Systems and methods for detecting light-emitting diode without flickering

An image sensor for detecting light-emitting diode (LED) without flickering includes a pixel array with pixels. Each pixel including subpixels including a first and a second subpixel, dual floating diffusion (DFD) transistor, and a capacitor coupled to the DFD transistor. First subpixel includes a first photosensitive element to acquire a first image charge, and a first transfer gate transistor to selectively transfer the first image charge from the first photosensitive element to a first floating diffusion (FD) node. Second subpixel includes a second photosensitive element to acquire a second image charge, and a second transfer gate transistor to selectively transfer the second image charge from the second photosensitive element to a second FD node. DFD transistor coupled to the first and the second FD nodes. Other embodiments are also described.

FIELD

An example of the present invention relates generally to image sensors. More specifically, examples of the present invention are related to image sensors that detect a high illumination element (e.g., light emitting diode (LED)) without flickering and methods of implementing thereof.

BACKGROUND

High-speed image sensors have been widely used in many applications in different fields including the automotive field, the machine vision field, and the field of professional video photography. Some applications in these fields require the detection and capture of LED lights, which has proven to be difficult. For example, automotive image sensors face the problem of LED flickering. Future automotive vehicle lights, traffic lights and signs will include LED that is pulsed at 90-300 Hz with high peak light intensity. This requires that the minimum exposure time be kept over 10 ms. A very high full well capacity (FWC) or very low light intensity are thus needed to avoid pixels to get saturated and lose useful information.

Current solutions to address the overflow and loss of useful information from saturated pixels include enhancing the FWC with a lateral overflow integrating capacitor (LOFIC). When the photodiode is filled after reaching a corresponding FWC, the excess charge is leaked into a floating drain. A large capacitor connected to the floating drain can then store the excess charge. However, the maximum FWC is thus limited by the floating drain capacitor rather than the photodiode FWC. Other solutions involve using non-linear sensor (e.g., logarithmic sensors) to enlarge the FWC, or using split diode pixels or sub-pixel sensors to maintain minimum exposure time by minimizing sensitivity of small photodiode. Further, for high dynamic range (HDR) applications, the existing image sensors struggle due to the limited charge storage and limited FWC.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown to avoid obscuring the understanding of this description.

FIG. 1is a block diagram illustrating an example imaging system100that detects a LED without flickering in accordance to one embodiment of the invention. Imaging system100may be a complementary metal-oxide-semiconductor (“CMOS”) image sensor. As shown in the depicted example inFIG. 1, imaging system100includes pixel array105coupled to control circuitry120and readout circuitry110, which is coupled to function logic115and logic control108.

The illustrated embodiment of pixel array105is a two-dimensional (“2D”) array of imaging sensors or pixel cells (e.g., pixel cells P1, P2, . . . , Pn). In one example, each pixel cell is a CMOS imaging pixel. As illustrated, each pixel cell is arranged into a row (e.g., rows R1to Ry) and a column (e.g., columns C1to Cx) to acquire image data of a person, place or object, etc., which can then be used to render an image of the person, place or object, etc. Several color imaging pixels may be included in the active area of an image sensor (e.g., pixel array105), such as red (R), green (G), and blue (B) imaging pixels. For example, the pixel array105may include four color imaging pixels (e.g., one red (R), one green (G), and one blue (B)) arranged into a Bayer pattern. Other color imaging pixels and other color patterns may be implemented into the pixel array105in accordance with the teachings of the present disclosure. For example, each pixel cell (e.g., pixel cells P1, P2, . . . , Pn) may include a plurality of subpixels respectively including a plurality of photosensitive elements (e.g., photodiodes) and a plurality of transfer gate transistors. Each of the subpixels in a pixel cell may include the same color imaging pixel (seeFIG. 2) or one of the subpixels in a pixel cell may include a color imaging pixel and the remaining subpixels in the pixel cell may include clear color imaging pixels (seeFIG. 3).

In one example, after each pixel has acquired its image data or image charge, the image data is read out by readout circuitry110through readout column bit lines109and then transferred to function logic115. In one embodiment, a logic circuitry108can control readout circuitry110and output image data to function logic115. In various examples, readout circuitry110may include amplification circuitry (not illustrated), a column readout circuitry210that includes analog-to-digital conversion (ADC) circuitry220(as illustrated inFIG. 6), or otherwise. Function logic115may simply store the image data or even manipulate the image data by applying post image effects (e.g., crop, rotate, remove red eye, adjust brightness, adjust contrast, or otherwise). In one embodiment, function logic115in imaging system100may periodically perform LED detection or detection of other high illumination devices or elements. In one example, readout circuitry110may read out a row of image data at a time along readout column lines (illustrated) or may read out the image data using a variety of other techniques (not illustrated), such as a serial read out or a full parallel read out of all pixels simultaneously.

In one example, control circuitry120is coupled to pixel array105to control operational characteristics of pixel array105. For example, control circuitry120may generate a shutter signal for controlling image acquisition. In one example, the shutter signal is a global shutter signal for simultaneously enabling all pixels within pixel array105to simultaneously capture their respective image data during a single acquisition window. In another example, the shutter signal is a rolling shutter signal such that each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows. The shutter signal may also establish an exposure time, which is the length of time that the shutter remains open. In one embodiment, the exposure time is set to be the same for each of the frames.

In another example, control circuitry120may comprise the horizontal and vertical scanning circuitry, which selects the row and/or column of pixels to be read out. Scanning circuitry may include, selection circuitry (e.g., multiplexers), etc. to readout a row or column of image data at a time along readout column bit lines109or may readout the image data using a variety of other techniques, such as a serial readout or a full parallel readout of all pixels simultaneously. When scanning circuitry selects pixels in pixel array105, the pixels convert light incident to the pixels to a signal and output the signal to column readout circuitry210. Column readout circuitry210may receive the signal from scanning circuitry or from pixel array105.

Referring toFIG. 6, a block diagram of the details of the readout circuitry110inFIG. 1in accordance to one embodiment of the invention is illustrated. The readout circuitry110includes the column readout circuitry210that includes ADC circuitry220. While not illustrated, in some embodiments, a plurality of column readout circuitry210may be included in readout circuitry110. It is also understood that column readout circuitry210may be similar for each column of pixel array105. ADC circuitry220may be as a double ramp ADC or other types of column ADC (i.e., SAR, cyclic, etc.). ADC circuitry220may convert each of the image data signal from pixel array105from analog to digital.

FIG. 2is a block diagram of a pixel in the pixel array105in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. While embodiments herein are described to detect a LED, it is understood that the embodiments may also be implemented to detect other high illumination elements or devices. To address the issues of overflow and loss of useful information from saturated pixels, a subpixel sensor combined with LOFIC and selective anti-blooming is used inFIG. 2. As shown inFIG. 2, each of the pixels in the pixel array105includes a plurality of subpixels. The subpixels respectively include photosensitive elements PD1-PDn(n>1), transfer gate transistors TX1TXn, and anti-blooming (AB) gates AB1-ABn. In this embodiment, each pixel includes four subpixels (e.g., n=4). Each photosensitive element PD1-PDnacquires an image charge. Each transfer gate transistor TX1-TXnselectively transfers the respective image charge from the photosensitive element PD1-PDnto a respective floating diffusion (FD) node. Each anti-blooming (AB) gate AB1-ABnis coupled to a respective photosensitive element PD1-PDn. For example, the first subpixel includes a first photosensitive element PD1to acquire a first image charge, a first transfer gate transistor TX1to selectively transfer the first image charge from the first photosensitive element PD1to a first FD node, and a first AB gate AB1coupled to the first photosensitive element PD1; and the second subpixel including a second photosensitive element PD2to acquire a second image charge, a second transfer gate transistor TX2to selectively transfer the second image charge from the second photosensitive element PD2to a second FD node, and a second AB gate AB2coupled to the second photosensitive element PD2. As shown inFIG. 2, each pixel also includes a dual floating diffusion (DFD) transistor coupled to the FD nodes and a capacitor C coupled to the DFD transistor. Capacitor C may be a lateral overflow integrating capacitor (LOFIC). In one embodiment, the DFD transistor is also coupled to the AB gates AB1-ABn.

Within a pixel transistor region, each pixel inFIG. 2includes a source-follower transistor SF, a row select transistor RS, and a reset transistor RST. Each of the transistors (e.g., source-follower transistor SF, a row select transistor RS, and a reset transistor RST) includes a gate and doped regions (i.e., drain and source).

Referring toFIG. 14, a block diagram of a pixel in the pixel array in the imaging system inFIG. 2that detects a LED without flickering is further illustrated in accordance to one embodiment of the invention. As shown inFIG. 14, the FD1-FD4are coupled to the AB1-AB4via the transfer gate transistors TX1-TX4. During a readout operation of the first photodiode PD1, transfer gate TX1receives a transfer signal, which causes the transfer of charge accumulated in photodiode PD1to the first FD node. In one embodiment, the FD nodes (e.g., first to fourth FD nodes) coupled respectively to the transfer gate transistors TX1-TX4are included in shared floating diffusion region FD. In one embodiment, AB gates AB1-ABare also coupled to the shared floating diffusion region FD via the transfer gate transistors TX1-TX4.

The reset transistor RST is coupled to reset (e.g., discharge or charge the FD to a preset voltage) under control of a reset signal received at the reset transistor RST's gate. The FD nodes are coupled to gate of the source-follower transistor SF. The source-follower transistor SF operates as a source-follower providing a high impedance output from the associated FD nodes. Finally, the row select transistor RS selectively couples the output of pixel circuitry in the pixel to the column bitline connection under control of a row select signal received.

Also included in the pixel transistor region are a shared source follower voltage supply connection, a column bitline connection, and a shared reset voltage supply connection. In one embodiment, connections are metal pads for connecting with metal routings that carry their respective signals among several pixels.

In one embodiment, the first AB gate AB1is biased to leak less than the first transfer gate transistor TX1and the remaining AB gates AB2-AB4are biased to leak more than the corresponding transfer gate transistors TX2-TX4. Accordingly, during signal integration, all transfer gate transistors TX1-TX4are turned off, and the first subpixel with the less leaky AB gate AB1will bloom into the floating drain (e.g., DFD transistor) after the first photosensitive element PD1is full or saturated. In other words, DFD transistor selectively couples to first AB gate AB1via the first transfer gate transistor TX1and capacitor C stores a bloomed charge from the first photosensitive element. Capacitor C may be implemented as a MOS capacitor, a metal-insulator-metal (MIM) capacitor or combinations of types of capacitor. Remaining subpixels including AB gates AB2-AB4that are more leaky than the corresponding transfer gate transistors TX2-TX4will bloom to a power rail VDD through the AB gates AB2-AB4. The dynamic range is thus increased by a factor that is equal to the number of subpixels in the pixel.

At the end of integration, the bloomed charge stored on capacitor C is read out using three transistors (3T) timing, and the first, second, third, and fourth image charges from photosensitive elements PD1-PD4are subsequently readout as photosensitive element signals, respectively, using four transistors (4T) timing. The multiple transfer gate transistors TX1-TXnof the subpixels may be turned on together to achieve FD charge binning or may be transferred separately to achieve high dynamic range (HDR) using differential integration of the subpixels.

FIG. 3is a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. To address the issues of overflow and loss of useful information from saturated pixels, a subpixel sensor combined with LOFIC and selective anti-blooming is further combined with the RGBC pattern is used inFIG. 3. Accordingly, the embodiment discussed inFIG. 2may further be combined with subpixel red (R), green (G), blue (B), and clear (C) color patterns inFIG. 3. As shown inFIG. 3, one of the subpixels in the pixel retains the normal Bayer color pattern, while the other remaining subpixels (e.g., remaining three subpixels) are replaced with clear color filter to increase the low-light sensitivity. InFIG. 3, first photosensitive element PD1in each pixel includes the Bayer color pattern while the second, third, and fourth photosensitive elements PD2-PD4include a clear color filter.

Similar to the embodiment inFIG. 2, inFIG. 3, the first AB gate AB1is biased to leak less than the first transfer gate transistor TX1and the remaining AB gates AB2-AB4are biased to leak more than the corresponding transfer gate transistors TX2-TX4. Referring toFIG. 14, a block diagram of the details of a pixel in the pixel array in the imaging system inFIGS. 2 and 3that detects a LED without flickering in accordance to one embodiment of the invention is illustrated. Accordingly, the subpixel with the RGB color filter is the one with an AB gate AB1that is less leaky than the corresponding transfer gate transistor TX1. During signal integration, all transfer gate transistors TX1-TX4are turned off, and the first subpixel with the less leaky AB gate AB1will bloom into the floating drain (e.g., DFD transistor) after the first photosensitive element PD1is full or saturated. In other words, DFD transistor selectively couples to first AB gate AB1via the first transfer gate transistor TX1and capacitor C stores a bloomed charge from the first photosensitive element. Thus, any excess charge in the RGB subpixels will bloom into the floating drain and be stored on LOFIC. Capacitor C inFIG. 3may also be implemented as a MOS capacitor, a MIM capacitor or combinations of types of capacitor. Remaining subpixels including AB gates AB2-AB4that are more leaky than the corresponding transfer gate transistors TX2-TX4will bloom to a power rail VDD through the AB gates AB2-AB4. The dynamic range is thus increased by a factor that is equal to the number of subpixels in the pixel.

Similar to the embodiment inFIG. 2, inFIG. 3, at the end of integration, the bloomed charge stored on capacitor C is read out using 3T timing, and the first, second, third, and fourth image charges from photosensitive elements PD1-PD4are subsequently readout as photosensitive element signals, respectively, using 4T timing. The RBG subpixels will thus be read out separately from the clear subpixels. The multiple transfer gate transistors TX2-TX4of the clear subpixels may be turned on together to achieve FD charge binning or may be transferred separately to achieve HDR using differential integration of the subpixels. The RGB subpixel signal and the clear subpixel signal may be combined in image signal processing (ISP) to produce a final image charge.

FIG. 4is a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. In this embodiment, a subpixel sensor combined with LOFIC and selective anti-blooming is used to address the issues of overflow and loss of useful information from saturated pixels without dedicated AB devices. InFIG. 4, pixel circuitry of each pixel includes four photosensitive elements (PD1-PD4), four transfer transistors (TX1-TX4), a capacitor C (or CLOFIC), a DFD transistor, a reset transistor RST, a source-follower transistor SF, and a row select transistor RS. During a readout operation of the first photodiode PD1, transfer transistor TX1receives a transfer signal, which causes transfer transistor TX1to transfer the charge accumulated in photodiode PD1to a FD node FD1. In this embodiment, each of the subpixels includes one photosensitive element PD1-PD4and one transfer transistor TX1-TX4.

Reset transistor RST is coupled between a reset voltage supply VRFD (or power rail VDD) and the FD node FD1to reset (e.g., discharge or charge the FD node FD1to a preset voltage) under control of a reset signal. As shown inFIG. 4, the FD nodes FD1, FD2, and FD3may be the same node. The FD node FD1is coupled to the gate of the source-follower transistor SF. The source-follower transistor SF is coupled between a source-follower voltage supply SFVDD (or power rail VDD) and row select transistor RS. The source-follower transistor SF operates as a source-follower providing a high impedance output from FD node FD1. Finally, row select transistor RS selectively couples the output of the pixel circuitry to the column bitline under control of a row select signal. In one embodiment, the transfer signals, the reset signal, and the row select signal are generated by control circuitry120. The transfer signals, the reset signal, the row select signal, the source-follower voltage supply SFVDD, the reset voltage supply VRFD, and ground may be routed in the pixel circuitry by way of metal interconnect layers (i.e., routings) included in the image sensor.

InFIG. 4, the top photosensitive element PD4is for LED detection. Photosensitive element PD4acquires a fourth image charge and the transfer transistor TX4selectively transfers the fourth image charge from the fourth photosensitive element PD4to a fourth FD node. DFD transistor and capacitor C is coupled to the fourth FD node FD4. Accordingly, during signal integration, capacitor C stores excess image charge from the fourth photosensitive element PD4that is leaking through the fourth transfer gate transistor TX4. When capacitor C saturates due to extreme light, the anti-blooming path for the fourth photosensitive element PD4and the saturated capacitor C is through the DFD transistor and the reset transistor RST. The remaining photodiodes PD1-PD3have anti-blooming path through remaining transfer gate transistors TX1-TX3and reset transistor RST.

At the end of signal integration, the image charge stored on the fourth photosensitive element PD4and the excess first image charge stored on the capacitor C may be read out using pseudo correlated double sampling (pseudo-CDS) 3T timing, and image charges on the photosensitive elements PD1-PD3that are binned together may be read out using Correlated Double Sampling (CDS). In one embodiment, the image charges may be readout three times: first, the fourth image charge on the fourth photosensitive element PD4and the excess image charge stored on capacitor C are readout; second, the image charges from photosensitive elements PD1-PD3that are binned together are then readout (e.g., High CG); and third, image charges from photosensitive elements PD1-PD3that are binned together and the excess image charge stored on capacitor C are then read out (e.g., Low CG).

In some embodiments, the fourth transfer gate transistor TX4is omitted to make it possible to reduce the size and sensitivity of photosensitive element PD4and thus, enhancing dynamic range.

As shown inFIG. 8, which illustrates a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention, the pixel layout as described inFIG. 4does not need to be 2×2 symmetric. Instead, in the embodiment inFIG. 8, the pixel layout may be generalized to split-diode pixels such as one-large photosensitive element PD1-3that replaces the photosensitive elements PD1-PD3inFIG. 4and one small photosensitive element PD4, where the small photosensitive element PD4is coupled to the capacitor C.

FIG. 5is a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. In this embodiment, a subpixel sensor combined with LOFIC and selective anti-blooming is used to address the issues of overflow and loss of useful information from saturated pixels without dedicated AB devices. In contrast to the embodiment inFIG. 4, the fourth transfer gate transistor TX4is omitted and a FD photosensitive element is coupled to a floating node FD1-3. Similar to the embodiment inFIG. 8, the pixel circuitry includes one large photosensitive element PD1-3and one small photosensitive element PD4that is coupled to the capacitor C. The differential integration for the small photosensitive element PD4and the one large photosensitive element PD1-3further extends the dynamic range of the image sensor. The integration times for photosensitive element PD4and the photosensitive element PD1-3may be different. In one embodiment, the exposure time of the small photosensitive element PD4is the sum of the integration time for the small photosensitive element PD4and the integration time of the one large photosensitive element PD1-3, while the exposure time of the one large photosensitive element PD1-3is the sum of the integration time of the one large photosensitive element PD1-3and the 3T readout time.

InFIG. 5, during signal integration, a bloomed charge from the small photosensitive element PD4is stored by capacitor C that is coupled to the DFD transistor and the overflow node (e.g., FD4), and at the end of integration, the bloomed charge stored on the capacitor C is read out using pseudo-CDS 3T timing and the image charge from the one large photosensitive element PD1-3is reading out using Dual Conversion Gain (DCG). In this embodiment, to perform DCG readout, first, FD is reset via RST transistor. DFD transistor is turned on to sample low conversion gain (LCG) reset. DFD transistor is then turned off to sample the high conversion gain (HCG) reset. The image charge on the photosensitive element PD1-3is transferred to the bitline while the DFD transistor is kept off to sample HCG signal, then DFD transistor is turned ON and later the residual charge from photosensitive element PD1-3is transferred while DFD transistor is kept ON, to sample LCG signal. Finally, both HCG and LCG CDS are completed without destruction of the total signal accumulated in PD1-3.

Moreover, the following embodiments of the invention may be described as a process, which is usually depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a procedure, etc.

FIGS. 7A-7Care flowcharts illustrating methods of detecting a LED without flickering in accordance to one embodiment of the invention.

FIG. 7Amay be a method implemented using the image sensor in the embodiment inFIG. 2. The method710starts with a pixel array including a plurality of pixels capturing an image frame (Block711). Each of the pixels including a DFD transistor, a capacitor coupled to the DFD transistor, and a plurality of subpixels including a first subpixel and a plurality of remaining subpixels. The first subpixel may include a first photosensitive element to acquire a first image charge, a first transfer gate transistor to selectively transfer the first image charge from the first photosensitive element to a first floating diffusion (FD) node, and a first anti-blooming (AB) gate coupled to the first photosensitive element. The first AB gate may be biased to leak less than the first transfer gate transistor. The remaining subpixels may include remaining photosensitive elements to acquire remaining image charges, a remaining transfer gate transistors to selectively transfer the remaining image charge from the remaining photosensitive element to remaining floating diffusion (FD) nodes, respectively, and remaining anti-blooming (AB) gates coupled to the remaining photosensitive elements, respectively. The remaining AB gates may be biased to leak more than the remaining transfer gate transistors. The DFD transistor may be coupled to the first FD node, the remaining FD nodes, the first AB gate, and the remaining AB gates included in each of the subpixels. In one embodiment, the pixel array is arranged in a Bayer color pattern as shown inFIG. 3, the first photosensitive element includes a color filter and the remaining photosensitive elements include a clear color filter. In this embodiment, a signal from the first subpixel is combined in image sensor processing (ISP) with signals from the remaining subpixel to produce a final image. In this embodiment, the signal from the first subpixel is an RGB color signal and the signals from the remaining subpixels are clear color signals. At Block712, during signal integration, the capacitor stores a bloomed charge from the first photosensitive element, and the remaining subpixels bloom to a power rail through remaining AB gates. The first transfer gate transistor and the remaining transfer gate transistors may be turned on together or at separate times. At Block713, at the end of integration, the bloomed charge stored on the capacitor is read out, and the first image charge and the remaining image charges, respectively, are read out. In some embodiments, at the end of integration, the bloomed charge stored on the capacitor is read out using three transistors (3T) timing while the first image charge and the remaining image charges, respectively, are read out using four transistors (4T) timing.

FIG. 7Bmay be a method implemented using the image sensor in the embodiment inFIG. 4. The method720starts at Block721with a plurality of subpixels capturing a plurality of image charges. The subpixels may include a first, a second, a third, and a fourth subpixel. The first subpixel includes a first photosensitive element to acquire a first image charge and a first transfer gate transistor to selectively transfer the first image charge from the first photosensitive element to a first floating diffusion (FD) node. The second subpixel includes a second photosensitive element to acquire a second image charge and a second transfer gate transistor to selectively transfer the second image charge from the second photosensitive element to a second FD node. The third subpixel includes a third photosensitive element to acquire a third image charge and a third transfer gate transistor to selectively transfer the third image charge from the third photosensitive element to the second FD node. The fourth subpixel includes a fourth photosensitive element to acquire a fourth image charge and a fourth transfer gate transistor to selectively transfer the fourth image charge from the fourth photosensitive element to the second FD node. A DFD transistor may be coupled to the first and the second FD nodes.

At Block722, during signal integration, a capacitor that is coupled to the first transfer gate transistor stores an excess first image charge. The excess first image charge is image charge from the first photosensitive element that is leaking through the first transfer gate transistor.

At Block723, at the end of integration, the first image charge stored on the first photosensitive element and the excess first image charge stored on the capacitor are read out using 3T timing, and the second, third, and fourth image charges that are binned together are reading out by Correlated Double Sampling (CDS). In one embodiment, read out by Correlated Double Sampling (CDS) may include reading out the second, third, and fourth image charges that are binned together, and reading out the second, third, and fourth image charges that are binned together and the excess first image charge. In one embodiment, if the capacitor saturates, the DFD transistor and a reset transistor coupled to a power rail and the second FD node provides an anti-blooming path for the first photosensitive element, and the second, third, and fourth transfer gate transistors and the reset transistor provide an anti-blooming path for the second, third, and fourth photosensitive elements.

FIG. 7Cmay be a method implemented using the image sensor in the embodiment inFIG. 5. The method730starts at Block731with a pixel including a plurality of subpixels capturing a plurality of image charges. The subpixels may include a first, a second subpixel, and a third subpixel. In one embodiment, the first subpixel includes a first photosensitive element to acquire a first image charge. The first photosensitive element is coupled to a first floating diffusion (FD) node. The second subpixel includes a second photosensitive element to acquire a second image charge, and a second transfer gate transistor to selectively transfer the second image charge from the second photosensitive element to the first FD node. The third subpixel includes a third photosensitive element to acquire a third image charge. The third photosensitive element may be coupled to an overflow node. In one embodiment, integration times for the second photosensitive element and the third photosensitive element are different. In one embodiment, a DFD transistor is coupled to the first FD node and to the overflow node. At Block732, during signal integration, a bloomed charge from the third photosensitive element is stored by a capacitor that is coupled to the DFD transistor and the overflow node. At Block733, at the end of integration, the bloomed charge stored on the capacitor is read out using pseudo-CDS 3T timing, and the second image charge is read out using Dual Conversion Gain (DCG).

FIG. 9is a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. To address the issues of overflow and loss of useful information from saturated pixels for HDR applications, the image sensor including the pixel array and ADC circuitry inFIG. 9uses a hybrid stack chip that includes a sensor chip (or top wafer) and a stack chip (or carrier wafer or bottom wafer). Similar to the embodiment inFIG. 8, the pixel circuitry includes one large photosensitive element PD1-3and one small photosensitive element PD4that is coupled to the capacitor C. The differential integration for the small photosensitive element PD4and the one large photosensitive element PD1-3further extends the dynamic range of the image sensor. The integration times for photosensitive element PD4and the photosensitive element PD1-3may be different.

In this embodiment, the subpixels including the photosensitive elements PD1-3and PD4and the transfer gate transistors TX1-3and TX4, the DFD transistor, the SF transistor and the reset transistor are disposed on a first semiconductor die (e.g., sensor chip), and the capacitor C (or CLOFIC1) is disposed on a second semiconductor die (e.g., stack chip). The first and the second semiconductor dies are stacked and coupled to form a stacked image sensor. In one embodiment, the stack chip includes the capacitor C (or integration capacitor) for 3T readout. In some embodiments, low cost capacitors of different possible Metal Oxide Conductor Capacitor (MOSCAP) designs may be used as capacitor C. In one embodiment, the readout for the small photosensitive element PD4is a 3T rolling shutter readout. In one embodiment, the large photosensitive element PD1-3readout is 4T rolling-shutter readout high-CG, with anti-blooming gate, with very low Full Well and very low Dark current. The low-pass difference (LPD) anti-blooming gate may be optionally used in the pixel circuitry corresponding to the large photosensitive element PD1-3at high light.

FIG. 10is a block diagram of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to one embodiment of the invention. Similar to the embodiment inFIG. 9, inFIG. 10, a stack chip is used. However, in lieu of a single capacitor, multiple overflow-capacitors are included on a carrier wafer of a stack chip in the embodiment ofFIG. 10. WhileFIG. 10includes a single subpixel including a photosensitive element PD1and transfer gate transistor TX1, it is understood, that the embodiment inFIG. 10may include a plurality of subpixels as discussed in herein. Thus, each of the pixels in the pixel array105may include a plurality of subpixels disposed on a first semiconductor die (e.g., sensor chip).

InFIG. 10, the first subpixel included on the sensor chip includes a first photosensitive element PD1to acquire a first image charge, and a first transfer gate transistor TX1to selectively transfer the first image charge from the first photosensitive element PD1to a first floating diffusion (FD) node that is coupled to a first dual floating diffusion (DFD) transistor. The first DFD transistor is disposed on the first semiconductor die (e.g., sensor chip). A first capacitor CLOFIC1is coupled to the first DFD transistor DFD and a second DFD transistor DFD1, and a second capacitor CLOFIC2is coupled to the second DFD transistor DFD1. The first capacitor CLOFIC1, the second capacitor CLOFIC2and the second DFD transistor DFD1are disposed on a second semiconductor die (e.g., stack chip). The first and the second semiconductor dies are stacked and coupled to form a stacked image sensor. Further, as shown inFIG. 10, a plurality of DFD transistors and capacitors CLOFICmay be coupled and included in the stack chip. The readout sequence of the embodiment inFIG. 10may include reading out in 4T readout at the first FD node for the darkest signal level, reading out the first capacitor CLOFIC1for higher signal level than that of the first FD node (e.g., overflow from the photosensitive element PD1), and reading out the second capacitor CLOFIC2for higher signal level than that of the first capacitor CLOFIC1, etc.

By using a plurality of capacitors on a carrier chip, the linear dynamic range of the image sensor is increased and improves HDR. Referring toFIG. 11, a graph that illustrates the relation between the signal-to-noise ratio (SNR) and the dynamic range (in dB) and the effects of the increased number of capacitors in the pixel inFIG. 10on the signal-to-noise ratio (SNR) in accordance to one embodiment of the invention is shown. FromFIG. 11, when the single capacitor is used the SNR drop is large. By using the multiple capacitors on the carrier chip of a stacked chip, the SNR drop is less and distributed.

FIGS. 12-13are block diagrams of a pixel in the pixel array in the imaging system inFIG. 1that detects a LED without flickering in accordance to two embodiments of the invention. In the embodiments inFIGS. 12-13, the image sensor is implemented as a stacked chip. Specifically, the image sensor includes an active source-follower stack-pixels with hybrid bond for low-noise.

Referring toFIGS. 12-13, a plurality of subpixels disposed on a first semiconductor die (e.g., top wafer or sensor chip). While the subpixels illustrated inFIGS. 12-13are the subpixels included in a single pixel, it is understood that the top wafer may include subpixels from a plurality of pixels included in the pixel array105.

As shown inFIGS. 12-13, each of the subpixels includes a photosensitive element PD1-PD4to respectively acquire an image charge, and a transfer gate transistor TX1-TX4to selectively transfer the respective image charge from the photosensitive element PD1-PD4to a floating diffusion (FD) node. InFIGS. 12-13, the FD node is a shared floating diffusion node or region. The FD node is coupled to a larger size source follower (SF) transistor disposed on a second semiconductor die (e.g., bottom wafer) via a hybrid bond. CFD(e.g., 1 fF) is parasitic capacitance at the bond pad. A reset transistor RST that is disposed on the second semiconductor die is further coupled to a power rail and the first FD node. In some embodiments, the second semiconductor die (or bottom wafer) includes low CG capacitors. In one embodiment, the SF transistor and the row select transistor RS are included in the pixel circuitry whereas a column circuitry included in the readout circuitry120includes the other half of the differential amplifier as illustrated inFIGS. 12-13. The active SF transistor can be implemented larger size and provides more unity gain (Av) and lower input capacitance (CIN) (e.g., CIN=(1-Av) CSF). As shown inFIGS. 12-13, a connection line couples the body of the SF transistor to the source of row select transistor RS. This connection line may ground the substrate of the SF transistor to reduce the body effect. Accordingly, the embodiments inFIGS. 12-13maintain high CG with a small floating diffusion FD capacitor (CFD).

The first and the second semiconductor dies are stacked and coupled to form a stacked image sensor. In one embodiment, the SF transistor is a large size. For instance, for 1.4 μm×4 share and a 2.8 μm pitch, the width and length of the SF transistor may be between 0.5 μm×0.5 μm to 2.4 μm×2.4 μm. For 1.1 μm×4 share and 2.2 μm pitch, the width and length of the SF transistor may be between 0.3 μm×0.3 μm to 1.8 μm×1.8 μm.

In the embodiment inFIG. 12, a plurality of capacitors C1-C3and a plurality of DFD transistors may also be disposed on the second semiconductor die. As shown inFIG. 12, a first dual floating diffusion (DFD) transistor DFD0is coupled to the FD node, a second DFD transistor DFD1is coupled to the first DFD transistor DFD0, and a third DFD transistor DFD2is coupled to the second DFD transistor DFD1. The first capacitor C1is coupled to the first DFD transistor DFD0and the second DFD transistor DFD1, the second capacitor C2is coupled to the second DFD transistor DFD1and the third DFD transistor DFD2, and the third capacitor C3is coupled to the third DFD transistor DFD2. In some embodiments, the first capacitor C1, second capacitor C2, and third capacitor C3 may be 8 fF, 16 fF, 32 fF, respectively.