported (Spindler et al., "High Brightness OLED Lighting", SID Display Week 2016, San Francisco, California, May 23 - 27, 2016).

[0025] Han et al., "Advanced Technologies for Large-Sized OLED Displays", Chapter 3, 10.5772 / intechopen.74869 (2018), which describes three stacked white OLED concepts and the progress of backplane technology including those concepts with two serially-connected transistors, although these technologies are separate rather than combined. The reference also states that such a two-transistor backplane "is difficult to adopt in large high-resolution panels due to large line loads and short charging times", and thus a different kind of backplane circuit was adopted for their devices.

[0026] Kwak et al., "Organic Light-Emitting Diode-on-Silicon Pixel Circuit Using the Source Follower Structure with Active Load for Microdisplays" (Japanese Journal of Applied Physics, 50, 03CC05 (2011)) describes a pixel circuit with an overvoltage protection circuit. The reference states that since the operating voltage of the OLED is higher than that of the MOSFET, an "overvoltage protection circuit" is required to prevent breakdown of the metal-oxide-semiconductor field-effect transistor (MOSFET). In Kwak et al., the protection circuit utilizes a grounded p-channel transistor.

[0027] The use of a protection circuit is also described in US9066379.

[0028] Vogel et al., SID 2017 Digest (Article 77 to 1, pp. 1125 to 1128) discloses the use of a protection circuit in a low-voltage OLED silicon microdisplay to extend the OLED voltage operating range.

[0029] Other references including overvoltage protection for OLEDs are disclosed in US6580657, WO2009072205, and CN200488960.

[0030] US9059123, US9299817, US9489886, US20080316659, and US20200202793 disclose the use of an n-p junction diode, such as a bipolar junction transistor, in a pixel control circuit of an OLED display.

[0031] Thus, while increasing the number of OLED light-emitting units within an OLED can provide higher brightness in a microdisplay, in many applications, increasing the size of the control circuit to be able to withstand higher power demands is not an option. There is a need to provide a small and compact pixel circuit that provides high brightness and contrast at a power level close to or at the maximum of the OLED without damaging the circuit. Summary of the Invention

[0032] A display is described that includes a light-emitting OLED stack on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein the control circuitry of the silicon-based backplane includes at least one driving transistor, wherein a first terminal of the driving transistor is electrically connected to an external power supply V DD , and a second terminal of the driving transistor is electrically connected to a segmented bottom electrode of the OLED stack; wherein a gate of the driving transistor is controlled by a data signal provided by a scanning transistor that is controlled by a signal from Select Line 1; and the control circuitry further includes a protection circuit that includes a bipolar junction transistor.

[0033] The above display, wherein there is a switching transistor between the scanning transistor and the gate of the driving transistor.

[0034] Any of the above displays, wherein the rated voltage of the driving transistor is 5V or less, or wherein both the driving transistor and the switching transistor are p-channel transistors.

[0035] Any of the above displays, where the OLED stack includes a single OLED light-emitting unit between a segmented bottom electrode and a top electrode, or where the OLED stack includes two or more OLED light-emitting units between a segmented bottom electrode and a top electrode. In either case, the OLED stack can form a microcavity, where the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels, or where the top electrode is transparent or semi-transparent so that light is emitted from the top of the OLED stack or both conditions are met. When there are two or more OLED light-emitting units, they can be separated from each other by a charge generation layer (CGL).

[0036] Any of the above displays, where the bipolar junction transistor is an NPN transistor, where the base is connected to a voltage source V 保护 or a current source I 保护 , the emitter is connected to a node, the node is connected to the bottom electrode of the OLED stack, and the collector is connected to an external power supply, or where the bipolar junction transistor is an NPN transistor, where the base is isolated, the emitter is connected to a node, the node is connected to the bottom electrode of the OLED stack, and the collector is connected to an external power supply, or where the bipolar junction transistor and the driving transistor are located in separate wells.

[0037] The display provides very high brightness and contrast with good stability and lifetime at a small pixel pitch size. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 shows a simple prior art control circuit for an OLED.

[0039] Figure 2 Shows a basic control circuit with a driving circuit and a protection circuit.

[0040] Figure 3A Shows an embodiment of the basic control circuit with an added switching transistor. Figure 3B Shows an alternative arrangement for the added switching transistor.

[0041] Figure 4 and Figure 5 Shows different embodiments of the intrinsic body diode connection for the circuit shown in FIG. 3.

[0042] Figure 6 Shows for Figure 2 the circuit shown a schematic side view of the intrinsic body diode connection.

[0043] Figure 7 Shows for Figure 3A the circuit shown a schematic side view of the intrinsic body diode connection.

[0044] Figure 8 Shows a control circuit having two driving transistors controlled in series by a single data line, wherein the gates of each driving transistor are controlled separately.

[0045] Figure 9 Shows a control circuit having two driving transistors in series, each driving transistor being controlled by a separate data line.

[0046] Figure 10 Shows a control circuit having two driving transistors in series controlled by a single data line, and the gates of each of the two driving transistors are controlled jointly.

[0047] Figure 11 Shows an alternative arrangement of Figure 10 including the LSC.

[0048] Figure 12 Shows a cross-sectional view of a single RGB microdisplay having three laterally adjacent monochromatic RGB OLED stacks, each individual stack having only one OLED light-emitting unit. 100 Each individual stack has only one OLED light-emitting unit.

[0049] Figure 13 Shows a cross-sectional view of an RGB microdisplay having three laterally adjacent monochromatic RGB OLED stacks, each individual stack having three OLED light-emitting units. 200 Each individual stack has three OLED light-emitting units.

[0050] Figure 14 Shows a cross-sectional view of a tandem microdisplay having a multimode microcavity OLED stack with two OLED light-emitting units and an RGB color filter array. 300 of the cross-sectional view.

[0051] Figure 15 Shows a cross-sectional view of a microdisplay having a multimode microcavity OLED stack with three OLED light-emitting units and an RGB color filter array. 400 of the cross-sectional view. DETAILED DESCRIPTION

[0052] For the purposes of the present disclosure, the terms "on" or "above" mean that the structure in question is located above another structure, i.e., on the side opposite to the substrate. "Top", "uppermost" or "upper" refer to the side or surface farther from the substrate, while "bottom", "lowermost" or "lower" refer to the side or surface closest to the substrate. Unless otherwise specified, "on" shall be construed to mean that the two structures may be in direct contact or that there may be an intermediate layer between them. For "layer", it should be understood that a single layer has two sides or surfaces (the uppermost and the lowermost); in some cases, "layer" may represent a plurality of layers considered as a whole and is not limited to a single layer.

[0053] For a light-emitting unit or light-emitting layer, R represents a layer that mainly emits red light (>600 nm, desirably in the range of 620 to 660 nm), G represents a layer that mainly emits green light (500 - 600 nm, desirably in the range of 540 to 565 nm), and B represents a layer that mainly emits blue light (<500 nm, desirably in the range of 440 to 485 nm). It is important to note that the R, G, and B layers can produce a certain amount of light outside the specified ranges, but the amount produced is always less than the primary colors. Y (yellow) represents a layer that emits a large amount of R and G light and much less B light. "LEL" refers to the light-emitting layer. Unless otherwise specified, wavelengths are expressed as vacuum values rather than in-situ values.

[0054] A single OLED light-emitting unit can produce light of a single "color" (i.e., R, G, B, or a combination of 2 or more primary colors such as Y, C (cyan), or W (white)). Monochromatic light can be produced within the OLED unit by a single layer having one or more emitters of the same color or by multiple layers each having the same or different emitters (whose primary emission falls within the same color). A single OLED unit can also provide a combination of two colors (i.e., R+G, R+B, G+B) in a single OLED unit by having: one layer with a single emitter that emits light of two colors, one layer with two different emitters, or a combination of multiple separate layers each emitting a single but different color. A single OLED unit can also provide white light (a combination of R, G, and B) by having: one layer that emits light of all three colors or a combination of multiple separate layers each emitting a single (but different) color whose sum is white. Each OLED light-emitting unit can have a single light-emitting layer or can have more than one light-emitting layer (either directly adjacent to each other or separated from each other by an intermediate layer). Each light-emitting unit can also contain various non-light-emitting layers, such as hole-transporting layers, electron-transporting layers, blocking layers, and other non-light-emitting layers known in the art, to provide desired effects such as promoting emission and managing charge transfer across the entire light-emitting unit.

[0055] Because an OLED light-emitting unit can include multiple layers, a single unit is sometimes referred to as a "stack" which may be confused with an OLED device having multiple units. In the present application, a "stacked" OLED has at least two OLED light-emitting units stacked on top of each other on a substrate, so there are multiple light sources within the device. In the stacked OLED of the present invention, each OLED light-emitting unit is separated from each other by a charge generation layer (CGL) rather than by a separate and independently controlled intermediate electrode. To be considered an OLED light-emitting unit, it must be separated from another light-emitting unit by a CGL. Therefore, a light-emitting layer adjacent to one of the OLED light-emitting units but not separated from it by a CGL is not considered a separate unit. Within the stack, all or some of the individual OLED light-emitting units can be the same, or they can all be different from each other. Within an OLED stack, each OLED light-emitting unit can be placed in any order between the top cathode and the bottom cathode. The stacked OLED can be monochromatic (each pixel of the OLED stack emits light of the same color primarily; for example, green light), or it can have multi-mode emission (where each pixel emits two or more colors of light (for example, yellow or white), or where different pixels emit different colors of light such that the overall emission includes two or more colors of light).

[0056] In some cases, the threshold voltage (V th ) of an OLED stack can be estimated by linear extrapolation of the I-V curve after significant light emission starts to return to the voltage axis. Since this method is inaccurate because the I-V response curve of an OLED may not be completely linear within its response range, the value calculated in this way is not precise. The general range of this metric is + / - 10%. More precisely, the threshold voltage can be defined as the voltage at which the current density is no greater than 0.2 mA / cm of the exposed anode layer 2 and there is at least some reliable detectable luminance (i.e., at least 5 cd / A). This is the method used in the present application.

[0057] In the following text, a transistor may be referred to as "on" or "off". In an "off" transistor, the signal sent to the gate is intended to cause no current to flow through the terminals; in other words, the signal (usually CV = 0) is an indication that the signal requires no current to flow through the transistor, such that the transistor is adjusted to be "off", rather than switched "off". In this case, even though the transistor may be "off", there may still be some current leakage. Similarly, in an "on" transistor, the signal sent to the gate is intended to cause at least some current to flow through the terminals; in other words, the signal (usually CV = greater than 0 but less than 255) is an indication that the image requires a certain level of emission from the pixel, such that the transistor is adjusted to be "on", rather than switched "on". In a similar manner, depending on the requirements of the image, a pixel or OLED may be referred to as "on" or "off", and thus an appropriate signal is sent to the pixel or OLED.

[0058] Silicon backplanes are derived from silicon wafers (also known as slices or substrates). They are thin slices of semiconductor (such as crystalline silicon (c-Si)) used to fabricate integrated circuits. The wafer serves as a substrate for microelectronic devices built within and on the wafer. The wafer undergoes many microfabrication processes, such as doping with various materials, ion implantation, etching, thin film deposition, and photolithographic patterning. Finally, the individual microcircuits are separated by wafer dicing and packaged into integrated circuits. Wafers are grown from crystals with a regular crystal structure, where silicon has a diamond cubic structure with a lattice spacing. When cut into wafers, the surface is aligned in one of several relative directions called crystal orientations. Silicon wafers are typically not 100% pure silicon, but are formed with an initial impurity doping concentration of boron, phosphorus, arsenic, or antimony, which are added to the melt and define the wafer as either bulk n-type or p-type. For background, see "Flat Panel Display Manufacturing" (Souk, L., Ed, 2018, Chapter 7). It is desirable for the silicon backplane to be a single crystal silicon wafer.

[0059] To provide a control circuit for the operation of a stacked OLED, transistors and other components such as capacitors, resistors, connection lines, or bus bars are provided on the surface of a silicon wafer. For example, see T. Arai, "High Performance TFT Technologies for the AM-OLED Display manufacturing", thesis, Nara Institute of Science and Technology, 2016; M.K. Han, Proc. of ASID’06, Oct. 8 - 12, New Delhi; US9066379; and US10163998. It should be understood that the transistors may or may not be integrated with the silicon wafer as part of the structure, or may be fabricated from a separate material deposited on the surface.

[0060] Transistors can be made using a variety of semiconductor materials. The characteristics of silicon-based transistors depend on the crystalline state of the silicon; that is, the semiconductor layer can be amorphous silicon, microcrystalline silicon, or it can be annealed to polycrystalline silicon (including low-temperature polycrystalline silicon (LTPS) and laser annealing).

[0061] The fabrication of a silicon backplane with a suitable control circuit is a well-known, understandable, and predictable technology. However, due to the cost and complexity of the manufacturing processes and equipment, it is often impractical to build a facility for manufacturing a particular backplane. Instead, in an industry where the functional characteristics of microelectronic devices have become more standardized, the foundry model has been widely adopted. This standardization allows for the separation of design from manufacturing. Designs that follow appropriate design rules can be more easily and inexpensively fabricated by different companies with compatible manufacturing methods. For this reason, the control circuits on silicon backplanes are typically limited to the use of standard components selected from a range of options provided by the backplane manufacturer. For example, a manufacturer of a silicon backplane may offer options to incorporate various standard designs of transistors rated at 1.8V, 2.5V, 3.3V, 5V, 8V, and 12V into a customer's design, but (without significant overhead) cannot offer transistors that are not included in the provided designs.

[0062] For the purposes of this application, "low voltage" (LV) is defined as those analog microelectronic components that are sized, designed, and rated to be able to operate safely and reliably at voltages of 5V or less. "High voltage" (HV) microelectronic devices are typically considered to be in the range of 18 to 25V. "Medium voltage" (MV) microelectronic devices are typically considered to be those microelectronic devices that are between LV and HV. It should be noted that these voltage ratings are set by the manufacturer, and the manufacturer does not recommend exceeding the maximum set voltage for each transistor.

[0063] Complementary metal-oxide-semiconductor (CMOS) technology uses p-type and n-type metal-oxide-semiconductor field-effect transistors (MOSFETs) to implement complex integrated circuits. Depending on the manufacturing process, there are different voltage domains that can be used (i.e., 1.8V, 2.5V, 3.3V, 5V, 12V, etc.). In all voltage domains, the MOSFET transistor has a drain, a source, a gate, and a body / well. The base of the MOSFET is the substrate; for an n-channel FET, the substrate is a p-type doped substrate or a well with a low doping rate; for a p-channel FET, the substrate is an n-type doped well with a low doping rate. The source region and the drain region are formed by highly doped regions with n-type or p-type for an n-channel FET or a p-channel FET. The controlled channel is formed between the source and the drain, isolated by a thin oxide and typically covered by a polysilicon layer that acts as the gate. All four terminals of the FET (source, drain, gate, substrate / well) are connected to a metal interconnect layer through metal contacts, and this metal interconnect layer is ultimately connected to the OLED.

[0064] The total emission area of the microdisplay is very small, and to achieve the necessary pixel pitch, the space available for the control circuit of each pixel is limited. For a full-color microdisplay, the space occupied by the control circuit for each individual pixel should be no more than 100 square micrometers, and preferably no more than 50 square micrometers. For a monochromatic microdisplay in which all pixels emit the same color, since fewer pixels are required, the space for the control circuit can be 3 to 4 times larger.

[0065] In a suitable low-voltage 5V transistor, the maximum voltage between any pair of terminals can be no more than 5V without damaging the device. For a short period of time, a typical 10% overvoltage safety margin is acceptable. Medium-voltage transistors with a rated voltage greater than 5V (e.g., a transistor rated at 7.5V) typically have the same setup as 5V transistors, but with a thicker gate oxide and larger geometries (channel width and length) to withstand higher voltages. Therefore, MV transistors will generally be larger and occupy more space than their corresponding LV transistors.

[0066] Although transistors can be manufactured in any size range regardless of their voltage rating, the total area of a low-voltage MOS transistor rated at 5V suitable for microdisplay applications does not exceed 20 square micrometers, and preferably does not exceed 10 square micrometers. The suitable channel area (channel length x channel width) of a 5V transistor for microdisplay applications should not exceed 1 square micrometer, and preferably does not exceed 0.30 square micrometer. Each of the two transistor contacts should not exceed 1 square micrometer, and preferably does not exceed 0.30 square micrometer.

[0067] For the purposes of this application, the BJTs suitable for the protection circuit are generally arranged vertically for both NPN and PNP types. For the NPN BJT, the collector is formed as a low-doped deep n-well with low p-doping within a common silicon substrate (body). The base is formed as a p-well inside the deep n-well and is connected through a highly doped p-region. The emitter of the BJT is formed by a highly doped n-region inside the p-well. The typical dimensions of the emitter region are about 500nm x 500nm, and preferably do not exceed 0.30 square microns to allow for smaller pixel sizes. The maximum voltage between any pair of all terminals of the BJT (body, base, emitter, collector) may not exceed 5V without damaging the device. A typical safety margin of 10% overvoltage is acceptable for short periods of time.

[0068] OLED displays (often referred to as "AMOLED") consist of an active matrix of OLED pixels that produce light (emit light) when electrically activated, and have been deposited or integrated onto an array of transistors or TFTs located on a silicon chip, where the array acts as a series of switches to control the current flowing to each individual pixel. Typically, this continuous current is controlled by at least two transistors at each pixel (to trigger the emission of light), one of which starts and stops the charging of a storage capacitor and a second transistor provides a voltage source at the level required to produce a constant current for the pixel.

[0069] This is illustrated in Figure 1, which represents the simplest form of prior art AMOLED pixel design. The simplest AMOLED pixel with pixel memory uses two transistors and a capacitor. The current drive transistor MP2 is typically driven from a supply voltage V DD Connected to the anode of the OLED. As shown, one transistor (MP2) drives the current of the OLED, and the other transistor MP1 (also called the scan transistor) acts as a switch to sample and hold the voltage onto the storage capacitor C1. There is a control circuit that controls the current V flowing through the drive transistor MP2. DD The data line (provides V 数据 ). There is a select line that controls MP1 and therefore controls the charging of capacitor C1. Typically, transistors have intrinsic capacitance, so depending on the intrinsic capacitance of the transistor and the leakage current through the transistor, additional capacitance may not be needed. For clarity, in the figures after Figure 1, any capacitor that exists need not be shown in the drawings.

[0070] It has been found that the high voltage / current required to provide high brightness from an OLED stack can be safely handled by a control circuit comprising a power supply V DDA driving circuit for the driving transistor between the bottom electrode of the OLED stack and a protection circuit, which also includes a bipolar junction transistor. This arrangement allows driving the OLED pixel without a large amount of current leaking through the transistor, so that high brightness is obtained without loss of contrast or damage to the LV circuit.

[0071] Figure 2 A basic control circuit arrangement suitable for displays of any size is shown in FIG. As shown by the dashed line, the circuit comprises a drive circuit including a drive transistor T1 (shown as a p-channel transistor) connected at a first terminal (source for a p-channel transistor) to V DD (external power supply) and is connected at a second terminal (in a p-channel transistor, the drain) to the bottom electrode of the OLED. The gate of drive transistor T1 is controlled via the data line and series select transistor T3 (the "scan" transistor), whose gate is controlled by select line Select 1. When a voltage is applied to the data line relative to V DD When T1 is selected to be “on” by placing a non-zero signal on the data line relative to V and turning T3 “on” via Select 1 so that the non-zero signal from the data line flows to the gate of T1, current flows to the bottom electrode of the OLED stack and light is emitted. DD By selecting T1 as "off" with a zero (or very small) signal and turning T3 on, no current will flow to the OLED stack.

[0072] exist Figure 2 In the embodiment of the present invention, the driving transistor of the driving circuit (T1) has the function of adjusting the voltage and current flowing to the OLED pixel to appropriate levels according to the displayed image. The switching transistor is preferably a p-channel MOSFET transistor. The driving transistor can be LV (5V or less) or MV (>5V), preferably LV, so as to minimize the space occupied by the control circuit. The scanning transistor of the driving circuit (T3) has the function of providing a data signal to the gate of the driving transistor and charging the optional capacitor C2 (if present) according to the scanning signal applied to its gate.

[0073] Figure 2 shows a storage capacitor C2, which is connected between V DD and a node on the data line between the gates of the scan transistor T3 and the drive transistor T1. When the scan transistor T3 is "on", the charge on this capacitor can change. However, the use of a capacitor (such as C2) is optional, and the circuit can have no capacitors or any number of capacitors as desired. The reference voltage of the capacitor can be V DD or some other voltage.

[0074] As Figure 2 shown, the basic control circuit further includes a protection circuit, as indicated by the dash-dotted line. The protection circuit includes a bipolar junction transistor (BJT). The protection circuit is designed to maintain the voltage at the anode of the OLED above a certain level whenever the OLED is intended to be "off" or not emitting light, and can help maintain a high contrast ratio. As the cathode voltage decreases (more negative voltage) so that the peak brightness of the OLED can be higher, the protection circuit is designed to supply additional current to pixels displaying black or low brightness in order to protect the drive transistor and the switching transistor from voltage levels that violate the device's maximum ratings.

[0075] As Figure 2 shown, the bipolar junction transistor BJT1 has a collector (c) connected to V DD , an emitter (e) connected to the node between T1 and the bottom electrode of the OLED stack, and a base (b) connected to a power supply 50, which is a voltage source V 保护 or a current source I 保护 . One benefit of using a BJT in the protection circuit is the current amplification from the base current to the collector current. If desired, the collector of the BJT can be connected to a separate power supply different from V DD . The power supply V 保护 or I 保护 can be shared among all pixels. It should be noted that V 保护 or I 保护 can be constant or can be non-constant, but can vary depending on whether the OLED is to emit light. This can be advantageous when used with a switching transistor to reduce the persistence of the display through the shutter. In Figure 2 , BJT1 is shown as an "NPN" type BJT transistor (preferred), but can also be a "PNP" transistor with appropriate design changes.

[0076] For an NPN type BJT, whenever the emitter voltage V E at the bottom electrode of the OLED is greater than the base voltage V B and V B is less than the collector voltage V C (V E >V B <V C ), the BJT will be in the off mode and essentially no current will flow through the BJT. However, whenever the voltage V E at the bottom electrode of the OLED is less than the voltage V B and V B is less than V C (V E <V B <V C),the BJT will be in the forward active mode. In this mode, the base-emitter junction is forward biased, and the base-collector junction is reverse biased, and the collector-emitter current will be approximately proportional to the base current.

[0077] Therefore, in Figure 2 if the V C of BJT1 is V DD and V 保护 (is V B ) is set to be less than the V th of the OLED (higher than V 阴极 ), whenever the voltage V E at the bottom electrode is higher than V th + V 阴极 , BJT1 is "cut off", but whenever V E drops below V th + V 阴极 (i.e., whenever T1 "cuts off"), the voltage at the bottom electrode is maintained near V th + V 阴极 . Therefore, the protection circuit is designed to provide sufficient current when needed to protect the driving transistor and the switching transistor so that whenever the cathode voltage drops below a certain value (more negative voltage), the voltage across its terminals does not exceed its rated value. In addition, by setting the base voltage B (V 保护 ) to be lower than the turn-on voltage of the OLED (V 阴极 plus V th ), the power loss is minimized whenever the voltage applied to the bottom electrode of the OLED is greater than or equal to V 阴极 plus V th .

[0078] In some embodiments using the protection circuit shown in Figure 2 , the base voltage (V B ) of the BJT is isolated from any external power supply. That is, there is no electrical connection between the power supply 50 ( V保护 or I 保护 ) and the base of the BJT. The BJT physically exists together with the existing collector and emitter connections, as shown in Figure 2 , but although the base connection still exists, it is not connected to any external source. In this case, V B is not deliberately maintained at any specific value, nor is any voltage or current deliberately applied to it. V B is allowed to be independent of the voltages V C and V Eis "floating" and is kept as part of the active control circuitry operating the OLED. However, there may be parasitic current paths within the backplane that bias the base internally with a high impedance. It should be noted that in this type of embodiment, due to the generation of parasitic currents within the circuitry, V B may vary during the operation of the OLED.

[0079] Since the protection effect can still be observed when the base of the BJT is isolated and not connected to an external power supply, the base voltage is not intentionally controlled, which means that in this embodiment the protection circuitry still provides some current and voltage control at the anode of the OLED. Without being limited to any particular theory or speculation, adjacent n-wells (e.g., the n-well of drive transistor T1) may be a source of holes that migrate to the p-well (base) of BJT BJT1. Once these holes are in the base, they will diffuse across the depletion region to the n-type emitter contact (OLED anode pad) that travels through the base-emitter diode in the forward direction. This will be supplemented by the diffusion of thermally excited electrons from the emitter into the base and then by the transport of the base and depletion region into the collector facilitated by the large electric field potential between the base and the collector. In this context, holes from adjacent n-wells supply charge (holes) to the base, which would normally come from an external V 保护 connection. When the OLED anode voltage drops to a very low level (e.g., to display black with a stacked OLED cell), the potential difference (at the OLED anode voltage) between an adjacent n-well of one of the drive circuit transistors (at V DD below) and the BJT base is very large, increasing the flow of holes from the drive transistor well into the BJT base. Due to the amplification of the BJT, this increase in base current increases the emitter current.

[0080] However, the protection effect provided by the protection circuitry is different for each frame and may need to be appropriately reset for each new frame of the image. This is not a problem when the BJT base is connected to an external power supply and actively controlled for each frame. For embodiments where the base of the BJT is isolated and not intentionally connected, a reset for each frame can be provided when the pixel is "turned off" by the scan transistor.

[0081] In other embodiments, the V B of the BJT can be self-biased (sometimes referred to as having a "base bias"). When the BJT is self-biased, no externally controlled input signal is applied to the base of the BJT, but rather the signal applied to the base of the BJT is from a constant power supply voltage (i.e., V DD) and the value of any bias resistors connected to the transistor. One method for implementing self - biasing of the base of a BJT is to form a "fixed base - bias circuit". In this arrangement, the base of the BJT is connected to a constant power supply (i.e., V DD ) through a single current - limiting resistor. The current path through the deep n - well can provide this resistance. When the V B of the BJT responds to changes in pixel brightness and follows V E , the base bias will change, and the base current (I B ) of the BJT will also change, thus providing a protection current in response to the OLED anode voltage. Alternatively, a simple voltage - divider network can provide the required bias. It should be noted that in this type of embodiment, the bias within each pixel and the resulting parasitic current will respond to the operation of the OLED. This response can provide effective protection for the transistor.

[0082] When used as part of a protection circuit, the pixel size of the BJT transistor design is smaller. Additionally, the inherent amplification factor of the forward - active - mode BJT means that a relatively small parasitic current can generate a much larger protection current. Therefore, the current from the reference / protection voltage source is much smaller, and the voltage drop across the reference - voltage interconnect is greatly reduced.

[0083] A stacked - OLED configuration can be designed such that the voltage range from the black level (below Vth; e.g., 2 μA / cm 2 ) to the white level (20 mA / cm 2 ) is relatively constant and less than about 6 V. This can result in a contrast ratio of about 10000:1 or greater, and may be slightly less due to a decrease in current efficiency at the high end of the current density. This voltage range is approximately within the allowable operating range of the LV transistor, and the protection circuit only becomes active at the bottom end of the current range when the drive transistor stops the current. Thus, at the low - current end of this range, the protection circuit also prevents the current density through the OLED from dropping below about 2 μA / cm 2 . However, it may also limit the ability to achieve higher contrast ratios. In exchange for this slightly elevated black level and reduced contrast, the protection circuit allows the pixel - drive circuit to be pushed by reducing the cathode voltage to achieve higher peak brightness or to compensate for efficiency losses due to OLED aging, and ensures that the LV transistors operate within their specified voltage range.

[0084] However, since there are no measures for providing the shutter function required to minimize motion blur, Figure 2The control circuit shown may not be suitable for all microdisplay applications. To prevent or minimize motion blur by providing a shutter function, especially when the drive transistor is "on" (conducting current), a switching transistor can be added to the drive circuit that controls the power supplied to its pixel without regard to the operation of the drive transistor, which prevents current from flowing to the OLED. At a time suitable for the pixel, the switching transistor can be selected such that even if the image to be displayed requires the pixel to be "on" (and thus the drive transistor "on"), the OLED pixel remains "off". This allows all or a rotating portion of the pixels in the display to be "off" (not emitting light) during a single frame, which minimizes the perception of motion blur in the microdisplay.

[0085] Figure 3A An embodiment of a control circuit with a switching transistor particularly suitable for microdisplay applications is shown. There is a "switching" transistor T2 whose gate is controlled by a signal from select line select 2, which is positioned along the data line between scan transistor T3 and drive transistor T1. As shown, one terminal of switching transistor T2 (source for a p-channel transistor) is connected to one terminal of scan transistor T3 (drain for a p-channel transistor), and the other terminal of T2 (drain) is connected to the gate of drive transistor T1. T3 and T2 are in series. In this embodiment, the purpose of the switching transistor in combination with the other components of the circuit is to switch the current flowing to the OLED pixel on and off in order to provide a shutter function.

[0086] For example, during normal non-shutter operation of the microdisplay to display an image, scan transistor T3 is selected according to select 1 to allow the signal from the data line (according to the image) to flow to the gate of T1. This allows the pixel to emit light. In some applications, activating T3 by select 1 can be along a row of pixels, while the data line correspondingly provides an appropriate signal for each pixel along that row. During this time, T2 is selected to be "on" according to the signal from select 2 such that the data signal from T3 is passed to the gate of T1. In some applications, select 2 can be common for a column of pixels.

[0087] To provide a shutter function, when a pixel is supposed to emit light normally, it is necessary to prevent the pixel from emitting light within a short period. To form a shutter period, T2 can be activated by a signal from Select 2 such that, in cooperation with T3, the gate of T1 receives a "cut-off" signal (i.e., CV = 0). Select 2 can be shared across multiple pixels (desirably a column of pixels) such that its signal is applied simultaneously to more than one pixel at a time, so that all pixels in the group do not emit light. This is different from the purpose of the scan transistor T3, which conducts only within a very small portion of the frame time (e.g., 1 / 1200 for a display with 1200 rows) to apply a data signal to the gate of the driving transistor of each individual pixel (and the storage capacitor C2 if it exists). T2 and T3 operate independently of each other, and their gates are controlled by different signal lines (Select 1 and Select 2). At some time during the course of each frame, both can be "conducting", both can be "cut-off", and one can be "conducting" while the other can be "cut-off".

[0088] The optional storage capacitor C2 is connected between V DD and the node on the data line between the switch transistor T2 and the gate of T1. When the scan transistor T3 is "conducting" (and T2 will also be "conducting" during this part of the operation), this capacitor can be charged so that the voltage at the gate of T1 is constant. In some embodiments, if it exists, C2 can be connected to the node between T3 and T2.

[0089] Figure 3B An alternative arrangement of a circuit with switch transistors is shown. In this arrangement, T3 and T2 are connected in parallel rather than in series. The gate of T3 is controlled by Select 1, and one terminal is connected to the data line, and the other terminal is connected to T1 through a common node with T2. The gate of T2 is controlled by Select 2, and one terminal is connected to the data line, and the other terminal is connected to T1 through a common node with T3. In this embodiment, during normal operation, for each pixel, T2 will be "cut-off", so that an appropriate data signal from the data line is provided to the gate of T1 through T3 as controlled by Select 1. During the shutter period, T3 will be "cut-off", so that an appropriate data signal from the data line (i.e., CV = 0) is provided to the gate of T1 through T2 as controlled by Select 2, so that those pixels connected to Select 2 do not emit light.

[0090] In Figure 3A and Figure 3BIn the two embodiments shown, T3 and T2, together with Selection 1 and Selection 2, work in synchronization to control the gate of T1. Basically, when the microdisplay is normally displaying an image, T1 is controlled according to T3, where T2 is set to not interfere. However, during the shutter period, T1 is controlled according to T2, where T3 is set to not interfere.

[0091] For example, for Figure 3A the circuit shown, where Selection 1 extends parallel to the data line, thus in a very short time, an entire column of pixels can be turned off by loading CV = 0 into each pixel. In Figure 3A , for the purpose of turning off a column for the "rolling shutter" function, the operation of turning off each column one by one would be: first, select all rows "on" through Selection 1, then set the data line of a column to CV = 0, and finally pulse Selection 2 to turn off the column.

[0092] Alternatively, for Figure 3A the circuit shown in, where C2 is connected to the node between T3 and T2, the global shutter function would be: first, load all data voltages onto C2 in sequence using T3 (as in normal operation), then start emission by activating all Selection 2 lines to turn T2 "on" for the entire display. To stop emission, set all data lines to CV = 0 at the desired time, then pulse all Selection 1 lines to turn on all T3 transistors and stop the emission (shutter) of the display, and finally turn off all T2 transistors and start over.

[0093] As is known in the art, MOSFET transistors require an intrinsic body diode connection to perform as required. Due to the structure of MOSFET transistors, parasitic diodes inherently exist, and they may affect the operation of the transistors. Generally, the intrinsic body diodes are connected internally or externally to a power supply to apply a bias voltage. These body connections are also referred to as "body connections" or "transistor wells" and other terms.

[0094] Figure 4 Shows Figure 3A an embodiment of, where IBD1, IBD2, and IBD3 are the intrinsic body diode connections (for T1, T2, and T3) to a separate voltage source V DD2 . For the intrinsic body diode connections, these transistors can share the same well. However, the same power supply V DD used to power T1 can also be used for the IBD; that is, V DD and V DD2 are common power supplies.

[0095] However, it may be desirable to float one or more transistors in their own separate wells on the silicon backplane to avoid exceeding the operating voltage range of any component. In particular, when both the drive transistor T1 and the switch transistor T2 are p-channel transistors, each transistor can be located in its own separate n-well. This allows a larger dynamic voltage range to control the OLED than can be achieved using two transistors in the same n-well. For transistors connected in series, the use of isolated, floating or different wells is known in the art, for example, see US9066379, US5764077, US7768299, US9728528 and JP2016200828.

[0096] This is Figure 5 (Similar to Figure 4 ) is shown in which T1 occupies a well that is connected to V via IBD1. DD2 and T2 occupies a different well, as shown by the dashed line, which is biased by a different separate connection IBD2 to the transistor source. Figure 5 In the embodiment of FIG. 1 , the same bias voltage is not applied to each IBD, and therefore, the n-wells are independent of each other.

[0097] Figure 6 is used for Figure 2 Schematic diagram of a cross-section of a transistor well of the circuit shown. It should be noted that T1 and T3 can each be located in separate, but not isolated or floating, n-wells connected to V DD and are all separated from the p-well of BJT1 (NPN BJT). For convenience, the source (s), gate (g), and drain (d) regions are labeled for T1 and T3. Similarly, the emitter (e), base (b), and collector (c) regions are labeled for BJT1. The collector (c) region of BJT1 is shown connected to V through a deep n-well. DD , but in some embodiments it may alternatively be connected directly to V DD . Figure 6 Also indicated are IBD5, which is the V to the deep n-well of the silicon substrate where all transistors are located, and IBD6. DD The intrinsic body diode connection of IBD6 is the intrinsic body diode connection of the entire silicon substrate to the ground. It should be noted that the BJT in the protection circuit can be an NPN or PNP transistor. If the BJT is an NPN transistor, the transistor is in the p-well, and if the BJT is a PNP transistor, it will be in the n-well. For clarity, capacitor C2 is not shown in cross section.

[0098] Figure 7 is used for Figure 4Cross-sectional schematic view of the transistor wells of the circuit shown. It should be noted that T1, T2, and T3 can each be located in separate, but not isolated or floating, n-wells that are connected to V DD and all are separated from the p-well (NPN BJT) of BJT1.

[0099] Although only p-channel transistors are shown in Figures 2 to 7 , n-channel transistors or a mix of n-channel and p-channel transistors can be used. In such a case, it will be necessary to appropriately re-arrange the circuit to account for the difference in polarity between n-channel and p-channel transistors. When all of the multiple transistors are of one type, multiple common transistors of the same type can share the same well to reduce the size of the design. However, to expand the operating range of the backplane circuit while observing the voltage limits of the individual transistors, it may be necessary to place transistors of the same type in separate well regions.

[0100] It is desirable for the switching transistor to be a p-channel transistor. When the switching transistor is a p-channel transistor, its source is connected to one terminal of the scan transistor and its drain is electrically connected to the gate of the drive transistor. In some embodiments, it is desirable that both the drive transistor and the switching transistor are p-channel transistors, where both are low-voltage transistors, or the drive transistor is low-voltage and the switching transistor is medium- or high-voltage.

[0101] Microelectronic components may be interposed between the scan transistor and the switching transistor or between the switching transistor and the gate of the drive transistor (other than the node connected to the capacitor). The interposed components can be in series, where the current between the gates of the scan transistor / switching transistor / drive transistor flows directly through the series components. There can also be other microelectronic components (excluding capacitors) that are indirectly electrically connected to any connection between the gates of the scan transistor / switching transistor / drive transistor such that current also flows to the additional component.

[0102] Figures 2 to 7 The embodiment shown has several advantages in its design and operation for driving an OLED microdisplay. In the design, the circuit can be very compact because all of the transistors can be relatively small LV transistors that are typically available in most foundries. All of the transistors can be p-channel transistors in which all of the n-wells are biased to V DD , which eliminates the need for isolated or floating wells. These features can allow for a very compact pixel circuit design for microdisplay designs with very high resolution in a small size.

[0103] In some embodiments, a second driving transistor may be present in the driving circuit between the power supply and the bottom electrode of the OLED. Having two or more driving transistors connected in series in the driving circuit allows the power load to be shared among multiple transistors.

[0104] An embodiment of such an arrangement is shown in Figure 8 which shows a driving circuit having two driving transistors T1 and T4 connected in series between a power supply V DD and the bottom electrode of the OLED. The gate of T1 is controlled via T3 and T2, as Figure 3A described. The gate of an additional driving transistor T4 is controlled via a scan transistor T5, the gate of which is controlled by Select 3. This allows independent control of the timing of the current passing through both T1 and T4. A second series switching transistor T6 (not shown) may be present between the gate of T5 and T4 to allow T4 to be turned off in a manner similar to switching transistor T2 and driving transistor T1. The gate of T6 may be controlled by Select 2 shared with T2 or a different signal line. T5 may also be connected to a data line different from T3.

[0105] Figure 9 Another embodiment of a driving circuit having two driving transistors T1 and T4 connected in series is shown. In this example, as Figure 3A described, the gate of the first driving transistor T1 is connected to a first data line Data 1 via T3 and T2. T4 is directly connected to a second data line Data 2. Data 1 and Data 2 are independent of each other. In different configurations, there is one Data 2 per pixel, one Data 2 per row or column of pixels, or more preferably, one Data 2 for all pixels. In this embodiment, T4 is directly controlled by Data 2 and there is no scan transistor corresponding to T3. This arrangement allows independent control of T1 and T4 within the pixel. A switching transistor (function similar to T2) may be added between Data 2 and the gate of T4.

[0106] Data 1 and Data 2 may provide the same or different signals at different times as needed. It is important to note that both T1 and T4 are driving transistors, the purpose of which is to control the power flowing to the OLED, and both will be "on" simultaneously during the course of a frame whenever the OLED provides brightness. Specifically, T4 is not a switching transistor providing a shutter function; that is, when T1 regulates the current flowing to the OLED, T4 also regulates the current flowing to the OLED for a period of time during the course of the frame during which T1 is "on". Sometimes this arrangement is referred to as a "cascode configuration". When T3 is appropriately set and the data signal is set not to be emitted from the pixel (i.e., CV = 0), only T2 provides the shutter function.

[0107] Figure 10 A schematic diagram of a control circuit having two driving transistors in series is shown, where the gates of the two driving transistors (T1 and T4) are controlled by a single common signal from a data line. In Figure 10 , this signal is controlled by T3 and T2 (similar to FIG. 3) and is applied simultaneously to the gates of both T1 and T4.

[0108] However, in some embodiments, even if there is only one common data line in the circuit, it may be desirable to ensure that the desired gate voltage is applied to both T1 and T4 to achieve the functions described below. As Figure 11 shown, this can be achieved using a level-shifting circuit (LSC) between the terminals of T2 and the gates of T1 and T4. A level-shifting circuit is a circuit for converting a signal from one logic level or voltage domain to another logic level or voltage domain, and is typically used to address voltage incompatibilities between parts of a system. For clarity, internal components or all connections to the LSC (such as V DD , ground, and other possible input connections or components such as capacitors not shown) are not shown. The LSC sets the gate voltages of the driving transistors T4 and T1 based on signals from the data line and Select 1 such that the total voltage across T4 and T1 is always divided approximately equally between the two driving transistors. It is well known that small logic transistors can be used for this function. In other embodiments (not shown), it may be desirable to divide the signal unevenly such that the voltages at the gates of T1 and T4 are different in some ratio between them. An LSC or a similar circuit can be used to perform this operation.

[0109] In the above embodiments having two driving transistors in series, the shutter transistor T2 for the driving transistors is optional. In embodiments having two or more driving transistors in series, they can be a combination of both LV and MV transistors. Preferably, they are all LV transistors to reduce the size of the pixel circuit. In some embodiments, it is also desirable that all transistors in the driving circuit are p-channel transistors. These multiple transistors can be p-channel, n-channel, or mixed transistors. Preferably, they are all p-channel. If there is a mixture of n-channel and p-channel transistors, then preferably there is at least one driving transistor that is a p-channel transistor, and more preferably, it is an LV transistor. If any one of the multiple transistors in series is of one type, then multiple common transistors of the same type can share the same well to reduce the size of the design. However, in order to expand the operating range of the backplane circuit while observing the voltage limits of the individual transistors, it may be necessary to place transistors of the same type in separate well regions.

[0110] Ideally, there should be no intervening transistors between the power supply and the bottom electrode of the OLED, except for the driving transistor in the electrical connection. By intervening, it means that in the electrical connection between the power supply and the OLED, the current does not directly flow through another transistor other than the driving transistor (i.e., through terminals, such as the source and drain of a p-channel transistor connected to the driving transistor, and the other terminal is connected to the OLED, such that at some point during the operation of the OLED, the current will flow through the intervening transistor). Ideally, there is a direct series connection of one (and only one) transistor between the power supply and the bottom electrode of the OLED (which means that the power flows through the terminals of the transistor (i.e., the source and drain if it is a p-channel)). Due to fewer components, the excessive size of the control circuit is minimized. Other (non-transistor) microelectronic components can be connected in series between the driving transistor and the power supply of the OLED.

[0111] There can be a branched (meaning not directly in series) connection between the driving transistor - OLED connection and the first terminal of the microelectronic component (i.e., non-intervening transistor or diode), where the OLED operating current does not directly flow through the non-intervening transistor or diode. This is a node outside the node between the driving transistor and the bottom electrode of the OLED, through which the BJT of the protection circuit is attached.

[0112] Other components in the control circuit can help control the current provided by the driving transistor, either the current intended to power the OLED when "on" or the current leakage through the driving transistor when the OLED is intended to be "off". Additionally, other microelectronic components such as other transistors, capacitors, and resistors can be included in this control circuit as needed. In particular, changes in the threshold voltage (V th ), carrier mobility, or series resistance will directly affect the uniformity of the current of the OLED driving transistor and thus affect the brightness of the display. A major factor affecting non-uniform current is the threshold voltage (V th) Variations. For pixels, other types of compensation may be required by the control circuit; for example, the aging, degradation, or aging of OLED materials over time, non-uniformity or non-uniformity across the active region, or voltage drops in metal connection lines. Additionally, the control circuit provided by the transistor may need to control, for example, the timing of the current delivered to the pixel via PWM. The design of control circuits for OLEDs, including various types of compensation and drive schemes, has attracted great interest, and many methods have been proposed. In addition to the protection circuits described, such compensation circuits may also be present in the control circuit. In addition to maintaining the minimum voltage at the bottom electrode below the threshold voltage of the OLED by including other appropriate circuit components, the protection circuit can also be designed to prevent other undesirable effects, such as short-circuit protection, electrostatic discharge, transient peaks, etc. However, in some instances, when using the stacked OLED of the present invention, including such known types of protection circuits in the pixel circuit may still be useful.

[0113] In the control circuit, power from an external power source can be delivered to the driving transistor as a variable current or voltage to drive the OLED stack to deliver a desired level of brightness. This is typically stored by charging a storage capacitor during a write operation. The power level can be controlled at the external power source, or if the delivered power is constant, the power can be set at an appropriate level by other microelectronic circuits within the backplane. This is called "current control" and is commonly used to power most OLED devices. Alternatively, the power supplied to the OLED stack can be constant, and the total amount of light emitted within a set time period (frame) is controlled by the time the OLED pixel is fully "on" (compared to the time it is "off"). This is called pulse width modulation or PWM control.

[0114] Since the control circuit described is capable of handling higher voltage and current requirements than at least the design or rated voltage of the driving transistor without significant leakage or damage, the use of OLED stacks with increased emission (and higher voltage) is enabled. To provide the necessary brightness, the OLED stack should have at least at least two OLED light-emitting units (commonly referred to as "series" OLED devices). However, series OLED devices may still not provide sufficient brightness as required. For this reason, although OLED stacks with three or more OLED light-emitting units have relatively higher V than series OLED stacks th requirements, OLED stacks with three or more OLED light-emitting units can be used. The circuit described can be used with stacked light-emitting OLEDs having a threshold voltage (V th ) greater than 7.5V; more desirably, the V of the light-emitting OLED stack this at least 10V or greater. Alternatively, the circuit can be used with a stacked OLED that provides a full-color microdisplay with luminance of at least 2500 nits or preferably at least 5000 nits.

[0115] In cases where the luminance of each individual pixel must be controlled by powering one of the pixel electrodes via a control circuit on a backplane, there are two basic methods for fabricating a pixelated OLED microdisplay. The first method involves having each pixel generate red, green, or blue light (R, G, B respectively) or the same color if it is a monochrome display. In this case, the light-emitting OLED stacks can be arranged such that all the light-emitting units of all the stacks above an individual bottom electrode segment emit light of the same color (selected from R, G, or B light) in order to form R, G, and B pixels. In some embodiments with this feature, each color pixel forms a microcavity, where the distance between the segmented bottom electrode and the top electrode depends on the color of the light emitted. In this case, the length of the microcavity will depend on the color of the light emitted and will be different for red, green, and blue pixels.

[0116] The second method is to have a common multimode (white) light-emitting OLED layer spanning all the pixels with a color filter array (CFA) in order to generate individual RGB pixels. The second method has an advantage over the first method because it does not have to form individual OLED pixels of different configurations and will thus reduce the manufacturing cost.

[0117] The number of individual OLED light-emitting units within a stacked OLED is limited only by the overall thickness of the OLED and the ability of the control circuit to handle the power required to operate the OLED. As the number of OLED units increases, the total amount of light emitted increases, but the thickness of the package, the complexity of the manufacturing process, and the threshold voltage also all increase. An OLED with at least three stacked light-emitting units will provide increased luminance compared to a series OLED (two OLED units). However, an OLED with at least four stacked OLED light-emitting units is preferred, and more preferably an OLED with at least five stacked OLED light-emitting units. It is conceivable to have an OLED with up to six to ten or more stacked OLED light-emitting units.

[0118] To minimize the increase in voltage required to drive the OLED stack, a charge generation layer (CGL; sometimes also referred to as a connector or an intermediate layer) is located between individual OLED light-emitting units. This is because the structure of the CGL is such that electrons and holes are generated upon application of a voltage and are injected into the adjacent organic emission layer. Thus, the use of a CGL may convert an injected electron into multiple photons, allowing for higher brightness. Specifically, it is desirable for the CGL to be located between each light-emitting unit within the stack. However, the light-generating unit need not have adjacent CGLs on both sides. The OLED light-emitting units at the top and bottom of the stack will typically have only one adjacent CGL. Although a CGL can be used when needed, it is generally not necessary to use a CGL between a light-emitting unit and one of the top or bottom electrodes.

[0119] Many different kinds of CGLs have been proposed and can be used in an OLED stack. For example, see US7728517 and US 2007 / 0046189. To form a CGL, an n-p semiconductor heterojunction located at the interface of an n-type and a p-type layer is generally required to generate charge. Thus, the CGL will have two or more layers. For example, an n-doped organic layer / transparent conductive layer, an n-doped organic layer / insulating material, an n-doped organic material layer / metal oxide layer, and an n-doped organic material layer / p-doped organic material layer have all been reported. A desirable metal oxide for the CGL is MoO3. In some cases, the n-layer and the p-layer can be separated by a thin intermediate layer. Generally, the CGL is arranged such that the n-layer is closer to the anode and the p-layer is closer to the cathode.

[0120] An ideal configuration of the CGL has three layers; an electron transport material doped with an n-type dopant (such as Li), a thin intermediate layer of the same (but undoped) electron transport material, and a hole transport material doped with a p-type dopant. Suitable electron transport and hole transport materials, along with n-type and p-type dopants suitable for the CGL, are well-known and commonly used. These materials can be organic or inorganic. The choice of suitable materials is not critical, and any material can be selected based on its properties. The thickness of the CGL should ideally be in the range of 200 to In many instances, the CGL will have an ETL on the anode side and an HTL on its cathode side to help improve charge transport and to help separate the charge generation dopant (if present) from the LEL in the light-emitting unit.

[0121] Although the use of a CGL helps to minimize the voltage increase when OLED light-emitting units are stacked together, the total voltage required for the stack still increases by approximately the voltage required for each individual unit.

[0122] In one embodiment, all of the OLED light-emitting units within the OLED stack above a single bottom electrode segment emit the same color; e.g., red, green, or blue. This results in a pixelated RGB display. Figure 12 A single display is shown 100 , which uses three different OLED sub-pixel stacks to form R, G, and B pixels. Each OLED sub-pixel stack contains only one OLED light-emitting unit and lacks a CGL.

[0123] In the display 100 , there is a silicon backplane 3, which includes an array of control circuits such as shown in Figures 2 to 7 , and other necessary components that will supply power to the sub-pixels according to input signals. Above the layer 3 with transistors and control circuits, there may be an optional planarization layer 5. Above the layer 5 (if present) are separate first electrode segments 9 connected by electrical contacts 7 that extend through the optional planarization layer to form electrical contacts between the separate bottom electrode segments 9 and the control circuits in layer 3. The separate bottom electrode segments 9 are electrically isolated from each other laterally by the pixel definition layer 1. Above the segmented bottom electrode segments 9 is a non-light-emitting OLED layer 11, such as an electron or hole injection (EIL or HIL) layer or an electron or hole transport (ETL or HTL) layer. The light-emitting OLED units 13 (the OLED units in each stack will emit different colors; i.e., B, G, or R) are above the OLED layer 11. Above the light-emitting units 13 are non-light-emitting OLED layers 23 such as electron or hole transport layers or electron or hole injection layers, and a transparent top electrode 25 through which light can pass. The OLED microcavity is protected from the environment by the encapsulation layer 27. In the illustrated embodiment, all of the organic layers within a single OLED stack are horizontally separated from adjacent stacks by the pixel definition layer 1, but the top electrode 25 and the encapsulation 27 are common and extend across the entire active area. However, the top electrode 25 does not need to be continuous and can be segmented as needed. This particular microdisplay is not a microcavity device; however, a similar pixelated RGB design that utilizes the microcavity effect can be used.

[0124] Figure 13 A similar RGB pixelated OLED stack is shown 200 , which has three light-emitting units and three laterally adjacent monochromatic RGB OLED stacks. The RGB pixelated OLED stack 200 is a three-stack device. In 200In [reference], above the first light-emitting unit 13, there are a first charge generation layer 15, a second light-emitting OLED unit 17, as well as a second charge generation layer 19 and a third light-emitting unit 21. The first CGL 15 is located between the first light-emitting unit 13 and the second light-emitting unit 17 and separates them, and the second CGL 19 is located between the second light-emitting unit 17 and the third light-emitting unit 21 and separates them. The remaining OLED stacks are the same as 100 in [reference]. In 200 each of the light-emitting units 13, 17, and 21 in the same stack emits the same color to form separate RGB pixels.

[0125] A well-known method for improving the brightness and color purity of OLED emission is by utilizing the optical microcavity effect. This effect is based on forming an optical resonator between a reflective surface and a semi-reflective surface, which allows some light to pass through. Depending on the optical distance between the two surfaces, multiple reflections between the two surfaces will form a standing wave, and due to the constructive and destructive interference effects that will occur depending on whether emission occurs at the antinode or node of the standing wave respectively, this will enhance the light of certain wavelengths and reduce the light of other wavelengths. Depending on the total space between the reflectors and the wavelength to be optimized, the antinodes will appear at different positions. However, the light emitted from the microcavity may exhibit severe angular dependence, where color shift and brightness loss may occur as the viewing angle deviates from perpendicular to the viewing surface. Since the incident angle of the projection optical device is limited, this is usually not a problem for NED applications.

[0126] It is desired to further increase the brightness of the OLED stack by using the microcavity effect. For example, as shown in Figures 12 to 13 a microdisplay 100 and 200 can be redesigned to generate the microcavity effect by using a reflective bottom electrode or a reflective layer under the bottom electrode, making the top electrode semi-transparent such that it has a certain degree of reflectivity and adjusting the distance between the uppermost surface of the reflective element (bottom electrode 9 or underlying reflective layer) and the lowermost surface of the top electrode to form a microcavity suitable for the light of that specific color.

[0127] Figure 14 shows a display 300 that uses a multi-mode (white) OLED microcavity shared across all pixels together with a color filter array (CFA) to form R, G, and B pixels. The multi-mode OLED emits light of more than one color. In an ideal case, the multi-mode OLED emits white light with approximately equal amounts of R, G, and B light. Typically, this will correspond to approximately 0.33, 0.33 of the CIE x CIE yValues. However, depending on the characteristics of the color filters used to form the RGB pixels, certain variations of these values are still acceptable or even desirable. In this embodiment, the multimode OLED stack includes two OLED light-emitting units that emit different colors, where each unit is vertically separated from the other by a CGL, and the distance between the reflective surface and the top electrode is constant over the active region. 300 is a series (two-unit or two-stack) OLED device because it has two light-emitting units separated by a single CGL.

[0128] In a tandem display 300 there is a silicon backplane 3 that includes an array of control circuits such as Figure 2 shown in FIGS. 3, and the necessary components to supply power to the sub-pixels in accordance with the input signals. An optional planarization layer 5 may be present on the layer 3 having the transistors and control circuits. Above the layer 5 (if present) are separate first electrode segments 9 connected by electrical contacts 7 that extend through the optional planarization layer to form electrical contacts between the separate bottom electrode segments 9 and the control circuits in the layer 3. In this embodiment, the bottom electrode segment 9 has two layers, a reflective layer 9B closer to the substrate 1 and an electrode layer 9A closer to the OLED layer. The separate bottom electrode segments 9 are electrically isolated from each other laterally. Above the segmented bottom electrode segments 9 is a non-light-emitting OLED layer 11, such as an electron or hole injection layer or an electron or hole transport layer. The first OLED light-generating unit 13A is above the OLED layer 11. The layer 15 is a first charge generation layer that is located between and separates the first OLED light-generating unit 13A and the second OLED light-generating unit 17A. Above the second light-emitting layer 17A is a non-light-emitting OLED layer 23, such as an electron or hole transport layer or an electron or hole injection layer, and a semi-transparent top electrode 25. This forms an OLED microcavity 30 that extends from the uppermost surface of the reflective surface 9B to the lowermost surface of the semi-transparent top electrode 25 that is also a semi-reflective electrode. The OLED microcavity is protected from the environment by a encapsulation layer 27. In this embodiment, there is a color filter array having color filters 29B, 29G, and 29R that filters the multimode emission generated by the OLED microcavity 30 such that B, G, and R light are emitted in accordance with the power supplied to the bottom electrode segments 9.

[0129] In 300 the first and second light-emitting OLED units 13A and 17A together generate a multimodal emission. The two OLED units can each generate white light, or one OLED unit can generate one or more colors different from each other such that they together generate a multimodal emission. For example, one OLED unit (i.e., 13A) can generate B light, while the other (i.e., 17A) can generate Y (R+G) light.

[0130] Although 300 is a tandem device, the same basic structure can be modified to produce a single unit microcavity device. For example, a single unit multimodal microcavity device can be prepared by replacing CGL 15 with a simple undoped organic intermediate layer, where 13A will subsequently include a yellow-emitting OLED layer and 17A will include a blue-emitting OLED layer. In such an example, layer 15 will be optional. Alternatively, if the light-emitting unit 13A emits white light, both layer 15 and the second light-emitting layer 17A can be completely omitted. In a white or multimodal emission device, color filters must be used to provide pixels of a specific color.

[0131] Figure 15 A similar multimodal microcavity OLED stack having three light-emitting units is shown 400 . In 400 , above the second light-emitting OLED unit 17A, there is a second charge generation layer 19, which is located between and separates the second light-emitting OLED unit 17A and the third light-emitting OLED unit 21A. The rest of the OLED stack is the same as in 300 .

[0132] In 400 , the first light-emitting OLED unit 13A, the second light-emitting OLED unit 17A, and the third light-emitting OLED unit 21A together produce multimodal emission. Each OLED unit can individually produce white light, or each can produce one or more colors different from the other two, such that together they produce multimodal emission. For example, one unit (i.e., 13A) can produce R light, another unit (i.e., 17A) produces G light, and the third unit (i.e., 21A) produces B light. Alternatively, two OLED units can produce B light (i.e., 17A and 21A), while the other (i.e., 13A) produces Y (R+G) light.

[0133] As Figures 13 to 15 shown, an OLED stack having two or more OLED light-emitting units can be used in displays of any size and can be used with a control circuit as shown in Figures 2 to 7 . However, these OLED stacks having multiple units are also particularly suitable for use as microdisplays incorporating a shutter function provided by the control circuit shown in FIGS. 3 to Figure 7 .

[0134] When the OLED stack includes three or more OLED light-emitting units, it is desirable for the driving transistor to be a low-voltage transistor. That is, it can be designed and sized to operate safely and effectively at voltages of 5V or less, regardless of the actual load in the circuit. However, if the OLED stack contains four or more OLED light-emitting units, the operating voltage may need to be well over 7.5V, and the driving transistor or switching transistor can be medium-voltage (e.g., designed for 7.5V to 12V) or high-voltage (designed for 18 to 25V).

[0135] As previously described, OLED displays and microdisplays are built on a silicon backplane that serves as a substrate. Generally, the backplane will be flat and of consistent thickness. Since the silicon backplane is typically opaque, the OLED stack is preferably top-emitting. However, transparent backplanes are known; in such cases, the OLED stack can be top-emitting or bottom-emitting. The top surface of the substrate faces the top surface of the OLED. The silicon backplane can have various types of underlying layers (i.e., planarization layers, optical management layers, light-blocking layers, etc.), which can be patterned or unpatterned and can be on the top or bottom surface.

[0136] The bottom electrode section (9 or 9a) can be an anode or a cathode and can be transparent, reflective, opaque, or semi-transparent. If the OLED is top-emitting, the bottom electrode can be made of a transparent metal oxide or a reflective metal such as Al, Au, Ag, or Mg or an alloy thereof and have a thickness of at least 30nm, desirably at least 60nm.

[0137] In a microcavity application where the first electrode is above the reflective layer, the first electrode should be transparent. However, in other applications, the first electrode layers 9A and 9B can be folded into a single reflective electrode such that its topmost reflective surface forms one side of the optical microcavity (i.e., Figure 14 30 in

[0138] When the OLED stack is a top-emitting microcavity and the bottom electrode is transparent, a reflective layer should exist below the bottom electrode, which defines the first side of the microcavity 30. When the transparent anode is located above the reflective surface, the transparent anode is part of the optical cavity. The reflective layer 9B can be a reflective metal such as Al, Au, Ag, Mg, Cu or Rh or their alloys, a dielectric mirror or a highly reflective coating. The dielectric mirror is composed of multiple thin material layers deposited on a substrate, such as magnesium fluoride, calcium fluoride and various metal oxides. The highly reflective coating consists of multiple layers of two materials, one with a high refractive index (such as zinc sulfide (n = 2.32) or titanium dioxide (n = 2.4)), and one with a low refractive index (such as magnesium fluoride (n = 1.38) or silicon dioxide (n = 1.49)). The thickness of the layer is typically a quarter-wave with respect to the wavelength of the light being reflected. It is desirable for the reflective layer to reflect at least 80% of the incident light, and most preferably at least 90% of the incident light. Preferred reflective layers are Al or Ag, with a thickness of 300 to Most preferably 800 to

[0139] Desirably, when the OLED stack is bottom-emitting, the bottom electrode is a transparent anode and should transmit as much visible light as possible, preferably having a transmittance of at least 70% or more desirably at least 80%. Although the bottom transparent electrode can be made of any conductive material, preferred are metal oxides such as ITO or AZO or thin metal layers such as Ag. Materials with lower conductivity (e.g., TiN) can be used provided they are made thin.

[0140] Electron-transporting and hole-transporting materials suitable for non-emissive layers (i.e., Figure 12 11 and 23 in) such as hole-injecting layers, hole-transporting layers or electron-injecting layers or electron-transporting layers are well-known and commonly used. These layers can be mixtures of such materials and can contain dopants to change their properties. Since these layers do not emit light, they do not contain light-emitting materials and are transparent. The choice of suitable materials is not critical, and any material can be selected based on its performance.

[0141] In embodiments utilizing the microcavity effect, since the spacing between various OLED units within the microcavity and the size of the microcavity are important for maximizing efficiency, the thicknesses of various non-emissive layers must typically be selected to provide the desired spacing. Desirably, the spacing between OLED units and the size of the microcavity are adjusted by using an appropriate thickness of an organic non-emissive layer such as a hole-transporting layer.

[0142] The light-emitting layer typically has a host material (or a mixture of host materials) and a light-emitting compound, and the host material is the main component of this layer. Ideally, the light-emitting compound is phosphorescent because they have higher efficiency. However, in some cases, some LELs can use fluorescent or TADF (thermally activated delayed fluorescence) compounds as the materials for light emission, while others use phosphorescent materials. Specifically, the blue OLED layer can use fluorescent or TADF compounds or a combination thereof, and the non-blue light-emitting layer can use green, yellow, orange, or red phosphorescent compounds or a combination thereof. The light-emitting layer can use a combination of light-emitting materials. The selection of suitable materials for LELs is well-known, not critical, and any material can be selected based on its performance and light-emitting characteristics. When using phosphorescent emitters, sometimes it is necessary to confine the excitons generated by the phosphorescent emitters within this layer. Therefore, if necessary, exciton blocking layers can be used on one or both sides of the phosphorescent LEL. Such materials and their applications are well-known. In addition, it may be desirable to add an HBL (hole blocking layer) and an EBL layer (electron blocking layer) around the light-emitting layer, especially the blue light-emitting layer, to improve the lifetime and brightness efficiency.

[0143] If the OLED stack is top-emitting, the top electrode (i.e., Figure 12 25 in and more desirably 125 to

[0144] It is desirable that the top electrode is a thin layer of metal or metal alloy. Suitable metals include Ag, Mg, Al, and Ca or their alloys. Among them, Ag is preferred because it has relatively low blue light absorption ability. To assist electron transport and stabilization, an adjacent layer of a transparent metal oxide such as ITO, InZnO, or MoO3 can be present on the electrode surface. Alternatively, metal halides such as LiCl, organometallic oxides such as lithium quinolate, or other organic materials can be used.

[0145] A protective layer or spacer layer ( Figures 10 to 13 not shown in

[0146] ) may be present above the upper electrode to prevent damage during encapsulation. Above the top electrode 25 and any optional protective layer (if present), the encapsulation 27 is deposited or placed. The encapsulation should at least completely cover the light-emitting area on the top and sides and be in direct contact with the substrate. The encapsulation should be impervious to air and water penetration. The encapsulation may be transparent or opaque. The encapsulation should not conduct electricity. The encapsulation may be formed in-situ or may be added as a separate preformed plate that also seals the side edges. An example of in-situ formation would be a thin-film encapsulation. Thin-film encapsulation involves depositing multiple layers of alternating layers of inorganic materials and polymer layers until the desired level of protection is achieved. The concepts and methods for forming thin-film encapsulation are well-known and any concepts and methods may be used as needed. Alternatively, a preformed plate or cover sheet attached above at least the sealed area and the enclosed area may be used to provide the encapsulation. The preformed plate may be rigid or flexible. It may be made of glass (including flexible glass), metal, or an organic / inorganic barrier layer. It should have a thermal expansion coefficient close to that of the substrate to achieve a more robust connection. The preformed encapsulation plate may need to be attached above the sealed area using a gas-permeable and waterproof adhesive (such as a silicone adhesive or an epoxy adhesive) or by a thermal means (such as ultrasonic welding or glass frit welding) that may require an additional sealant such as solder or glass frit. The side edges and bottom edge of the cover sheet may be specifically designed to fit better into the sealed area or to promote a better seal. The cover sheet and the sealed area may be designed together such that they are partially assembled or locked in place before the seal is formed. Additionally, the cover sheet may be pretreated to promote better adhesion to the sealed area.

[0147] Although this application describes using OLEDs as the light-emitting elements in a display, the same control circuit can be used in any self-emitting display technology that requires a relatively high voltage to emit light. The present invention is not limited to OLEDs, but is applicable to any other display technology that requires greater than 5V, preferably greater than 7.5V, or even greater than 10V to provide at least 1000 nits or preferably at least 5000 nits of light emission.

[0148] A display having a control circuit in a silicon backplane can be of any size and can be used in many different applications; for example, billboards and advertising displays, televisions, mobile applications (such as cell phones), or vehicle interiors.

[0149] The above description describes many different embodiments, which may involve different combinations of different individual features. As desired, unless incompatible, the individual features from any embodiment can be combined in any order or degree without limitation.

[0150] In the above description, reference is made to the accompanying drawings which form a part of the description and in which specific embodiments that can be practiced are shown by way of illustration. These embodiments are described in detail to enable those skilled in the art to practice the present invention, and it should be understood that other embodiments can be utilized and structural, logical, and electrical changes can be made without departing from the scope of the present invention. Therefore, the description of any exemplary embodiment should not be construed as a limitation. Although the present invention has been described for purposes of illustration, it should be understood that such details are for that purpose only and that those skilled in the art can make variations without departing from the spirit and scope of the present invention.

[0151] Partial list

[0152] MP1 switching transistor

[0153] MP2 driving transistor

[0154] C1, C2 capacitors

[0155] V DD and V DD2 and 50 external power supply

[0156] Select 1 - Select 3 selection lines

[0157] T1 first driving transistor

[0158] T2 switching transistor

[0159] T3 series selection (scan) transistor

[0160] T4 second driving transistor

[0161] T5 series selection (scan) transistor

[0162] T6 switching transistor

[0163] V 阴极 cathode voltage

[0164] IBD1 to IBD6 intrinsic body diodes

[0165] V 保护 external voltage source

[0166] I 保护 external current source

[0167] V REF reference voltage

[0168] I REF reference current

[0169] BJT1 Bipolar Junction Transistor

[0170] V C Collector Voltage

[0171] V E Emitter Voltage

[0172] V B Base Voltage

[0173] LSC Level Shifting Circuit

[0174] 1 Pixel Definition Layer

[0175] 3 Silicon Backplane

[0176] 5 Optional Planarization Layer

[0177] 7 Electrical Contact

[0178] 9 First Electrode Section

[0179] 9A First Electrode Layer

[0180] 9B Reflective Layer

[0181] 11, 23 Non - Emissive OLED Layer

[0182] 13 First Emissive OLED Unit

[0183] 13A Bottom - Emitting OLED Unit

[0184] 15, 19, 24 Charge Generation Layer

[0185] 17 Second Emissive OLED Unit

[0186] 17A Second Emissive OLED Unit

[0187] 21 Third Emissive OLED Unit

[0188] 21A Top - Emitting OLED Unit

[0189] 25 Top Electrode

[0190] 27 Encapsulation

[0191] 29 Color Filter Array

[0192] 29B Blue Color Filter

[0193] 29G Green Color Filter

[0194] 29R Red Color Filter

[0195] 30 Microcavity

[0196] 100, 200 RGB pixelated OLED

[0197] 300 , 400 Multi-mode OLED microcavity device

Claims

1. A display includes a light-emitting OLED stack located on top of a silicon-based backplane having individually addressable pixels and control circuitry, wherein: The control circuit of the silicon-based backplane includes at least one driving transistor, wherein a first terminal of the driving transistor is electrically connected to an external power supply (V DD ), and a second terminal of the driving transistor is electrically connected to a segmented bottom electrode of the OLED stack; wherein a gate of the driving transistor is controlled by a data signal provided by a scanning transistor, and the scanning transistor is controlled by a signal from a first select line; and The control circuit further includes a protection circuit, and the protection circuit includes a bipolar junction transistor which has a collector terminal electrically connected to the external power supply (V DD ). wherein the emitter of the bipolar junction transistor is connected to the segmented bottom electrode of the OLED stack.

2. The display according to claim 1, wherein There is a switching transistor between the gate of the scan transistor and the gate of the drive transistor, wherein the gate of the switching transistor is controlled by a signal from a second select line different from the first select line.

3. The display according to claim 1 or 2, wherein, The drive transistor has a rated voltage of 5 V or less.

4. The display according to claim 2, wherein Both the drive transistor and the switching transistor are p-channel transistors.

5. The display according to claim 1 or 2, wherein, The OLED stack includes a single OLED light-emitting unit between the segmented bottom electrode and the top electrode.

6. The display according to claim 5, wherein, The OLED stack forms a microcavity in which the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

7. The display according to claim 5, wherein, The top electrode is transparent or semi-transparent so that the OLED stack emits light from the top.

8. The display according to claim 1 or 2, wherein, The OLED stack includes two or more OLED light-emitting units between the segmented bottom electrode and the top electrode.

9. The display according to claim 8, wherein, The OLED light-emitting units are each separated from one another by a charge generation layer (CGL).

10. The display according to claim 8, wherein, The OLED stack forms a microcavity in which the physical distance between the segmented bottom electrode and the top electrode is constant across all pixels.

11. The display according to claim 8, wherein, The top electrode is transparent or semi-transparent so that the OLED stack emits light from the top.

12. The display according to claim 1 or 2, wherein, The bipolar junction transistor is an NPN transistor, where the base is connected to a voltage source (V 保护 ), or a current source (I 保护 ), the emitter is connected to a node connected to the segmented bottom electrode stacked with the OLED, and the collector is connected to the external power supply.

13. The display according to claim 1 or 2, wherein, The bipolar junction transistor is an NPN transistor, wherein the base is isolated, the emitter is connected to a node connected to the segmented bottom electrode of the OLED stack, and the collector is connected to the external power supply.

14. The display according to claim 1 or 2, wherein, The bipolar junction transistor and the drive transistor are located in separate wells.

15. The display according to claim 1, wherein, There is a switching transistor connected in parallel with the scan transistor between the data line and the gate of the drive transistor, wherein the gate of the switching transistor is controlled by a signal from a second select line different from the first select line.

16. The display according to claim 15, wherein, The drive transistor has a rated voltage of 5 V or less.

17. The display according to claim 15, wherein, Both the drive transistor and the switching transistor are p-channel transistors.

18. The display according to claim 1 or 2, wherein, There are two or more driving transistors connected in series, where a first terminal of a previous driving transistor is electrically connected to the external power supply (V DD ), and a second terminal of a subsequent driving transistor is electrically connected to a segmented bottom electrode of the OLED stack.

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