Improved image sensor reset circuit for reducing image lag

Abstract: The present invention is an imaging system that includes sensor logic, an image processor receiving and transmitting data to the sensor logic, a reverse bias controller, and a pixel array having hysteresis or image lag. In the present invention, one embodiment to minimize lag is to eliminate the charge contributed during the scanning period and maintaining an optimum reverse voltage at the photodiode node. These actions minimize the trapping of electrons and holes at the defect sites. The reverse bias condition lowers the charge capture rate at defect sites by pulling the charge to positive and negative terminals immediately after illumination induced electron and hole generation.

Description: Field of the Invention The present invention is directed towards the field of imaging electronics particularly towards minimizing residual charge in amorphous silicon pixel arrays. BACKGROUND Amorphous silicon (a-Si) has been used to build three dimension pixels for optical sensors. This material enables CMOS sensors with greater than 90% fill factor. For high dynamic range scenes involving motion, image lag results in the smearing of bright spot into dark spot. Figure 1 shows a scene where a bright spot has moved from A to C, leaving a trace of residual light behind. Image lag is maximized when a scene illumination is quickly changed from bright to dark. An active pixel array (APS) sensor of the prior art comprises a first-n-channel transistor, the gate of which receives the voltage controller output and the drain of which is connected to V DD . An a-Si diode is reverse-biased between the source of the first n-channel transistor and ground. For a second n-channel transistor, the gate is connected to the source of the first n-channel transistor and the drain is connected to V DD . For a third n-channel transistor, the drain is connected to the source of the second n-channel transistor, the gate receives ROW data, and the source is connected to the output V OUT . In the article of Aflatooni K., et. al.: "Dependence of image transient behaviour on operating parameters of amorphous silicon image sensors" in Materials Research Society Symp. Proc., vol. 557, April 5, 1999, pages 827 to 832, the transient behaviour in imaging characteristics using amorphous silicon X-ray image sensors is described. The results of a systematic study on the image lag and its dependence on various operating conditions of the image sensors, such as applied bias, light intensity, frame time, and applied gate voltage is described. The charge retention in the intrinsic region of the photodiode is found to be an important source of the image lag and it is correlated to the forward transient characteristics of the diode. This correlation can be explained by the common underlying mechanism leading to depletion region formation and charge transport through the diode. The time dependent decay of the time lag, originated mainly from charge retention in the diode, presents a power-low time dependence, which is consistent with the release of trapped charges from a continuous distribution of defect states. It is the object underlying the present invention to provide an improved imaging system enabling for minimizing time lag upon recording a scene. This object is achieved by an imaging system according to claim 1. SUMMARY The present invention is an imaging system that includes sensor logic, an image processor receiving and transmitting data to the sensor logic, a reverse bias controller, and a pixel array having image lag. The reverse bias controller effects the image lag. In one embodiment, the image lag is minimized by eliminating or reducing the charge contributed during the scanning period and maintain a reverse voltage at the diode node. These actions minimize the trapping of electrons and holes at the defect sites. The reverse bias condition at int2 lowers the charge capture rate at defect sites by pulling the charge to positive and negative terminals immediately after illumination induced electron and hole generation. The reverse bias controller includes a PRESET line. The PRESET line controls the int2 reverse bias condition. For a first n-channel field effect transistor (n-FET), the drain is connected to the PRESET line, the gate is connected to a RESET line. An a-Si diode is reverse biased between the source of the first nFET and ground. For a second n-FET, the drain is connected to V DD , the gate is connected to the source of the first n-FET. For a third n-FET, the drain is connected to the source of the second n-FET, the gate is connected to the ROW DECODER within the image processor, and the source is PIXEL out. The useful range of the pixel is from V dd -V t to GND+1V t . A voltage clamp limits the integration level at int1 to GND+1V t by placing a nominally 2V t at the gate of the reset transistor during int1. In another embodiment, the reverse bias controller acts as a clamping circuit to create a programmable voltage of 3 V t +/- V t . A higher clamp voltage will limit the dynamic range, while a lower level allows the diode to integrate to voltages as low as ½ V t . The clamp voltage can be adjusted according to the signal path gain so as to maximize reverse bias on the PD node while not limiting the dynamic range. This clamp circuit limits the possibility of forward biasing the a-Si diode due to very high illuminations during int1 and has a programmable feature to optimize the reverse bias condition level. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a scene where a bright spot occurs using pixel array of the prior art. Figure 2 illustrates an amorphous silicon pixel (diode) of the prior art. Figure 3 illustrates a functional block diagram for a pixel array controller of the present invention. Figure 4 illustrates a VGA array where INT2>INT1 in a very bright lighting scene. Figure 5 illustrates a circuit schematic diagram for a pixel sensor controlled by the voltage controller shown in Figure 3. Figure 6 illustrates a functional block diagram for the voltage controller 16 shown in Figure 3. Figure 7 shows a schematic diagram embodiment for the embodiment shown in Figure 3. DETAILED DESCRIPTION OF THE DRAWINGS In the present invention, every pixel is conditioned to minimize amorphous silicon image lag. Figure 2 illustrates a PIN-diode operation where image lag is due to the presence of crystal imperfections in amorphous silicon (a-Si) material structure. When a photon hits a-Si material, electron-hole pairs are generated. Electrons are attracted to the positive terminal and holes to the negative terminal. Due to imperfections in the a-Si material, e.g. deep and shallow level defects, some of these carriers are captured at the defect sites and not released until later. Depending on temperature and electric field, it may take up to 100 seconds for very deep level defects to release captured charge. Figure 3 illustrates a functional block diagram for a pixel array controller 10 of the present invention. Sensor logic 12 is connected to an image processor 14. The image processor 14 transmits a gain signal to a reverse bias controller 16. The reverse bias controller 16 emits a RESET signal to an a-Si pixel array 18. The pixel array 18 is further connected to a power supply VDD and a preset voltage selector PRESET. Image sensors are made by integrating a plurality of single pixels into an array 18. Common Interchange Formant (CIF) format is 352x288 pixels while V Graphic Array (VGA) format is 640x480 pixels. Figure 4 shows a VGA array with two integration times: Int1 and Int2. Int1 is the time required to sense a scene. Int2 is the time between read operations of a single pixel. Alternatively, Int2 is just the wait time to maintain an optimum voltage level and pixel cycling. Int2 is expressed as follows: Int 2 = [ Total_frame_read_time ]- Int 1 When the duration of Int2 is twice that of Int1, the amount of discharge on the photodiode (PD) node during Int2 is twice as much as Inti. If Int2 is twice the duration of Int1, the amount of discharge on the PD node during int2 will be twice as much as during int1 under the same illumination. As a result of the charge accumulation, the voltage level at PD node can forward bias the a-Si diode. This corresponds to a high charge capture rate at the deep defect sites. Image lag is the main barrier to acceptance of a-Si in CMOS sensor technology. There are several causes image lag including the material deposition techniques, deep and shallow level defects, and circuit interactions. In the present invention, one embodiment to minimize lag is to eliminate the charge contributed during the scanning period and maintaining an optimum reverse voltage at the photodiode node PD (see Fig. 5). These actions minimize the trapping of electrons and holes at the defect sites. The reverse bias condition at int2 lowers the charge capture rate at defect sites by pulling the charge to positive and negative terminals immediately after illumination induced electron and hole generation. One embodiment of the pixel circuit 18 n of the present invention, shown in Figure 5, includes a PRESET line. The PRESET line controls the int2 reverse bias condition. For a first n-channel field effect transistor (n-FET) 20, the drain is connected to the PRESET line, the gate is connected to a RESET line. An a-Si diode 22 is reverse biased between the source of the first nFET 20 and ground. For a second n-FET 24, the drain is connected to V DD , the gate is connected to the source of the first n-FET 20. For a third n-FET 26, the drain is connected to the source of the second n-FET 24, the gate is connected to the ROW DECODER within the image processor, and the source is PIXEL out. In this embodiment, voltage controller 16 provides the RESET signal. Initially, the reset line is set to 3.3 V. This creates a voltage on the PD node of ∼2.2V. The PD node is positively charged. When the scene is illuminated, the electron moves to the source of the RESET transistor and lowers the PD node from the RESET value of ∼2.2V to close to oV. The clamp voltage is applied to the gate of the RESET transistor. Due to the subthreshold behavior of the RESET transistor, the PD node will be clamped to the same voltage as the clamp voltage. This is due to a fight at the PD node between photon generated current and RESET transistor subthreshold current. The useful range of the pixel is from V dd -V t to GND+1V t . A voltage clamp limits the integration level at int1 to GND+1V t by placing a nominally 2V t at the gate of the reset transistor during int1. In an alternate embodiment of the present invention, shown in Figure 6, the reverse bias controller 16 includes a voltage reference table 30 and a clamp circuit 32. Figure 7 shows a schematic diagram corresponding to the voltage controller 16 shown in Figure 6. While voltage reference table 30 is implemented as a circuit, the concept is easily implemented in software via a lookup table. Clamp circuit 32 is a clamp digital-analog converter (DAC). This circuit creates a programmable voltage of 3 V t +/- V t . A higher clamp voltage will limit the dynamic range, while a lower level allows the diode to integrate to voltages as low as ½ V t . The clamp voltage can be adjusted according to the signal path gain so as to maximize reverse bias on the PD node while not limiting the dynamic range. This clamp circuit limits the possibility of forward biasing the a-Si diode due to very high illuminations during int1 and has a programmable feature to optimize the reverse bias condition level.

Claims: An imaging system for minimizing image lag, the imaging system providing a pixel output signal (PIXEL OUT), and the imaging system comprising: sensor logic (12); an image processor (14) connected to the sensor logic (12) and generating a gain signal (GAIN) on the basis of data received from the sensor logic (12); a pixel array (18) having a plurality of pixel circuits (18n), each pixel circuit (18n) comprising a photodetector (22) connected and reverse biased between a power supply (PRESET) and ground and a pixel output connected to a photodetector node (PD) arranged between the power supply (PRESET) and the photodetector (22); and a reverse bias controller (16) connected between the image processor (14) and the pixel array (18) and receiving the gain signal (GAIN) from the image processor (14), the reverse bias controller (16) generating a bias control signal (RESET) in response to the gain signal (GAIN) received from the image processor (14), and the reverse bias controller (16) providing the bias control signal (RESET) to the photoconductor node (PD) of the pixel circuit (18n) of the pixel array (18), thereby maintaining the reverse bias condition of the photodetector (22). The imaging system, as defined in claim 1, wherein the pixel array (18) is made from amorphous silicon. The imaging system, as defined in claim 1 or 2, wherein the reverse bias controller (16) includes: a voltage reference table (30), receiving the gain signal (GAIN) and generating a voltage output; and voltage clamp digital-analog converter (32), receiving the voltage output from the voltage reference table (30) and generating the bias control signal (RESET). The imaging system, as defined in one of claims 1 to 3, wherein each pixel circuit (18n) of the pixel array (18) comprises a first n-channel field effect transistor (20) having a drain receiving a supply signal (PRESET) and a gate receiving the bias control signal (RESET); a second n-channel field effect transistor (24) having a drain connected to a power supply (VDD) and a gate connected to the source of the first n-channel field effect transistor (20); and a third n-channel field effect transistor (26) having a drain connected to the source of the second n-channel field effect transistor (24), a gate connected to the image processor, and a source providing the pixel output signal (PIXEL OUT); wherein the photodetector (22) is an amorphous silicon diode, connected between the source of the first n-channel field effect transistor (20) and ground, the photodetector node (PD) being between the source of the first n-channel field effect transistor (20) and the amorphous silicon diode (22).

Inventors: BORG MATTHEW M, BUDRYS AUDRIUS J, GIBSON JONATHAN S, HADDAD HOMAYOON, KOPLEY THOMAS EDWARD, MENTZER RAY A, THEIL JEREMY A, BORG, MATTHEW M., BUDRYS, AUDRIUS J., GIBSON, JONATHAN S., HADDAD, HOMAYOON, KOPLEY, THOMAS EDWARD, MENTZER, RAY A., THEIL, JEREMY A.
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 2001
Main classification: H04N5
IPC codes: H01L27, H04N3, H04N5