Pixel circuit, display device, and electronic device

Abstract

Embodiments of the present application provide a pixel circuit, a display device and an electronic device, and relate to the technical field of display. The pixel circuit is used for ultrasonic fingerprint identification, and includes a reset branch, a capacitive integration branch and an output control branch. The reset branch is configured to reset input voltages of the capacitive integration branch and an input voltage of the output control branch. The capacitive integration branch is configured to perform integration processing on an echo signal of ultrasonic waves, and includes a first transistor, a second transistor and a first capacitor. The control electrode of the second transistor is coupled with a first output end of the reset branch, and configured to receive a reset signal output by the reset branch. The input end of the output control branch is coupled with a second output end of the reset branch, and configured to receive the reset signal output by the reset branch. The input end of the output control branch is also coupled with a first electrode of the first capacitor, and configured to receive and process an output signal of the capacitive integration branch, and transmit the output signal to an output end of the pixel circuit.

Description

Technical Field

[0001] This application relates to the field of display technology, and more particularly to a pixel circuit, display device, and electronic device. Background Technology

[0002] Currently, in-screen fingerprint recognition technology has become one of the most important key technologies for mobile phones and other terminal products. There are three main types of fingerprint recognition technologies currently used in products: capacitive fingerprint recognition, optical fingerprint recognition, and ultrasonic fingerprint recognition.

[0003] Among them, ultrasonic fingerprint recognition technology has significant advantages over under-display optical fingerprint recognition technology and capacitive fingerprint recognition technology: this technology can achieve under-display recognition and no longer depends on the light transmittance of the screen, and can be compatible with the new technology of organic light-emitting diode (OLED) screens.

[0004] This is mainly due to the strong penetrating power of ultrasound. By receiving ultrasound waves of different intensities reflected back from the valleys and ridges of the finger, the piezoelectric layer generates electrical signals of different intensities. The imaging chip then forms a fingerprint image based on these electrical signals. However, current ultrasonic under-display fingerprint recognition technology also has the following problems: First, the manufacturing cost of the module is relatively high; second, some effective signals are lost after the echo signal of the ultrasound passes through the thin-film transistor (TFT) pixel circuit, resulting in a small difference in signal amplitude between the valleys and ridges of the finger. Summary of the Invention

[0005] This application provides a pixel circuit, a display device, and an electronic device, mainly used to process ultrasonic echo signals, especially to improve the problem of small signal amplitude difference between the finger valley and finger ridge.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] A first aspect of this application provides a pixel circuit for ultrasonic fingerprint recognition. The pixel circuit includes a reset branch, a capacitance product branch, and an output control branch. The reset branch resets the input voltage of the capacitance product branch and the input voltage of the output control branch. The capacitance product branch integrates the echo signal of the ultrasonic wave and includes a first transistor, a second transistor, and a first capacitor. The control electrode of the first transistor is coupled to an integration enable terminal, the first electrode of the first transistor is coupled to a first power supply voltage terminal, the second electrode of the first transistor is coupled to the first electrode of the second transistor, the second electrode of the second transistor is coupled to the first electrode of the first capacitor, and the control electrode of the second transistor is coupled to the first output terminal of the reset branch to receive a reset signal output by the reset branch. The second electrode of the first capacitor is coupled to a reference ground voltage terminal. The input terminal of the output control branch is coupled to the second output terminal of the reset branch to receive the reset signal output by the reset branch. The input terminal of the output control branch is also coupled to the first electrode of the first capacitor to receive and process the output signal of the capacitance product branch and transmit it to the output terminal of the pixel circuit. The capacitance branch can accumulate the charge change caused by the input signal throughout the entire signal processing time, and finally convert it into the voltage between the capacitor plates. This voltage is then transmitted to the final output terminal through the output control branch and used as the output voltage of the pixel circuit.

[0008] In one possible implementation, the reset branch includes a reset switch and a reset parallel transistor. The control terminal of the reset switch is coupled to the reset signal terminal, its first terminal is coupled to the reference voltage terminal, and its second terminal is coupled to the control terminal of the second transistor. The control terminal of the reset parallel transistor is coupled to the reset signal terminal, its first terminal is coupled to the first terminal of the first capacitor, and its second terminal is coupled to the second terminal of the first capacitor. The reset switch and reset parallel transistor, acting as switching transistors in the initialization circuit, control when the reset signal is applied to the pixel circuit to reset the node voltage. This prevents residual charge from the previous circuit from affecting the current node voltage, thus avoiding interference from residual charge during subsequent capacitor integration and ensuring the accuracy of signal reception, processing, and transmission in the circuit.

[0009] In one possible implementation, the output control branch includes a follower switch and a transfer switch. The control terminal of the follower switch is coupled to the first terminal of the first capacitor, the first terminal of the follower switch is coupled to the first power supply voltage terminal, and the second terminal of the follower switch is coupled to the first terminal of the transfer switch. The control terminal of the transfer switch is coupled to the read enable terminal, and the second terminal of the transfer switch is coupled to the output terminal of the pixel circuit. The function of the output control branch is to orderly transmit the voltage processed by the capacitor branch to the final output terminal.

[0010] In one possible implementation, the voltage at the first power supply voltage terminal is a DC power supply voltage. The main advantage is that the voltage value at the first power supply voltage terminal is constant, which can reduce energy loss during level transitions of variable power supply voltage, and reduce interference between the voltage plates of the first capacitor C1 from other signals (such as power supply voltage).

[0011] In one possible implementation, the pixel circuit also includes a piezoelectric sensing device for emitting and receiving ultrasonic waves, with the control electrode of the second transistor and the second electrode of the reset switch transistor both coupled to the piezoelectric sensing device.

[0012] In one possible implementation, the signal at the integral enable terminal is used to control the on and off states of the capacitance branch.

[0013] In one possible implementation, the signal at the read enable terminal is used to control the on / off state of the output control branch.

[0014] A second aspect of this application provides a method for driving a pixel circuit, comprising: generating a reset signal, resetting the voltage of each node in the circuit, receiving an electrical signal from a piezoelectric induction device, integrating the electrical signal to obtain an integrated signal, and outputting the integrated signal to an image processing chip.

[0015] A third aspect of the present application provides a display device, which includes a pixel circuit and a display screen, the pixel circuit and the display screen being coupled together.

[0016] A fourth aspect of this application provides an electronic device, which includes a display device and a printed circuit board, the display device being disposed on the printed circuit board. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of an electronic device provided in an embodiment of this application;

[0018] Figure 2 This is a schematic diagram of the structure of an ultrasonic sensing unit provided in an embodiment of this application;

[0019] Figure 3 A schematic diagram illustrating the principle of ultrasonic wave reflection by a finger, provided in an embodiment of this application;

[0020] Figure 4 This is a schematic diagram illustrating the principle of the peak detection technology provided in the embodiments of this application;

[0021] Figure 5 An exemplary TFT pixel circuit structure diagram provided for embodiments of this application;

[0022] Figure 6A timing diagram of the TFT pixel circuit for an ultrasonic fingerprint provided in an embodiment of this application;

[0023] Figure 7 A schematic diagram of the echo signals of the finger valley and finger ridge provided in the embodiments of this application;

[0024] Figure 8 This is a schematic diagram of the capacitor integration circuit provided in the embodiments of this application;

[0025] Figure 9 This is a functional module diagram of the TFT pixel circuit provided in the embodiments of this application;

[0026] Figure 10 An exemplary TFT pixel circuit structure diagram provided for embodiments of this application;

[0027] Figure 11 A schematic diagram of the reflected wave flow of the finger valley and finger ridge provided in the embodiments of this application;

[0028] Figure 12 An exemplary TFT pixel circuit timing diagram provided for embodiments of this application;

[0029] Figure 13 Another exemplary TFT pixel circuit structure diagram provided for embodiments of this application;

[0030] Figure 14 Another exemplary TFT pixel circuit structure diagram provided for embodiments of this application;

[0031] Figure 15 Another exemplary TFT pixel circuit timing diagram provided for embodiments of this application. Detailed Implementation

[0032] It should be noted that the terms "first" and "second" used in the embodiments of this application are only used to distinguish features of the same type and should not be construed as indicating relative importance, quantity, order, etc.

[0033] The terms "exemplary" or "for example" used in the embodiments of this application are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0034] The terms "coupling" and "connection" used in the embodiments of this application should be interpreted broadly. For example, they can refer to a physical direct connection or an indirect connection achieved through electronic devices, such as a connection achieved through resistors, inductors, capacitors or other electronic devices.

[0035] First, some basic concepts involved in the embodiments of this application will be explained:

[0036] The principle of ultrasonic fingerprint recognition technology: When ultrasonic waves come into contact with a finger, the signal intensity of the reflected waves will differ because the finger has ridges and valleys. Therefore, by detecting the signal intensity of the reflected waves, the location of the ridges and valleys can be determined, thereby realizing fingerprint recognition.

[0037] Display module: A display component used for displaying images, typically located on the light-emitting side of an electronic device. Display modules can be designed as full-screen, curved screen, irregularly shaped screen, dual-sided screen, or foldable screen, and can also be designed as a combination of full-screen and curved screen, or a combination of irregularly shaped screen and curved screen.

[0038] This application provides an electronic device. This electronic device can be, for example, a consumer electronics product, a home electronics product, an in-vehicle electronics product, a financial terminal product, or a communication electronics product. Consumer electronics products include mobile phones, tablets, laptops, e-readers, personal computers (PCs), personal digital assistants (PDAs), desktop monitors, smart wearable products (e.g., smartwatches, smart bracelets), virtual reality (VR) terminal devices, augmented reality (AR) terminal devices, and drones. Home electronics products include smart door locks, televisions, remote controls, refrigerators, and rechargeable small household appliances (e.g., soymilk makers, robot vacuum cleaners). In-vehicle electronics products include in-vehicle navigation systems and in-vehicle high-density digital video discs (DVDs). Financial terminal products include automated teller machines (ATMs) and self-service terminals. Communication electronics products include servers, storage devices, radar, base stations, and other communication equipment.

[0039] For ease of explanation, we will use an electronic device as an example to illustrate the following. Figure 1 This is a structural block diagram of the electronic device provided in an embodiment of this application. Figure 1 As shown, the electronic device 1 may include one or more of the following components: processor 11, memory 12, and display module 13.

[0040] The processor 11 may include one or more processing cores. The processor 11 connects to various parts within the electronic device 1 using various interfaces and lines. It executes various functions and processes data by running or executing instructions, programs, code sets, or instruction sets stored in the memory 12, and by calling data stored in the memory 12. For example, the processor 11 may be implemented using at least one hardware form of digital signal processing (DSP), field-programmable gate array (FPGA), or programmable logic array (PLA). The processor 11 may integrate one or more of the following: central processing unit (CPU), graphics processing unit (GPU), neural network processing unit (NPU), application processor (AP), and modem. The CPU primarily handles the operating system, user interface, and applications. The GPU is responsible for rendering and drawing the content required to be displayed by the display module 13. The NPU is used to implement artificial intelligence (AI) functions. The modem is used to handle wireless communication. It is understandable that the aforementioned modem may not be integrated into the processor 11, but may be implemented using a separate chip.

[0041] The memory 12 may include random access memory (RAM) or read-only memory (ROM). For example, the memory 12 may include a non-transitory computer-readable storage medium. The memory 12 can be used to store instructions, programs, code, code sets, or instruction sets. The memory 12 may include a program storage area and a data storage area. The program storage area may store instructions for implementing an operating system, instructions for at least one function (such as touch functionality, sound playback functionality, image playback functionality, etc.), instructions for implementing the various method embodiments of this application, etc. The data storage area may store data created based on the use of the electronic device 1 (such as audio data, phonebook, etc.).

[0042] Display module 13 is a display component used for displaying images, and is typically located on the light-emitting side of electronic device 1. Display module 13 can be designed as a full-screen, curved screen, irregularly shaped screen, dual-sided screen, or foldable screen. Display module 13 can also be designed as a combination of full-screen and curved screen, or a combination of irregularly shaped screen and curved screen; this embodiment does not limit this.

[0043] In some embodiments, continue to refer to Figure 1 The display module 13 shown includes a display driver integrated circuit (DDIC) 131, a display screen 132, a touch panel integrated circuit (TPIC) 133, and an ultrasonic sensing unit 134.

[0044] DDIC131 is used to drive display screen 132 for image display. In addition, DDIC131 is connected to processor 11 via mobile industry processor interface (MIPI) to receive image data and instructions from processor 11.

[0045] TPIC133 is used to drive the display screen 132 to receive touch operations, which are triggered by the user using a finger, stylus, or any suitable object. In this embodiment, TPIC133 is also electrically connected to DDIC131 to receive synchronization signals (e.g., frame synchronization signals, line synchronization signals, etc.) sent by DDIC131. Furthermore, TPIC133 is also connected to processor 11 via a MIPI interface to report touch signals to processor 1110.

[0046] For example, the display screen 132 can be a low-temperature poly-silicon (LTPS) display screen, an active-matrix organic light-emitting diode (AMOLED) display screen, a low-temperature polycrystalline oxide (LTPO) display screen, a liquid crystal display (LCD), or a micro light-emitting diode (micro LED) display screen. Of course, this application embodiment does not limit the type of display screen 132; any display screen with touch display function is applicable to this application embodiment, and the above example is merely illustrative.

[0047] The ultrasonic sensing unit 134 is used to transmit and receive ultrasonic waves and generate electrical signals under the action of piezoelectric induction, which are transmitted to DDIC131. DDIC131 processes the received electrical signals to acquire and identify fingerprint images.

[0048] In addition, those skilled in the art will understand that the structure of the electronic device 1 shown in the above figures does not constitute a limitation on the electronic device 1. The electronic device 1 may include more or fewer components than shown, or combine certain components, or have different component arrangements. For example, the electronic device 1 may also include components such as a microphone, speaker, radio frequency circuit, input unit, sensor, audio circuit, wireless fidelity (WiFi) module, power supply, and Bluetooth module, which will not be described in detail here.

[0049] Figure 2 The ultrasonic sensing unit 134 provided in this application embodiment is described as follows: the ultrasonic sensing unit 134 includes a thin-film transistor (TFT) circuit substrate 20, a first electrode 21, a common electrode 22 made of silver (Ag) as the main material (hereinafter referred to as "Ag common electrode"), and a piezoelectric layer 23 located between the first electrode 21 and the Ag common electrode 22.

[0050] The piezoelectric layer comprises a piezoelectric material. When an AC voltage is input to the first electrode 21 and the Ag common electrode 22 (e.g., the first electrode 21 is grounded, and an AC square wave is input to the Ag common electrode 22), the piezoelectric material deforms (or the piezoelectric material causes the substrates of its upper and lower films to vibrate together), thereby generating and transmitting ultrasonic waves. The piezoelectric material can be a polyvinylidene fluoride (PVDF) film piezoelectric material, or other inorganic or organic piezoelectric materials such as AlN / PZT / ZnO.

[0051] The TFT circuit board 20 includes TFT pixel circuits and pixel electrodes on the surface of the board. The TFT pixel circuits are used to process the echo signal of the finger's reflected wave and transmit it to the image processing chip for fingerprint imaging.

[0052] When emitted ultrasonic waves encounter external obstructions, such as a finger reflecting the ultrasonic waves, they form reflected waves. Figure 3 As shown, the reflected wave is emitted to the piezoelectric layer 23 and converted into an alternating voltage. The Ag common electrode 22, as the receiving end, receives the voltage signal converted from the reflected wave. Since there are valleys and ridges in the fingerprint, the energy of the reflected waves formed by the two are different, and the intensity of the reflected wave signals are also different.

[0053] When a finger is detected covering the touch screen, the ultrasonic sensor unit 134 emits ultrasonic waves to the valley of the finger, generating a reflected wave with stronger energy and a stronger reflected wave signal. Conversely, when the ultrasonic sensor unit 134 emits ultrasonic waves to the ridge of the finger, generating a reflected wave with weaker energy and a weaker reflected wave signal, the valley of the finger and the ridge of the finger are characterized by the different intensities of their respective reflected wave signals.

[0054] Therefore, in some embodiments, the TFT pixel circuit design in the ultrasonic sensing unit 134 uses peak detection technology to detect the echo signals of the finger valley and finger ridge.

[0055] The specific principle of peak detection is as follows: Figure 4 As shown: When the output voltage Vout is less than the input voltage Vin, the diode conducts, and the output voltage Vout increases to: the input voltage Vin minus the diode's turn-on voltage Vth. When the output voltage Vout is greater than the input voltage Vin, the diode is turned off, and the output voltage Vout maintains its previous value until the next voltage signal with a higher value arrives. Ultimately, the output voltage Vout will remain near the peak value of the input voltage Vin.

[0056] Peak detection technology utilizes the unidirectional conductivity of a diode to extract the peak value of the input signal waveform. The characteristic of this circuit is that the output voltage Vout remains at the peak value of the previous time period until a new, larger peak value appears or the circuit is reset.

[0057] Figure 5 This application provides an exemplary TFT pixel circuit for ultrasonic fingerprint recognition. All transistors are N-type TFTs. Transistor M802 is a reset switch used to reset the first voltage terminal Vg of the circuit under the control of the reset signal Reset, avoiding the influence of residual charge on the current signal reception and processing, and ensuring the consistency and accuracy of each signal processing. Diode D801 is used to implement peak detection function. Transistor M801 is used to receive peak-related signals and is turned on during the read phase, controlling the source voltage to change with the gate voltage. Transistor M803 is a read switch. When the read signal Read is valid, transistor M803 outputs the peak-related signal to the image processing chip.

[0058] like Figure 6 As shown, the timing of the TFT pixel circuit of the ultrasonic sensing unit 134 can be divided into three stages: Transmit Mode, Receive Mode, and Read Mode.

[0059] In Transmit Mode, the chip and peripheral electronic components transmit a high-voltage periodic input signal Tx Drive to the electrode side of the piezoelectric material. The piezoelectric material is subjected to electrostatic high voltage on both the upper and lower sides, which is converted into high-frequency mechanical vibration and generates ultrasonic waves.

[0060] In Receive Mode, with the Reset signal active, transistor M802 is turned on, setting the voltage at the power supply terminal DBias to its initial value. Through the transmission of transistor M802, the level at the first voltage terminal Vg is also initialized, and diode D801 is in the off state. Afterwards, the Reset signal is set to an inactive level, and transistor M802 is turned off. The ultrasonic echo signal reflected back from the finger passes through the various dielectric layers and reaches the piezoelectric material layer. The nearly stationary piezoelectric material layer is driven to vibrate at high frequency by the reflected ultrasonic echo signal, which then converts the high-frequency vibration into a high-frequency AC pulse signal. The stronger the ultrasonic echo signal, the larger the amplitude of the high-frequency AC pulse signal. This high-frequency AC pulse signal is applied to the first voltage terminal Vg side through capacitor Cf. At this time, the voltage at the power supply terminal DBias changes from low to high. When the voltage at the first voltage terminal Vg is less than the voltage at the power supply terminal DBias, diode D801 conducts, and the voltage at the first voltage terminal Vg increases to: the voltage at the power supply terminal DBias minus the threshold voltage of diode D801. When the voltage at the first voltage terminal Vg is greater than the voltage at the power supply terminal DBias, diode D801 is cut off, and the voltage at the first voltage terminal Vg remains unchanged. Because the echo signals of the ultrasonic waves reflected back from the fingerprint valleys and ridges have different amplitudes, the amplitudes of the AC pulse signals converted from the echo signals are also different, and the peak voltages of the first voltage terminal Vg of the TFT pixel circuits corresponding to the valleys and ridges are also different.

[0061] In Read Mode, the power supply terminal DBias is switched to an invalid level, diode D801 is cut off, the read signal Read is set to an active level, transistor M803 is turned on, and transistor M801 is set to source-follower mode. The source voltage of transistor M801 follows the gate voltage. Therefore, the voltage at the output terminal Vout changes with the voltage change at the first voltage terminal Vg, thereby realizing the mapping of different ultrasonic echo signal intensities through different output voltage magnitudes.

[0062] In the above embodiment, since diode D801 only conducts when the voltage at the first voltage terminal Vg is less than the voltage at the power supply terminal DBias, the voltage level at the power supply terminal DBias is transmitted to the first voltage terminal Vg through diode D801. Therefore, as... Figure 7As shown, the first voltage terminal Vg only stores the portion of the ultrasonic echo signal that is greater than the power supply terminal DBias voltage value DBIAS(H). For the portion of the ultrasonic echo signal that is less than the power supply terminal DBias voltage value DBIAS(H), there is a loss of effective information. Therefore, only a portion of the valley-ridge signal difference information can be collected and compared.

[0063] Furthermore, because the echo signal of ultrasound is a high-frequency sinusoidal signal with a certain attenuation coefficient, the diode in the circuit has a slow response rate and cannot keep up with the changes in the echo signal in real time. This makes it difficult to perform timely detection actions, resulting in the peak value ultimately detected by the first voltage terminal Vg not necessarily being the maximum value of the entire echo signal, thus further losing effective information. Ultimately, this leads to a small difference in the amplitude of the output valley and ridge signals, further causing deviations in subsequent fingerprint imaging.

[0064] Secondly, the pixel circuit contains diode structures, requiring specialized fabrication processes, resulting in high process complexity. Furthermore, the signal at the power supply side (DBias) needs to toggle rapidly, placing even higher demands on the chip.

[0065] Based on this, this application proposes a novel TFT pixel circuit structure for ultrasonic fingerprint recognition. It utilizes capacitance integration to process the ultrasonic echo signals reflected from the valleys and ridges of the fingerprint, thereby improving the signal difference between the valleys and ridges. Secondly, since the circuit structure and operating principle do not involve diode fabrication processes, the process flow can be simplified. Furthermore, the power supply voltage uses a constant DC voltage, eliminating the need for rapidly switching power supplies, saving circuit power consumption, and reducing the impact of the power supply voltage on the transistor's operating state and the node voltage and output voltage of the pixel circuit.

[0066] The capacitance integration method for processing ultrasonic echo signals reflected from finger ridges has significant advantages over traditional peak detection techniques. The basic circuit working principle is as follows: Figure 8 As shown.

[0067] The input signal Vin serves as the gate voltage of the transistor DTFT. When the gate-source voltage difference of the transistor DTFT is greater than the threshold voltage, the transistor DTFT turns on. Regardless of whether the transistor DTFT operates in the linear region or the saturation region (i.e., regardless of the relative magnitudes of the gate and drain voltages of the transistor DTFT), the input signal Vin controls the magnitude of the output current ID of the transistor DTFT. Since current represents the amount of charge passing perpendicularly through any cross-section per unit time, integrating the output current ID of the transistor DTFT over time yields the amount of charge received by capacitor C1 during the time interval t.

[0068] Capacitance is a physical quantity that describes the ability of a capacitor to store charge. The capacitance of a capacitor is defined as the ratio of the amount of charge carried by the capacitor to the potential difference between the two plates of the capacitor. Therefore, the potential difference Vout between the two plates of capacitor C1 is the ratio of the amount of charge carried by the capacitor to the capacitance C of capacitor C1 itself, that is, Vout = Q / C.

[0069] In summary, the capacitor integration method accumulates the charge change caused by the input signal Vin throughout the entire signal processing time. The input signal Vin is received by the gate of the transistor DTFT. The voltage change caused by the input signal Vin affects the drain output current ID of the transistor DTFT. The drain output current ID of the transistor DTFT affects the charge accumulation of capacitor C1, and thus affects the output voltage between the two plates of capacitor C1. The higher the voltage of the input signal Vin, the larger the output current ID of the transistor DTFT, and ultimately, the higher the output voltage Vout between the two plates of capacitor C1.

[0070] like Figure 9 As shown, the TFT pixel circuit mainly comprises three functional modules: an initialization module, a capacitor integration module, and an output module. The initialization module initializes the voltages of key nodes in the TFT pixel circuit to avoid the influence of residual charge and ensure the accuracy of signal reception and processing. The capacitor integration module is mainly used to integrate the echo signals of ultrasonic waves, collecting the echo signals through capacitors. The output module converts the output voltage of the capacitor integration module into the final read voltage, which is then transmitted to the chip for fingerprint image imaging.

[0071] Figure 10 An exemplary TFT pixel circuit provided in this application embodiment has a specific circuit structure including: a reset branch 1001, a capacitance branch 1002, and an output control branch 1003.

[0072] The initialization module corresponds to the reset branch 1001, which is used to reset the input voltage of the capacitance branch 1002 and the input voltage of the output control branch 1003.

[0073] The capacitance integration module corresponds to capacitance integration branch 1002, which is used to integrate the echo signal of the ultrasonic wave.

[0074] The output module corresponds to the output control branch 1003, which is used to receive the reset signal output by the reset branch 1001, and to receive and process the output signal of the above-mentioned capacitance branch 1002, and transmit it to the output terminal of the pixel circuit.

[0075] In some embodiments, the capacitance branch 1002 includes, but is not limited to, a first transistor M3, a second transistor M4, and a first capacitor C1. For example, the control terminal of the first transistor M3 is coupled to the integration enable terminal, the first terminal of the first transistor M3 is coupled to the first power supply voltage terminal AP, the second terminal of the first transistor M3 is coupled to the first terminal of the second transistor M4, the second terminal of the second transistor M4 is coupled to the first terminal of the first capacitor C1, the control terminal of the second transistor M4 is coupled to the first output terminal of the reset branch 1001 for receiving the reset signal output by the reset branch 1001, and the second terminal of the first capacitor C1 is coupled to the reference ground voltage terminal.

[0076] For example, the aforementioned capacitance branch 1002 may further include multiple first transistors, multiple second transistors, and multiple first capacitors. The multiple transistors and multiple capacitors may be cascaded in series or in parallel, but at least one of the first transistors must have its first terminal coupled to the first power supply voltage terminal AP, and at least one of the second transistors must have its control terminal coupled to the first output terminal of the reset branch 1001, and at least one of the first capacitors must have its second terminal coupled to the reference ground voltage terminal.

[0077] The signal from the integration enable terminal EM is used to control the on / off state of the capacitive product branch 1002. The first transistor M3 acts as the switching transistor for the capacitive product branch. When the signal from the integration enable terminal EM is valid, the first transistor M3 is turned on, and the capacitive product branch 1002 is officially started. The second transistor M4 acts as the switching transistor for driving the capacitor to integrate. The gate of the second transistor M4 is coupled to the piezoelectric induction device Q1 and receives the electrical signal generated by the piezoelectric induction device Q1. The voltage at the gate of the second transistor M4 affects the drain output current id of the second transistor M4. By integrating the drain output current id, the amount of charge stored in the first capacitor C1 within a specific time period is obtained. Then, the voltage between the two plates of the first capacitor C1 is obtained based on the amount of charge stored in the first capacitor C1.

[0078] For example, the first terminal of the first transistor M3 is coupled to the first power supply voltage terminal AP. The main advantage is that the voltage value of the first power supply voltage terminal is constant, which can reduce the energy loss of the variable power supply voltage during level transitions, and reduce the interference of other signals (such as power supply voltage) on the voltage between the two plates of the first capacitor C1.

[0079] Transistor operating states are divided into three types: cutoff region, linear region, and saturation region. When the difference between the gate voltage and source voltage is less than the threshold voltage, the transistor operates in the cutoff region. When the difference is greater than the threshold voltage, the transistor is turned on. In the on-state, the transistor's operating state is further divided into the linear region and the saturation region. When the gate voltage is greater than the drain voltage, the transistor operates in the linear region, where both the gate and drain voltages affect the drain current. When the gate voltage is less than the drain voltage, the transistor is in the saturation region, where the drain current depends only on the gate voltage.

[0080] Therefore, the first terminal of the first transistor M3 is coupled to the first power supply voltage terminal AP, ensuring that the voltage of the first power supply voltage terminal AP is a DC voltage and that the voltage value is greater than the gate voltage of the second transistor M4. At this time, when the first transistor M3 is turned on, the voltage signal of the first power supply voltage terminal AP is transmitted to the drain of the second transistor M4 through the first transistor M3. Since the drain voltage of the second transistor M4 is greater than the gate voltage of the second transistor M4, the second transistor M4 operates in the saturation region. The drain current of the second transistor M4 depends only on the gate voltage of the second transistor M4, thus avoiding the influence of the power supply voltage on the drain current of the second transistor M4.

[0081] Of course, the first terminal of the first transistor M3 is not limited to being coupled only to the DC power supply terminal; it can also be coupled to the variable AC power supply terminal. However, it is important to note that the value of the variable AC power supply voltage must be greater than the gate voltage value of the second transistor M4 at any given time. This ensures that the second transistor M4 operates in saturation, so that the drain current of the second transistor M4 depends only on its gate voltage. This weakens the influence of the externally supplied power supply voltage on the drain currents of the second transistors M3 and M4. Consequently, the amount of charge stored in the first capacitor C1 is only related to the input signal coupled to the gate of the second transistor M4, ensuring that the output signal of the final pixel circuit is strongly correlated with the input signal and is not affected by other signals.

[0082] In some embodiments, the reset branch 1001 includes, but is not limited to, a reset switch M1 and a reset parallel transistor M2. The control terminal of the reset switch M1 is coupled to the reset signal terminal RESET, the first terminal of the reset switch M1 is coupled to the reference voltage terminal VREF, and the second terminal of the reset switch M1 is coupled to the control terminal of the second transistor M4. The control terminal of the reset parallel transistor M2 is coupled to the reset signal terminal RESET, the first terminal of the reset parallel transistor M2 is coupled to the first terminal of the first capacitor C1, and the second terminals of the reset parallel transistor M2 and the second terminals of the first capacitor C1 are both coupled to the reference ground voltage terminal GND.

[0083] For example, the reset branch 1001 described above may also include multiple reset switches and multiple reset parallel transistors. The multiple reset switches and multiple reset parallel transistors may be cascaded in series or in parallel. However, the first terminal of at least one of the multiple reset switches must be coupled to the reference voltage terminal VREF, the control terminal of at least one of the multiple reset switches must be coupled to the reset signal terminal RESET, the second terminal of at least one of the multiple reset switches must be coupled to the input terminal of the capacitance branch, and the control terminal of at least one of the multiple reset parallel transistors must be coupled to the reset signal terminal RESET, the second terminal of at least one of the multiple reset parallel transistors must be coupled to the reference ground voltage terminal GND, and the first terminal of at least one of the multiple reset parallel transistors must be coupled to the input terminal of the output control branch.

[0084] Reset switch M1 and reset parallel transistor M2, acting as switching transistors in the initialization module, control when the reset signal is applied to the pixel circuit, resetting the node voltages at port A and port B of the pixel circuit. When the RESET signal is valid, reset switch M1 and reset parallel transistor M2 are turned on. The signal at the reference voltage terminal VREF is transmitted to port A through reset switch M1, and the signal at the reference ground voltage terminal GND is transmitted to port B through reset parallel transistor M2. The reason for resetting the node voltages at the ports is to prevent residual charge from the previous circuit from affecting the current node voltage, thus avoiding interference from residual charge during subsequent capacitor integration and ensuring the accuracy of signal reception, processing, and transmission in the circuit.

[0085] In some embodiments, the output control branch 1003 includes, but is not limited to, a follower switch M6 and a transmission switch M5. The control terminal of the follower switch M6 is coupled to the first terminal of the first capacitor C1. The first terminal of the follower switch M6 is coupled to the first power supply voltage terminal AP. The second terminal of the follower switch M6 is coupled to the first terminal of the transmission switch M5. The second terminal of the transmission switch M5 is coupled to the output terminal of the pixel circuit. The control terminal of the transmission switch M5 is coupled to the read enable terminal READ.

[0086] For example, the output control branch 1003 may include multiple follower switches and multiple transmission switches, which may be cascaded in series or in parallel. The control terminal of at least one of the transmission switches must be coupled to the read enable terminal READ, and the second terminal of at least one of the transmission switches must be coupled to the output terminal VOUT. The control terminal of at least one of the follower switches must be coupled to the first terminal of the first capacitor C1.

[0087] The follow-up switch M6 and the transmission switch M5 are used as switching transistors in the output module to convert the output signal of the received capacitance branch into the final output voltage and transmit it to the output terminal VOUT of the pixel circuit. The image chip coupled to the output terminal processes the output signal to perform fingerprint image imaging.

[0088] The control electrode of the transmission switch M5 is coupled to the read enable terminal READ. The signal at the read enable terminal READ is used to control the on and off state of the output control branch 1003. When the signal at the read enable terminal READ is valid, the transmission switch M5 is turned on, transmitting the signal following the source of the switch M6 to the output terminal VOUT of the pixel circuit.

[0089] The gate of the follower switch M6 is coupled to the first terminal of the reset parallel transistor M2, receiving the RESET signal to clear the influence of residual charge from the previous circuit on the gate of the follower switch M6. The gate of the follower switch M6 is also coupled to the output terminal of the capacitance branch 1002, receiving the output signal of the capacitance branch 1002. Therefore, changes in the source voltage of the follower switch M6 are affected by the output signal of the capacitance branch 1002.

[0090] For example, the first terminal of the follower switch M6 is coupled to the first power supply voltage terminal AP. The main advantage is that the voltage value of the first power supply voltage terminal AP is constant, which reduces the interference of the power supply voltage on the drain current of the follower switch M6. Otherwise, the noisy signal output by the follower switch M6 will be transmitted to the output terminal Vout through the transmission switch M5, which will directly cause the signal at the output terminal Vout of the pixel circuit to be interfered with.

[0091] Of course, the first terminal of the follower switch M6 is not limited to being coupled only to the DC power supply terminal; it can also be coupled to the variable AC power supply terminal. However, it is important to note that the value of the variable AC power supply voltage must be greater than the gate voltage of the follower switch M6 at any given time. This ensures that the follower switch M6 operates in saturation, so that the drain current of the follower switch M6 depends only on the gate voltage of the follower switch M6. This weakens the influence of the external power supply voltage on the drain current of the follower switch M6, thereby avoiding interference from the power supply voltage on the output signal Vout.

[0092] The pixel circuit also includes a piezoelectric sensing device Q1, which is used to transmit and receive ultrasonic waves. The control electrode of the second transistor M4 and the second electrode of the reset switch M1 are both coupled to the piezoelectric sensing device Q1.

[0093] The piezoelectric sensing device Q1 includes a TFT pixel electrode, a piezoelectric layer, and a common electrode layer (such as an Ag electrode). During the transmission phase, a high-voltage pulse signal is output to the common electrode layer via an integrated circuit chip and peripheral electronic components, setting the TFT pixel electrode to a fixed level. The piezoelectric layer, subjected to electrostatic high voltage from the upper and lower electrodes, transforms this into high-frequency mechanical vibration, emitting ultrasonic waves that are transmitted to the OLED screen. When a finger presses against the top layer of the OLED screen cover, some of the ultrasonic waves are reflected back, while a small portion continues to propagate forward. During the reception phase, the ultrasonic waves are reflected back from the finger. The reflected ultrasonic waves pass through the various layers of the piezoelectric sensing device and reach the piezoelectric layer. The nearly stationary piezoelectric layer is then driven to vibrate at high frequency by the reflected ultrasonic waves, converting this high-frequency vibration into a high-frequency pulse electrical signal. This signal is processed by the TFT pixel circuit and then transmitted to the image processing chip. Finally, the image processing chip obtains the final ultrasonic fingerprint image.

[0094] like Figure 11 As shown, fingerprints have valleys and ridges, and the reflected waves generated by these two types of waves have different energies and signal intensities. When an ultrasonic wave is emitted to a finger valley, the reflected wave energy is stronger and the signal intensity is greater; when the ultrasonic wave is emitted to a finger ridge, the reflected wave energy is weaker and the signal intensity is lower. The difference in the intensity of the reflected wave signals from the finger valleys and ridges causes different amplitudes of vibration in the piezoelectric sensor, resulting in different magnitudes of the electrical signals generated by the sensor. After processing by the pixel circuit, these signals are transmitted to the image processing chip, ultimately resulting in different images of the fingerprint valleys and ridges.

[0095] Therefore, the difference between the finger valleys and finger ridges is ultimately characterized by the intensity of their respective reflected wave signals and the intensity of the electrical signals generated by the driving piezoelectric sensing device, which are then presented through fingerprint imaging.

[0096] like Figure 12 As shown below, the working principle and circuit state of the pixel circuit provided in the embodiments of this application are described. The circuit state can be mainly divided into three stages: the transmission stage, the reception stage, and the reading stage.

[0097] During the transmission phase, the chip inputs a high-voltage periodic signal TX-DRIVE to the piezoelectric material and electrode side, causing the piezoelectric material to deform and generate ultrasonic waves. When the RESET signal at the reset signal terminal is valid, the reset switch M1 and the reset parallel M2 are turned on. The VREF signal at the reference voltage terminal is transmitted to port A through the reset switch M1 for initialization, and the GND signal at the reference ground voltage terminal is transmitted to port B through the reset parallel M2 for initialization.

[0098] During the receiving phase, the RESET signal is invalid, the EM signal is enabled, the first transistor M3 is turned on, and the signal from the first power supply voltage terminal AP is transmitted to the drain of the second transistor M4 through the first transistor M3. The voltage value of the first power supply voltage terminal AP is set to be greater than the peak value of the finger reflection wave signal to ensure that during the same finger press, the gate voltage of the second transistor M4 is less than the drain voltage of the second transistor M4, so that the second transistor M4 is in the saturation region. The drain current of the second transistor M4 is only related to the gate voltage of the second transistor M4. During the same press, the signal intensity of the reflected wave from the finger valley and the finger ridge is different. Therefore, the electrical signal intensity generated by the piezoelectric sensing device Q1 is different, and the gate voltage and drain current of the second transistor M4 are different. Since the capacitor does not have unidirectional conductivity like the diode, the capacitor is in a charging and discharging state throughout the entire finger press time. Therefore, the entire reflected wave signal effectively controls the second transistor M4, and there is no circuit that only conducts when the voltage of the reflected wave signal is greater than a certain value. This scheme comprehensively collects information on the entire reflected wave signal throughout the entire finger pressing period. Because the reflected wave signal amplitudes of the fingerprint valleys and ridges are different, the charges ultimately stored between the plates of the first capacitor C1 are different, and the voltage at port B is also different.

[0099] During the reading phase, when the READ signal is active, the transmission switch M5 and the follower switch M6 are turned on. The voltage at port B is transmitted from the gate of the follower switch M6 to the source, and then through the transmission switch M5 to the output terminal VOUT. Therefore, the voltage at the output terminal VOUT of the pixel circuit changes with the voltage at port B, thereby realizing the transformation of different reflected wave signal intensities into different output voltage magnitudes.

[0100] like Figure 13 As shown, in some other embodiments, the reference ground voltage terminal GND can be replaced with an arbitrary voltage terminal VSET. The voltage value of the arbitrary voltage terminal VSET is less than the voltage value of the reference voltage terminal VREF minus the threshold voltage value Vth, which ensures that the second transistor M4 is turned on normally during the receiving stage.

[0101] like Figure 14 As shown, in some other embodiments, the source voltage terminal VL of the reset parallel transistor M2 is not coupled to any voltage terminal VSET, and the voltage value of the source voltage terminal VL is set to be different from the voltage value of any voltage terminal VSET, ensuring that the voltage value of the source voltage terminal VL is at least less than: the reference voltage terminal VREF minus the threshold voltage value Vth, wherein the voltage value of any voltage terminal VSET can be any fixed value.

[0102] like Figure 15As shown, in some other embodiments, the signal of the integration enable terminal EM can also be set to an effective level during the reading phase. Since the echo signal of the reflected wave has a short duration, there is no effective waveform information in the echo signal of the reflected wave during the reading phase. Therefore, whether the signal of the integration enable terminal EM is effective during the reading phase has little impact on the output of the circuit.

[0103] In this embodiment, the transistor is an N-type metal-oxide-semiconductor (NMOS) field-effect transistor. In practical applications, P-type metal-oxide-semiconductor field-effect transistors, complementary metal-oxide-semiconductor field-effect transistors, and other circuit structures can also be used, and the timing can be reversed accordingly.

[0104] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A pixel circuit, characterized in that, The pixel circuit is used for ultrasonic fingerprint recognition, and the pixel circuit includes: a reset branch, a capacitance branch, and an output control branch; The reset branch is used to reset the input voltage of the capacitance product branch and the input voltage of the output control branch. The capacitance product branch is used to integrate the echo signal of the ultrasonic wave. The capacitance product branch includes a first transistor, a second transistor, and a first capacitor. The control terminal of the first transistor is coupled to the integration enable terminal. The first terminal of the first transistor is coupled to the first power supply voltage terminal. The second terminal of the first transistor is coupled to the first terminal of the second transistor. The second terminal of the second transistor is coupled to the first terminal of the first capacitor. The control terminal of the second transistor is coupled to the first output terminal of the reset branch for receiving the reset signal output by the reset branch. The second terminal of the first capacitor is coupled to the reference ground voltage terminal. The input terminal of the output control branch is coupled to the second output terminal of the reset branch and is used to receive the reset signal output by the reset branch; the input terminal of the output control branch is also coupled to the first pole of the first capacitor and is used to receive and process the output signal of the capacitance product branch and transmit it to the output terminal of the pixel circuit.

2. The pixel circuit according to claim 1, characterized in that, The reset branch includes a reset switch and a reset parallel transistor; The control electrode of the reset switch transistor is coupled to the reset signal terminal, the first electrode of the reset switch transistor is coupled to the reference voltage terminal, and the second electrode of the reset switch transistor is coupled to the control electrode of the second transistor. The control terminal of the reset parallel transistor is coupled to the reset signal terminal, the first terminal of the reset parallel transistor is coupled to the first terminal of the first capacitor, and the second terminal of the reset parallel transistor is coupled to the second terminal of the first capacitor.

3. The pixel circuit according to claim 1 or 2, characterized in that, The output control branch includes a follower switch and a transmission switch; The control terminal of the follower switch is coupled to the first terminal of the first capacitor, the first terminal of the follower switch is coupled to the first power supply voltage terminal, and the second terminal of the follower switch is coupled to the first terminal of the transmission switch. The control terminal of the transmission switch is coupled to the read enable terminal, and the second terminal of the transmission switch is coupled to the output terminal of the pixel circuit.

4. The pixel circuit according to claim 1, characterized in that, The voltage at the first power supply voltage terminal is a DC power supply voltage.

5. The pixel circuit according to claim 2, characterized in that, The pixel circuit also includes a piezoelectric sensing device for emitting and receiving ultrasonic waves. The control electrode of the second transistor and the second electrode of the reset switch are both coupled to the piezoelectric sensing device.

6. The pixel circuit according to claim 1, characterized in that, The signal at the integral enable terminal is used to control the on and off states of the capacitance branch.

7. The pixel circuit according to claim 3, characterized in that, The signal at the read enable terminal is used to control the on / off state of the output control branch.

8. A driving method for a pixel circuit, characterized in that, The driving method for the pixel circuit is applied to the pixel circuit as described in any one of claims 1-7, wherein the driving method for the pixel circuit includes: Generate a reset signal to reset the voltage at each node in the circuit; The electrical signal output from the piezoelectric induction device is received, and the electrical signal is subjected to capacitance integration processing to obtain an integrated signal. The integral processing signal is output to the image processing chip.

9. A display device, characterized in that, It includes a pixel circuit as described in any one of claims 1-7, and a display screen; the pixel circuit and the display screen are coupled together.

10. An electronic device, characterized in that, It includes the display device as described in claim 9, and a printed circuit board, wherein the display device is disposed on the printed circuit board.

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