Pixel element and method for operating a pixel element

Abstract

A pixel element (1) comprises a transmission gate (11) and a sampling structure (20), the sampling structure (20) having a first sampling stage (21) and a second sampling stage (22). The transmission gate (11) and the first and second sampling stages (21, 22) are configured to operate in conjunction with a light source in response to a control signal. The first sampling stage (21) is configured to sample a first sampling value, the first sampling value being dependent on radiation incident on the photosensitive element (10) from an object or scene illuminated by a light source emitting light at a first output power, while the second sampling stage (22) is configured to sample a second sampling value, the second sampling value being dependent on radiation incident on the photosensitive element (10) from an object or scene illuminated by a light source emitting light at a second output power. The first output power is different from the second output power, in particular significantly different.

Description

[0001] The present disclosure relates to a pixel element, in particular to a pixel element having two storage nodes, and to a method for operating such a pixel element.

[0002] CMOS image sensors are used in a wide range of applications, such as camera modules and smartphones, tablets, and laptops. In many of these applications, background subtraction processing is applied, at least in certain capture modes, to improve image quality. This is particularly true in applications where a light source is used to illuminate the object or scene to be captured, such as for facial recognition or fingerprint identification. Due to the limited dynamic range of the image sensor's pixels, excessive background illumination can result in a low signal from the object or scene in the captured image.

[0003] Conventional pixel elements and image sensors typically overcome excessive background illumination and achieve an enhanced signal-to-noise ratio in the pixel element by increasing the power of the light source to achieve higher illumination levels. However, such solutions are only minimally applicable to systems typically characterized by limited power budgets. Alternative solutions use background subtraction of the pixel signal in post-processing, which requires additional circuitry and computing power, and may require multiple exposures, resulting in significant power consumption.

[0004] An object to be achieved is to provide an improved pixel concept which overcomes the limitations of conventional pixel elements.

[0005] This object is achieved by the subject matter of the independent claims. Embodiments and developments of the improved concepts are defined in the dependent claims.

[0006] The improved concept is based on the idea of providing a pixel element in which background light cancellation can be performed within the pixel element during the image acquisition phase, thereby relaxing the need for off-chip data buffers or storage while improving contrast and detection probability in the image. To this end, the pixel element according to the improved concept includes two sampling stages for sampling light signals at different light levels.

[0007] A pixel element according to the improved concept includes a photosensitive element for generating charge in response to incident radiation, a sensing node, and a transfer gate connected between the photosensitive element and the sensing node, the transfer gate for controlling the transfer of charge to the sensing node. The pixel element also includes a first buffer amplifier having an input connected to the sensing node; and a sampling structure connected to an output of the first buffer amplifier, wherein the sampling structure includes a first sampling stage and a second sampling stage. The first sampling stage and the second sampling stage are selectively operable to sample a sampled value of the sensing node.

[0008] The transmission gate and the first and second sampling stages are configured to operate in conjunction with a light source in response to a control signal. The first sampling stage is configured to sample a first sampled value that depends on radiation incident on the photosensitive element from an object or scene illuminated by the light source emitting light at a first output power. Similarly, the second sampling stage is configured to sample a second sampled value that depends on radiation incident on the photosensitive element from an object or scene illuminated by the light source emitting light at a second output power. The first output power is different from the second output power, in particular significantly different.

[0009] The pixel elements are configured to capture optical information incident on the respective pixel elements and to generate electrical information representative of the optical information. In particular for pixel elements and image sensors manufactured according to standard CMOS technology, the pixel elements operate by converting light intensity into photocurrent using a photodiode (e.g., a pin-type photodiode). Silicon-based photodiodes are a common choice for this purpose, as these diodes are sensitive over a wide wavelength range between 190 nm and 1100 nm, and therefore cover relevant parts of the electromagnetic spectrum in the visible and infrared domains.

[0010] The photosensitive element, the transmission gate connected to the photosensitive element, the sensing node (also commonly referred to as a floating diffusion) connected to the transmission gate, and the first buffer amplifier connected between the sensing node and the sampling structure are standard components of a typical CMOS pixel element and are not further elaborated in this disclosure.

[0011] The sampling structure is capable of storing multiple (e.g., two) sampled values of the sensing node. To this end, the sampling structure includes a first sampling stage having a first storage element and a second sampling stage having a second storage element. For example, the first storage element and the second storage element are implemented as capacitors. In addition, the sampling structure is configured to selectively operate the first sampling stage and the second sampling stage. This means that the sampling structure is configured to activate and suspend the path connecting the first buffer amplifier and the path connecting the storage element of the sampling stage. For example, the sampling structure includes a plurality of sampling switches, each sampling switch being implemented, for example, as a transistor that receives a switching signal at a corresponding gate for selectively operating the first sampling stage and the second sampling stage.

[0012] The transmission gate and the first and second sampling stages are configured to operate in conjunction with a light source in response to a control signal, meaning that reading out the photocurrent from the photosensor and storing the signal in the respective sampling stages can be synchronized with a light source (e.g., an LED or a laser diode). For example, the light source is configured to emit broadband light or monochromatic light in the visible domain or narrowband light in the infrared domain, e.g., at or near 940 nm.

[0013] In this manner, a first sampled value can be stored in a storage element of the first sampling stage, the first sampled value depending on the photocurrent generated during the first exposure period and the first output power of the light source. Similarly, a second sampled value can be stored in a storage element of the second sampling stage, the second sampled value depending on the photocurrent generated during the second exposure period and the second output power of the light source. The first output power differs from the second output power. In other words, the first exposure period differs from the second exposure period in terms of the illumination level generated by the light source. For example, the first output power differs from the second output power by an order of magnitude. Alternatively, the first output power or the second output power is zero. Furthermore, in some cases, the first output power can correspond to the second output power.

[0014] Using such a pixel element according to the improved concept, a background image subtraction process can be implemented, wherein the first sampling stage and the second sampling stage sample and store corresponding light signals captured by the photosensitive element with the same exposure time but different lighting conditions. For example, the first light signal is based on the exposure, during which the light source used to illuminate the object or scene to be captured is activated in terms of emitting light; while the second light signal is based on the exposure, during which the light source is disabled in terms of emitting light. The difference signal resulting from the background image subtraction process can be calculated on-chip or off-chip. However, the subtraction operation previously performed within the pixel element during the image acquisition stage can eliminate the need for off-chip data buffers or storage.

[0015] In particular, the pixel structure proposed according to the improved concept realizes the background light elimination function within the pixel element, such as a global shutter operating pixel.

[0016] In some embodiments, the pixel element or the plurality of pixel elements further comprises a controller that controls the transfer gate and the first and second sampling stages in response to a control signal.

[0017] In such an embodiment, the controller is configured to operate the readout and storage of the light signal from the photosensitive element. Thus, a control signal controls the transfer gate to transfer charge from the photosensitive element to the floating diffusion, and switches the first sampling stage and the second sampling stage to transfer charge from the sensing node to a corresponding one of the first sampling stage and the second sampling stage via the first buffer amplifier.

[0018] In some embodiments, the control signal is configured to set the output power of the light source.

[0019] In order to acquire an image with background light cancellation, a light source, such as a laser or LED, is controlled synchronously with a pixel element or an image sensor comprising a plurality of pixel elements according to the improved concept during an exposure phase.

[0020] In some embodiments, the control signal is configured to operate the transmission gate and the first sampling stage, in particular the first switch of the first sampling stage, to sample a first sampling value when the light source emits light at a first output power.

[0021] In some embodiments, the control signal is configured to operate the transmission gate and the second sampling stage, in particular the second switch of the second sampling stage, to sample a second sampling value when the light source emits light at a second output power.

[0022] For synchronization purposes, the control signals in these embodiments are arranged to coordinate the operation of the pixel elements so as to enable storage of two light signals generated based on separate exposures of the light sensitive elements having the same exposure time but different illumination of the light sources.

[0023] In some embodiments, the first output power is significantly greater than or less than the second output power.

[0024] To implement the background subtraction process, the light signals stored at the first sampling stage and the second sampling stage are based on the aforementioned exposures at different illumination levels. To this end, the first exposure is performed at a specific first output power (i.e., luminous level) of a light source that is controlled synchronously with the pixel elements via a control signal. Similarly, the second exposure is performed at a specific second output power that is different from the first output power. For example, the first output power or the second output power is zero. The order of which of the first output power and the second output power is greater can be arbitrarily selected.

[0025] In some embodiments, the transfer gate is configured to transfer substantially all charge from the photosensitive element to the sensing node when the transfer gate is turned on.

[0026] In these embodiments, the charge capacity of the sensing node can be selected to be greater than the charge capacity of the photosensitive element (eg, a pin-type photodiode) so that when the transfer gate is turned on, all photocharge generated during exposure is transferred to the sensing node.

[0027] In some embodiments, the well capacity of the sensing node is greater than the well capacity of the photosensitive element.

[0028] In these embodiments, the control signal can apply multiple pulses to the transfer gate, which allows the pixel element to accumulate more charge, for example, particularly when background light is high and saturation is a concern.

[0029] In some embodiments, the first sampling stage and the second sampling stage of the sampling structure are arranged in series.

[0030] In an alternative embodiment, the first sampling stage and the second sampling stage of the sampling structure are arranged in parallel.

[0031] The improved concept is applicable to a pixel structure characterized by comprising at least two selectively operable sampling stages. In particular, a pixel design having a typical serial or parallel sampling stage can be adopted to realize a pixel element according to the improved concept.

[0032] In some embodiments, the pixel element further includes a readout circuit having an input connected to the sampling structure and configured to generate a first output signal by reading out a first sample value from the first sampling stage, and to generate a second output signal by reading out a second sample value from the second sampling stage.

[0033] As well as being selectively operable to sample and store the optical signal from the sensing node, the first sampling stage and the second sampling stage can also be selectively operable to be read out by a readout circuit. For example, the readout circuit can be configured to first read out the signal stored in the first sampling stage and then read out the signal stored in the second sampling stage, or vice versa.

[0034] In some other embodiments, the readout circuit is further configured to generate a differential signal according to the first output signal and the second output signal.

[0035] The readout circuit can also be configured to generate a differential signal, i.e., perform a background subtraction operation. To this end, the readout circuit can include a circuit for reading out the optical signals stored at the first sampling stage and the second sampling stage, and a circuit for calculating a differential signal from the two optical signals (e.g., by calculating a difference).

[0036] In some embodiments, the pixel element further includes a reset switch configured to operate in conjunction with the light source in response to a control signal, wherein the reset switch is connected to the sensing node for resetting the sensing node and / or the photosensitive element to a predetermined voltage.

[0037] Before each exposure cycle, a reset switch can be controlled by a control signal to reset the photosensitive element to a known predetermined state, so that the initial state of each exposure is the same.

[0038] The above objects are further solved by an electronic device comprising an array of pixel elements according to one of the above embodiments, a light source and a control circuit having a controller configured to generate control signals for operating the array of pixel elements together with the light source.

[0039] For example, the electronic device includes an image sensor, such as a CMOS image sensor, which includes an array of pixel elements according to the improved concept. Each pixel element or image sensor includes a controller for performing the above-mentioned operations of controlling the sampling and storage of two different light signals at a first sampling level and a second sampling level of each pixel element, respectively. For example, the electronic device is a mobile device, such as a smartphone, a portable computer, a wearable device, etc., which includes an image sensor for imaging and / or recognition purposes. In addition, for example, the pixel element according to the improved concept can be used in a time-of-flight (ToF) sensor for distance measurement.

[0040] The foregoing objects are further addressed by a method of operating a pixel element. The method includes generating charge via a photosensitive element in response to incident radiation and transferring the charge to a sensing node via a transfer gate. The method also includes selectively operating a first sampling stage and a second sampling stage of a sampling structure to sample a value at the sensing node. The method also includes operating the transfer gate and the first and second sampling stages in response to a control signal in conjunction with a light source, particularly an output power of the light source.

[0041] The method further comprises sampling, by the first sampling stage, a first sampled value, the first sampled value being dependent on radiation incident on the photosensitive element from an object or scene illuminated by a light source emitting light at a first output power. The method further comprises sampling, by the second sampling stage, a second sampled value, the second sampled value being dependent on radiation incident on the photosensitive element from an object or scene illuminated by a light source emitting light at a second output power. The first output power is different from the second output power, in particular significantly different.

[0042] Other embodiments of the method will become apparent to the skilled reader based on the above-described embodiments of the pixel element.

[0043] The following description of the drawings of exemplary embodiments may further illustrate and explain various aspects of the improved concepts. Components and parts of the pixel elements having the same structure and the same effect are shown with equivalent reference numerals. As long as the components and parts of the pixel unit correspond to each other in terms of their functions in different figures, the following figures will not be repeated.

[0044] In the figure:

[0045] Figures 1 to 4 An exemplary embodiment of a pixel element according to the improved concept is shown; and

[0046] Figure 5 and Figure 6 A timing diagram is shown for explaining the operating principle of the pixel element according to the improved concept.

[0047] Figure 1 A schematic diagram shows an exemplary embodiment of a pixel element 1 according to the improved concept. The pixel element includes a photosensitive element 10 for generating an electrical signal, the electrical signal being dependent on electromagnetic radiation incident on the photosensitive element 10 and generating charge within the photosensitive element 10 based on the photoelectric effect. For example, the photosensitive element 10 is a photodiode, in particular a pinned photodiode.

[0048] The pixel element 1 further includes a transfer gate 11 and a sensing node 12, wherein the transfer gate 11 is connected between the photosensitive element 10 and the sensing node 12. The transfer gate 11 is configured to provide an electrical connection between the photosensitive element 10 and the sensing node 12 upon receiving a specific control signal that turns on the transfer gate 11. The sensing node 12 (also commonly referred to as a floating diffusion) is a node that is electrically isolated from the other nodes of the pixel element. Typically, a floating diffusion is implemented as a quasi-neutral region isolated by a pn junction. Therefore, its potential is entirely determined by the amount of charge stored therein and its capacitance. Its capacitance can be determined by the floating diffusion capacitor 15.

[0049] Pixel element 1 also includes a reset switch 14 for resetting sense node 12 and / or photosensitive element 10 to a predetermined voltage or potential. For example, photosensitive element 10 is reset to a specific voltage before exposure. Similarly, sense node 12 is reset to a specific voltage before charge is transferred from photosensitive element 10 via transfer gate 11 and after charge is transferred from sense node 12 to sampling structure 20.

[0050] Furthermore, the pixel element 1 comprises a first buffer amplifier 13, also commonly called a source follower, connected to the sensing node 12. The first buffer amplifier 13 is configured to amplify the typically small signal of the sensing node 12 before providing said signal to the sampling structure 20.

[0051] The sampling structure 20 includes a first sampling stage 21 and a second sampling stage 22 connected in a series configuration. In this embodiment, each sampling stage 21 and 22 includes a storage capacitor for storing an electrical signal as capacitance. The sampling structure 20 also includes a first sampling switch S1 and a second sampling switch S2 for selectively operating the first and second sampling stages 21 and 22. In this embodiment, when a signal is to be transmitted from the sensing node 12 to the first sampling stage 21, the first sampling switch S1 is closed, while the second sampling switch S2 is open. Similarly, when a signal is to be transmitted from the sensing node 12 to the second sampling stage 22, both the first sampling switch S1 and the second sampling switch S2 are closed. Because, in some cases, both sampling stages 21 and 22 are electrically connected to the first buffer amplifier 13 and, therefore, to the sensing node 12, an effective total capacitance different from the capacitance of the second sampling stage 22 must be considered, particularly when reading and further processing the signals from the first and second sampling stages.

[0052] The sampling structure further comprises a second buffer amplifier 23 for further amplifying the signals read out from the first and second sampling stages 21 , 22 via the readout switch 16 and the column line 30 of the pixel element 1 before providing said signals to the evaluation circuit.

[0053] According to the improved concept, the pixel element 1 is controlled to be synchronized with the light source to capture images and achieve background light elimination. The pixel element 1 can be operated as follows:

[0054] 1. Perform the first exposure of the photosensitive element 10 under ambient light conditions. At the beginning of this exposure period, the light source emission is disabled and photocarriers are integrated in the photodiode of the pixel element.

[0055] 2. At the end of the exposure period, the sensing node 12 is reset to a predetermined level by pulse resetting the switch 14 .

[0056] 3. Subsequently, by applying a pulse to the transmission gate 11 to turn on the transmission gate 11, the charge is transferred to the sensing node 12 through the transmission gate 11. By applying a pulse to the first sampling switch S1 and the second sampling switch S2, the signal is buffered by the first buffer amplifier 13 and stored on the storage capacitor of the second sampling stage 22 as the background signal V BG .

[0057] 4. After this charge transfer, the photodiode is depleted, i.e., all the charge is transferred from the photodiode to the storage capacitor via the sensing node 12 and the first buffer amplifier 13. An additional reset phase can be generated to ensure that all the charge is actually drained from the photosensitive element 10. This reset is achieved by pulsing both the reset transistor 14 and the transfer gate 11.

[0058] 5. Perform a second exposure of the photosensitive element 10 under illumination conditions, that is, when the light source is able to emit light. Similarly, the photocarriers are integrated on the photodiode.

[0059] 6. After resetting the sensing node 12 and resetting the switch 14 by pulsing, the charge is transferred to the sensing node 12 through the transfer gate 11 by applying a pulse to the transfer gate 11 to turn on the transfer gate 11. The signal is again buffered by the first amplifier 13 and in this step is stored in the storage capacitor of the first sampling stage 21 as the signal V by applying a pulse to the first sampling switch S1. SIG .

[0060] For global shutter operation of an image sensor comprising a plurality of pixel elements 1 according to the improved concept, all of the above operations occur synchronously for all pixel elements 1 .

[0061] The pixel element 1 can also include a readout circuit for performing the following operations. The readout circuit can be arranged before or after the column line 30 and is not shown in the figure for illustration purposes. In addition, the pixel element 1 can include a controller for generating control signals for operating the transfer gate 11, the reset switch 14, and the first and second sampling switches S1 and S2.

[0062] 7. After the image acquisition sequence, the frame readout can begin. To read out the acquired frame, the background signal V sampled by the second sampling stage 22 is read out through the second buffer amplifier 23 and the reset switch 16. BG , and the resulting output signal V at column line 30 out,1 Ability to write:

[0063] V out，1 = BG .

[0064] 8. Then, the second sampling switch S2 is pulsed to be in a closed state, thereby enabling charge sharing between the storage capacitors of the first and second sampling stages 21, 22. After the second sampling switch S2 is pulsed to be in an open state, the resulting output signal V transmitted to the column line 30 is out,2 Ability to write:

[0065] V out，2 =(VSIG +V BG )·(C1 / (C1+C2)),

[0066] Wherein, C1 and C2 represent the capacitances of the storage capacitors of the first sampling stage 21 and the second sampling stage 22, respectively.

[0067] 9. To perform background elimination, two signals V can be calculated by analog circuits or digital circuits. out,1 and V out,2 The difference between V out And output it from the pixel element as:

[0068] V out =(V SIG -V BG )·(C1 / (C1+C2)).

[0069] In an image sensor comprising a plurality of pixel elements 1 according to the improved concept, this readout operation is performed sequentially row by row by scanning the array of pixel elements 1 , wherein this can be achieved without disturbing the signal stored on the photosensitive element 10 of each pixel element 1 .

[0070] It is worth noting that the described order can be alternating, particularly with respect to exposure. For example, the first exposure can be performed with the light source enabled for light emission, while the second exposure is the background light exposure. Similarly, both exposures can be performed with the light source enabled for light emission, for example, at different output powers.

[0071] Figure 2 Shows something like Figure 1 FIG2 is a schematic diagram of an exemplary embodiment of a pixel element 1. In this figure, the first and second buffer amplifiers 13 and 23, as well as the reset switch 16, are shown implemented as transistors, a common choice of switches used in pixel elements. Furthermore, the pixel element 1 in this embodiment also includes a pre-charge transistor 17, which is configured as a load transistor and serves as a current source for the first buffer amplifier 13.

[0072] Alternatively or additionally, the pre-charge transistor 17 may be used to discharge one or both of the storage capacitors of the first and second sampling stages 21 , 22 of the sampling structure 20 before sampling the voltage signal from the floating diffusion 12 .

[0073] Figure 3 and Figure 4 An exemplary embodiment of a pixel element 1 according to the improved concept is shown, wherein a sampling structure 20 comprises a first sampling stage and a second sampling stage 21, 22, and Figure 1 and Figure 2The first sampling stage 21 and the second sampling stage 22 are arranged in a parallel configuration as compared to a series configuration. As a result, the sampling structure includes additional first and second readout switches R1 and R2 for transferring the respective charges from the storage capacitors of the first and second sampling stages 21 and 22 to the column output 30 via the second buffer amplifier 23 and the readout switch 16.

[0074] For the parallel configuration, the two sampling stages 21, 22 can be operated selectively and independently without the charge transfer of the series configuration described above. Therefore, this configuration simplifies the background elimination process at the expense of additional components (i.e., transistors). However, the operating principle of the pixel element 1 having the first and second sampling stages 21, 22 arranged in parallel remains similar to the operation described above with respect to the series configuration.

[0075] It is noteworthy that the above-described operating principles can also be applied to alternative pixel element architectures featuring additional components, such as additional photosensitive elements 10 or transistors, but including at least two selectively operable sampling stages. In particular, the above-described operating principles can be applied to various architectures based on or derived from 3T (three transistor) and 4T (four transistor) pixel element architectures.

[0076] Figure 5 and Figure 6 A timing diagram is shown to illustrate the operating principle of a pixel element according to the improved concept, which performs double exposure to achieve a background removal process by synchronizing the exposure with the illumination light source.

[0077] Figure 5 The timing diagram shows the applied Figure 1 and Figure 2 The control signals for the various components of a pixel element and a light source (e.g., a camera flash or laser diode) are shown. The x-axis represents time, while the y-axis shows the control signals applied to the various components in a stacked fashion. Each control signal can be understood as a signal with two discrete levels: on and off, up or down, 1 or 0. Furthermore, the dashed lines depict the three phases of pixel element operation.

[0078] In this example, during the first "laser off" phase, a first exposure is performed, during which the light source's emission is disabled (i.e., turned off). During the second "laser on" phase, a second exposure is performed, during which the light source's emission is enabled (i.e., turned on and outputting light at a specific output power). Finally, during the third "readout" phase, the signal is read out from the two exposures stored in the first and second sampling stages.

[0079] In detail, before the first exposure, the initial pulse of the transmission gate 11 and the reset switch 14 initiates the predetermined starting state of the photosensitive element 10 and the sensing node 12 by means of the RST pulse and the TX pulse of the control signal. The closing of the transmission gate 11 and the reset switch 14 marks the beginning of the first exposure phase. During this entire phase, the emission of the light source (here a laser, for example a laser emitting at approximately 940 nm in the infrared domain for facial recognition purposes) is disabled. The re-pulsing of the reset switch 14 by the RST pulse ensures that the sensing node 12 is indeed in its initial state. The TX pulse of the transmission gate 11 during the "laser off" phase marks the end of the period during which charge is integrated in the photosensitive element 10 and the charge is transferred from the photosensitive element 10 to the sensing node 12.

[0080] Subsequently, both sampling switch S1 and sampling switch S2 are pulsed to transfer charge from sensing node 12 to the storage capacitor of second sampling stage 22 via first buffer amplifier 13. At the end of the "laser off" phase, second sampling switch S2 is opened, and photosensitive element 10 and sensing node 12 are reset to the initial state again.

[0081] The closure of transfer gate 11 and reset switch 14 now marks the beginning of the second exposure phase. During this entire phase, emission from the light source is enabled. Re-pulsing reset switch 14 via the RST pulse reassures that sensing node 12 is indeed in its initial state. The TX pulse of transfer gate 11 during the "laser on" phase marks the end of the second period, during which charge is integrated within photosensitive device 10 and transferred from photosensitive element 10 to sensing node 12.

[0082] While sampling switch S1 remains closed, charge is transferred from sensing node 12 directly to the storage capacitor of first sampling stage 21 via first buffer amplifier 13. At the end of the "laser on" phase, first sampling switch S1 is opened. Thus, both exposure phases are completed, and the respective signals are stored on the storage capacitors of first and second sampling stages 21 and 22, respectively.

[0083] The steps of the two exposure phases are defined as global signals, since they are performed simultaneously, for example, on a plurality of pixel elements 1 of an image sensor. Therefore, both exposure cycles constitute a global shutter exposure of the pixel element array.

[0084] During the readout phase marked "ROT" in the figure, all optical signals stored in the sampling stages 21, 22 of the pixel elements 1 of the pixel element array are read out row by row, which is generally referred to as a rolling shutter readout process. During this phase, the readout switch 16 is pulsed by the RS pulse of the control signal to read out the optical signal stored on the storage capacitor of the second sampling stage 22 via the second buffer amplifier 23 and the column line 30. Subsequently, the second sampling switch S2 is pulsed to transmit the optical signal stored on the storage capacitor of the first sampling stage 21. The closing of the readout switch 16 of the corresponding row of pixel elements 1 then starts the readout of the next row of pixel elements 1 in a corresponding manner. Figure 5 In FIG. 1 , for an array of pixel elements having n rows, the row numbers starting from 0 to n are represented by numbers in brackets.

[0085] As previously described, for example, a readout circuit disposed before or after the column line 30 may further process the two signals read out from each pixel element by performing the background light cancellation process described above.

[0086] Figure 6 The timing diagram shows the applied Figure 3 and Figure 4 The different elements of a pixel element are shown, having a first and a second sampling stage 21 , 22 arranged in a parallel configuration, and the control signals applied to the light source.

[0087] In principle, the proposed storage of two light signals exposed under different illumination conditions combined with the described readout mechanism can generally be applied to conventional pixel element architectures with two storage nodes without any changes at the device level except for the synchronization with the light source.

[0088] This patent application claims priority from European patent application 20182563.5, the disclosure of which is incorporated herein by reference.

[0089] It is worth noting that the procedures for the two exposure phases remain unchanged. However, since the light signals can be read out independently from the sampling stages 21, 22 via dedicated first and second readout switches R1, R2, the RT phase is the same as the Figure 5 Here, the readout switch 16 is opened, and the first readout switch R1 and the second readout switch R2 are opened in sequence to perform the above-mentioned separate readouts.

[0090] It will be understood that the invention is not limited to the disclosed embodiments and the embodiments that have been particularly shown and described above. On the contrary, the features described in the independent dependent claims or in the description may be advantageously combined. In addition, the scope of the invention includes those changes and modifications that are obvious to a person skilled in the art and that fall within the spirit of the appended claims. The term "comprising", as used in the claims or in the description, does not exclude other elements or steps of the corresponding features or processes. If the terms "a" or "an" are used with a feature, a plurality of such features is not excluded. In addition, any reference signs in the claims should not be construed as limiting the scope.

[0091] Reference numerals

[0092] 1 pixel component

[0093] 10 Photosensitive element

[0094] 11 Transmission Gate

[0095] 12 Sensing Nodes

[0096] 13. First buffer amplifier

[0097] 14 Reset switch

[0098] 15 Floating diffusion capacitor

[0099] 16 Readout switch

[0100] 17 Precharge transistor

[0101] 20 Sampling Structure

[0102] 21 First sampling stage

[0103] 22 Second sampling stage

[0104] 23 Second buffer amplifier

[0105] 30 lines

[0106] R1, R2 First readout switch and second readout switch

[0107] S1, S2 first sampling switch and second sampling switch

[0108] RST reset pulse

[0109] TX transmit pulse

Claims

1. A pixel element (1), comprising: - a photosensitive element (10) for generating an electrical charge in response to incident radiation; - Sensing node (12); - a transmission gate (11), connected between the photosensitive element (10) and the sensing node (12), for controlling the transmission of charges to the sensing node (12); - a first buffer amplifier (13) having an input connected to the sensing node (12); a sampling structure (20) connected to the output of the first buffer amplifier (13), the sampling structure (20) comprising a first sampling stage (21) and a second sampling stage (22), the first sampling stage (21) and the second sampling stage (22) being selectively operable to sample a sample value of the sensing node (12); as well as a readout circuit having an input connected to the sampling structure (20) and configured to generate a first output signal by reading out a first sampled value from the first sampling stage (21) and to generate a second output signal by reading out a second sampled value from the second sampling stage (22), wherein - the transmission gate (11) and the first sampling stage (21) and the second sampling stage (22) are configured to operate in conjunction with a light source in response to a control signal; - the first sampling stage (21) is configured to sample first sampled values, the first sampled values being dependent on radiation incident on the photosensitive element (10) from an object or a scene illuminated by a light source emitting light at a first output power; - the second sampling stage (22) is configured to sample second sampled values, the second sampled values being dependent on radiation incident on the light-sensitive element (10) from an object or scene illuminated by a light source emitting light at a second output power; - the readout circuit is further configured to generate a differential signal based on the first output signal and the second output signal; and - The first output power is different from the second output power.

2. The pixel element (1) according to claim 1, further comprising a controller configured to control the transmission gate (11) and the first sampling stage (21) and the second sampling stage (22) in response to a control signal.

3. The pixel element (1) according to claim 1 or 2, wherein: The control signal is configured to set the output power of the light source.

4. The pixel element (1) according to claim 1 or 2, wherein: The control signal is configured to operate the transmission gate (11) and the first sampling stage (21) to sample a first sampling value when the light source emits light at a first output power.

5. The pixel element (1) according to claim 1 or 2, wherein: The control signal is configured to operate the transmission gate (11) and the second sampling stage (22) to sample a second sampling value when the light source emits light at a second output power.

6. The pixel element (1) according to claim 1 or 2, wherein: The first output power is greater than or less than the second output power.

7. The pixel element (1) according to claim 1 or 2, wherein: The first output power or the second output power is zero.

8. The pixel element (1) according to claim 1 or 2, wherein: The transmission gate (11) is configured to transfer all charges from the photosensitive element (10) to the sensing node (12) when the transmission gate (11) is turned on.

9. The pixel element (1) according to claim 1 or 2, wherein: The first sampling stage (21) and the second sampling stage (22) of the sampling structure (20) are arranged in series.

10. The pixel element (1) according to claim 1 or 2, wherein: The first sampling stage (21) and the second sampling stage (22) of the sampling structure (20) are arranged in parallel.

11. The pixel element (1) according to claim 1 or 2, further comprising a reset switch (RST), wherein the reset switch (RST) is configured to operate together with the light source in response to a control signal, and the reset switch (RST) is connected to the sensing node (12) for resetting the sensing node (12) and / or the photosensitive element (10) to a predetermined voltage.

12. The pixel element (1) according to claim 1 or 2, wherein: The control signal is configured to operate the transmission gate (11) and the first switch (S1) of the first sampling stage (21) to sample a first sampling value when the light source emits light at a first output power.

13. The pixel element (1) according to claim 1 or 2, wherein: The control signal is configured to operate the transmission gate (11) and the second switch (S2) of the second sampling stage (22) to sample a second sampling value when the light source emits light at a second output power.

14. An electronic device comprising an array of pixel elements (1) according to any one of the preceding claims, a light source and a control circuit, the control circuit having a controller configured to generate control signals for operating the array of pixel elements (1) together with the light source.

15. A method for operating a pixel element (1), the method comprising: - generating an electric charge in response to incident radiation by a photosensitive element (10); - transferring the charge to the sensing node (12) via the transfer gate (11); - selectively operating a first sampling stage (21) and a second sampling stage (22) of a sampling structure (20) to sample a sample value of the sensing node (12); as well as - operating the transmission gate (11) and the first sampling stage (21) and the second sampling stage (22) together with a light source in response to a control signal; - sampling, by means of said first sampling stage (21), first sampled values which depend on radiation incident on the light-sensitive element (10) from an object or a scene illuminated by a light source emitting light at a first output power; - sampling, by means of said second sampling stage (22), a second sampled value, said second sampled value being dependent on radiation incident on the light-sensitive element (10) from an object or a scene illuminated by a light source emitting light at a second output power; as well as - generating a first output signal by reading out first sampled values from the first sampling stage (21) and generating a second output signal by reading out second sampled values from the second sampling stage (22) by means of a readout circuit having an input connected to the sampling structure (20); in - the readout circuit is further configured to generate a differential signal based on the first output signal and the second output signal; and - The first output power is different from the second output power.

16. The method of operating a pixel element (1) according to claim 15, wherein: In response to a control signal, the transmission gate (11) and the first sampling stage (21) and the second sampling stage (22) operate with the output power of the light source.

Images