High conversion gain image sensor

This disclosure relates generally to image sensors, and in particular but not exclusively, relates to CMOS image sensors having a high conversion gain.

Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.

FIG. 1 is a circuit diagram illustrating pixel circuitry of two four-transistor (“4T”) pixel cells Pa and Pb (collectively pixel cells 100 ) within an image sensor array. Pixel cells Pa and Pb are arranged in two rows and one column and time share a single readout column line (bit line). Each pixel cell 100 includes a photodiode PD, a transfer transistor T 1 , a reset transistor T 2 , a source-follower (“SF”) or amplifier (“AMP”) transistor T 3 , and a row select (“RS”) transistor T 4 .

During operation, transfer transistor T 1 receives a transfer signal TX, which transfers the charge accumulated in photodiode PD to a floating diffusion node FD. Reset transistor T 2 is coupled between a power rail VDD and the floating diffusion node FD to reset the pixel (e.g., discharge or charge the FD and the PD to a preset voltage) under control of a reset signal RST. The floating diffusion node FD is coupled to control the gate of AMP transistor T 3 . AMP transistor T 3 is coupled between the power rail VDD and RS transistor T 4 . AMP transistor T 3 operates as a source-follower providing a high impedance connection to the floating diffusion FD. Finally, RS transistor T 4 selectively couples the output of the pixel circuitry to the readout column line under control of a signal RS.

In normal operation, the photodiode PD and node FD are reset to the supply voltage VDD by temporarily asserting the reset signal RST and the transfer signal TX. The image accumulation window (exposure period) is commenced by de-asserting the transfer signal TX and permitting incident light to charge the photodiode PD. As photogenerated electrons accumulate on the photodiode PD, its voltage decreases (electrons are negative charge carriers). The voltage or charge on photodiode PD is indicative of the intensity of the light incident on the photodiode PD during the exposure period. At the end of the exposure period, the reset signal RST is de-asserted to isolate node FD and the transfer signal TX is asserted to couple the photodiode to node FD and hence the gate of AMP transistor T 3 . The charge transfer causes the voltage of node FD to drop from VDD to a second voltage indicative of the amount of charge (e.g., photogenerated electrons accumulated on the photodiode PD during the exposure period). This second voltage biases AMP transistor T 3 , which is coupled to the readout column line when the signal RS is asserted on RS transistor T 4 .

The conversion gain of pixel cells 100 is defined as the ratio (R) of the change in voltage ΔV at node FD after charge transfer to the change in charge ΔQ of photodiode PD.

R=ΔV/ΔQ   (Equation 1)

The capacitance of photodiode PD is determined by calculating the amount of charge Q stored for a given amount of voltage applied V.

Q=C·V   (Equation 2)

Combining equations 1 and 2, conversion gain (R) is inversely proportional to the capacitance of node FD.

R=ΔV/ΔQ=ΔV/C·ΔV= 1 /C   (Equation 3)

In general, it is desirable to design pixel cells 100 to have a high conversion gain R to improve low-light sensitivity. For traditional image sensors, conversion gain can be increased by reducing the capacitance of the floating diffusion. However, as pixel cell sizes continue to shrink, pixel saturation or overexposure in bright environments is becoming more acute. Furthermore, as pixel sizes get smaller and smaller, shared structure pixels are widely used. For shared structure pixels, the floating node FD is connected to several transfer gates. Therefore, the shared structure pixel floating node capacitance cannot be very small. For large pixels, large transfer gates are necessary to avoid image lag and therefore the floating node capacitance cannot be very small. This limit on the size of the floating node capacitance makes it difficult to achieve a high conversion gain in large image sensor pixels.

Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1 is a circuit diagram illustrating two conventional pixel cells within an image sensor array.

FIG. 2 is a block diagram illustrating an imaging system, in accordance with an embodiment of the invention.

FIG. 3 is a circuit diagram illustrating a high conversion gain image sensor, in accordance with an embodiment of the invention.

FIG. 4 is a circuit diagram illustrating a high conversion gain image sensor including a voltage boost circuit, in accordance with an embodiment of the invention.

FIG. 5 is a circuit diagram illustrating a high conversion gain image sensor including a voltage boost circuit and a boost capacitor, in accordance with an embodiment of the invention.

FIG. 6 is a flow chart illustrating operation of a high conversion gain image sensor having an increased voltage swing at the floating diffusion node, in accordance with an embodiment of the invention.

FIG. 7 is a timing diagram illustrating operation of a high conversion gain image sensor, in accordance with an embodiment of the invention.

FIG. 8 is a circuit diagram illustrating configurable pixel circuitry for implementing a standard gain image sensor and a high conversion gain image sensor, in accordance with an embodiment of the invention.

FIG. 9 is a circuit diagram illustrating a standard conversion gain image sensor, in accordance with an embodiment of the invention.

Embodiments of an apparatus and method of operation of a high conversion gain pixel cell are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

FIG. 2 is a block diagram illustrating an imaging system 200 , in accordance with an embodiment of the invention. The illustrated embodiment of imaging system 200 includes an image sensor array 205 , readout circuitry 210 , function logic 215 , and control circuitry 220 .

Image sensor array 205 is a two-dimensional (“2D”) array of image sensors or pixels (e.g., pixels P 1 , P 2 . . . , Pn). In one embodiment, each pixel is a complementary metal-oxide-semiconductor (“CMOS”) imaging pixel. Image sensor array 205 may be implemented as either a front side illuminated image sensor array or a backside illuminated image sensor array. In one embodiment, image sensor array 205 includes a color filter pattern, such as a Bayer pattern or mosaic of red, green, and blue additive filters (e.g., RGB, RGBG or GRGB), a color filter pattern of cyan, magenta, yellow, and key (black) subtractive filters (e.g., CMYK), a combination of both, or otherwise. As illustrated, each pixel is arranged into a row (e.g., rows R 1 to Ry) and a column (e.g., column C 1 to Cx) to acquire image data of a person, place, or object, which can then be used to render a 2D image of the person, place, or object.

After each pixel has acquired its image data or image charge, the image data is readout by readout circuitry 210 and transferred to function logic 215 . Readout circuitry 210 may include amplification circuitry, analog-to-digital (“ADC”) conversion circuitry, or otherwise. Function logic 215 may 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, readout circuitry 210 may readout a row of image data at a time along readout column lines (illustrated) or may readout the image data using a variety of other techniques (not illustrated), such as a column readout, a serial readout, or a full parallel readout of all pixels simultaneously.

Control circuitry 220 is coupled to image sensor array 205 to control operational characteristic of image sensor array 205 . For example, control circuitry 220 may generate a shutter signal for controlling image acquisition. In one embodiment, the shutter signal is a global shutter signal for simultaneously enabling all pixels within image sensor array 205 to simultaneously capture their respective image data during a single acquisition window (exposure period). In an alternative embodiment, the shutter signal is a rolling shutter signal whereby each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows.

FIG. 3 is a circuit diagram illustrating a high conversion gain image sensor 300 , in accordance with an embodiment of the invention. Image sensor 300 represents one possible embodiment of image sensor circuitry (pixel circuitry) for implementing pixels P 1 -Pn of pixel array 205 . The illustrated embodiment of image sensor 300 includes a pixel cell 305 , sample and hold circuitry 310 , a bit line 315 , and a current source 320 . The illustrated embodiment of pixel cell 305 includes a photosensitive element PD (e.g., photodiode, CCD, etc.), a transfer transistor T 5 , a floating diffusion (circuit node FD), a reset transistor T 6 , a source-follower or amplifier (“AMP”) transistor T 7 , and a row select (“RS) transistor T 8 . The illustrated embodiment of sample and hold circuitry 310 includes a hold transistor T 9 , a reference capacitor Cref, a select transistor T 10 , a hold transistor T 11 , a signal capacitor Csig, and a select transistor T 12 .

During operation, reset transistor T 6 selectively resets the voltage on node FD coupled to the gate terminal of AMP transistor T 7 under control of a reset signal RST. The PD generates an image charge in response to incident light, which is subsequently transferred to node FD through transfer transistor T 5 under control of the transfer signal TX. The voltage at circuit node FD biases the gate terminal of AMP transistor T 7 . AMP transistor T 7 provides a high impedance connection to circuit node FD and generates an output signal on bit line 315 indicative of the bias voltage on its gate terminal at circuit node FD. In one embodiment, sample and hold circuitry 310 is time shared by a plurality of pixel cells 305 coupled to bit line 315 . RS transistor T 8 is used select which row within pixel array 205 to transfer an image signal into sample and hold circuitry 310 at a given time under control of the row select signal RS.

Image sensor 300 may operate by first acquiring a black level reference signal from pixel cell 305 , next acquiring an image signal from pixel cell 305 , and then using the black level reference signal as a sort of offset value to cancel out circuit noise (e.g., thermal noise). The black level reference signal may be acquired by asserting the HDBLK signal to sample the black level reference signal output from pixel cell 305 on bit line 315 with reference capacitor Cref. The sampled black level reference signal may subsequently be output from sample and hold circuitry 310 through select transistor T 10 under control of select signal SEL 1 . Similarly, the image signal may be acquired by asserting the HDSIG signal to sample the image signal output from pixel cell 305 on bit line 315 with signal capacitor Csig. The sampled image signal may subsequently be output from sample and hold circuitry 310 through select transistor T 12 under control of select signal SEL 2 .

The illustrated embodiment of image sensor 300 is configured to operate as a high conversion gain pixel. In this embodiment, current source 320 is coupled between a high level voltage/power rail VDD and a channel terminal of AMP transistor T 7 . A channel terminal is a generic term intended to represent either a source or drain terminal. Furthermore, RS transistor T 8 is coupled between the other channel terminal of AMP transistor T 7 and a low level voltage/power rail GND. Finally, reset transistor T 6 couples circuit node FD (also gate terminal of AMP transistor T 7 ) to a node N 2 on bit line 315 between the current source 320 and the high side channel terminal of AMP transistor T 7 . In contrast, standard conversion gain pixels typically couple circuit node FD directly to VDD during reset.

Capacitor Ceq coupling the circuit node FD to node N 2 is not a physical capacitor; but rather, represents the equivalent coupling capacitance between the gate and high side channel terminal of AMP transistor T 7 . This inherent capacitive coupling helps boost the voltage at circuit node N 2 by transferring charge driven into the high side channel terminal of AMP transistor T 7 to its gate terminal.

FIG. 4 is a circuit diagram illustrating a high conversion gain image sensor 400 including pixel cell 305 and boost circuitry 401 , in accordance with an embodiment of the invention. Image sensor 400 represents another possible embodiment of image sensor circuitry that could be used to implement pixels P 1 -Pn of pixel array 205 . Image sensor 400 is similar to image sensor 300 except boot circuitry 401 is coupled to node N 1 between the low side channel terminal of AMP transistor T 7 and RS transistor T 8 .

In the illustrated embodiment, boost circuitry 401 includes a PMOS type transistor T 13 having channel terminals coupled between node N 1 and a boost voltage rail Vbst. In one embodiment, Vbst is equivalent to VDD. In other embodiments, Vbst may be higher or lower than VDD. Transistor T 13 is selectively enabled under control of a boost signal BST, to temporarily boost the voltage at node N 1 on the low side channel terminal of AMP transistor T 7 . Boost circuitry 401 may be implemented with alternative circuitry capable of selectively applying a pre-determined voltage to node N 1 . In short, boost circuitry 401 temporarily boosts the voltage at node N 1 , which indirectly boosts the voltage at circuit node FD, thereby providing greater margin for voltage swing at circuit node FD. Further discussion of this voltage boost technique is provided below in connection with FIG. 6 .

FIG. 5 is a circuit diagram illustrating a high conversion gain image sensor 500 including a pixel cell 505 and boost circuitry 401 , in accordance with an embodiment of the invention. Image sensor 500 represents another possible embodiment of image sensor circuitry that could be used to implement pixels P 1 -Pn of pixel array 205 . Image sensor 500 is similar to image sensor 300 except for the inclusion of boot circuitry 401 and a boost capacitor Cbst coupled between node N 1 and circuit node FD. Boost capacitor Cbst is an actual physical or discrete capacitor which may be implemented with a variety of capacitive circuit elements including a metal-insulator-metal capacitor, a polysilicon capacitor, or otherwise. The presence of boost capacitor Cbst coupling the gate terminal of AMP transistor T 7 to one of its channel terminals further promotes voltage boost at circuit node FD via charge transfer from node N 1 .

FIG. 6 is a flow chart illustrating operation of a high conversion gain image sensor, in accordance with an embodiment of the invention. Process 600 is described with reference to timing diagram 700 ( FIG. 7 ) and the circuit diagrams illustrated in FIGS. 3 , 4 , and 5 . The order in which some or all of the process blocks appear in process 600 should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated.

In a process block 605 , the pixel cell (e.g., pixel cell 305 or 505 ) is reset. In one embodiment, the pixel cell is reset by asserting the signal RS to enable RS transistor T 8 and by asserting the reset signal RST to couple circuit node FD to node N 2 . FIG. 7 illustrates both RS and RST high enabled during times t 1 -t 2 . Resetting causes the voltage at circuit node FD to rise towards VDD as it is charged by the current output from current source 320 on bit line 315 . The voltage at circuit node FD will continue to rise towards VDD until the gate voltage on AMP transistor T 7 is such that the current flowing through AMP transistor T 7 matches the current output from current source 320 on bit line 315 . At this point, the voltage at node N 2 reaches a steady state value.

After the reset window (time t 1 -t 2 ), the reset signal is de-asserted while the row select signal RS continues to be asserted as a mechanism to select the particular pixel cell. In a process block 610 , a reference signal is acquired from the pixel cell to capture the steady state voltage value at node N 2 . The reference signal is used to offset the image signal later acquired. It should be appreciated that the order in which the image signal and reference signal are acquired can be swapped. The reference signal may be thought of as a black level reference signal, since no charge has been transferred from the PD to circuit node FD at this point. As such, the voltage at circuit node FD remains relatively constant—similar to a dark room environment during image acquisition. To acquire the reference signal, the voltage at circuit node N 2 is sampled and held by temporarily coupling capacitor Cref to bit line 315 via transistor T 9 under control of the signal HDBLK (time t 3 -t 4 ).

In a process block 615 , the RS signal is de-asserted to open circuit RS transistor T 8 (time t 5 ). When RS transistor T 8 is open circuited, the voltage at node N 1 will begin to rise from near GND towards VDD, while bit line 315 will also pull near VDD. In the embodiment of pixel cell 305 ( FIGS. 3 and 4 ), the circuit equivalent capacitance Ceq acts to boost the voltage at circuit node FD by coupling charge driven onto bit line 315 by current source 320 to circuit node FD. In the embodiment of pixel cell 505 , the presence of the additional boost capacitor Cbst between node N 1 and circuit node FD further promotes a voltage boost at circuit node FD (process block 630 ).

If the image sensor includes boost circuit 401 (image sensors 400 and 500 ) (decision block 620 ), then the boost voltage Vbst is applied to circuit node N 1 by asserting the signal BST (process block 625 , time t 5 ′-t 7 ′). Application of the boost voltage Vbst to node N 1 provides additional voltage boost to circuit node FD through the capacitive coupling of boost capacitor Cbst (process block 630 ).

In a process block 635 , charge accumulated on the PD during an exposure period is transferred to circuit node FD via transfer transistor T 5 under control of the transfer signal TX (time t 6 -t 7 ). In a process block 640 , the row select signal RS is re-asserted to permit current to flow through the channel of AMP transistor T 7 to ground (time t 8 ). Transfer of the photo-electrons causes the voltage on circuit node FD to drop by an amount indicative of the intensity of light incident on the PD. The drop in the voltage at circuit node FD after charge transfer causes the gate-source voltage Vgs of AMP transistor T 7 to decrease. In turn, this results in a drop of the channel current through AMP transistor T 7 to a value below the bit line current output from current source 320 . Bit line 315 will then be pulled towards VDD by the unbalanced current and builds charge at node N 2 . At the same time, the voltage at circuit node FD will also rise due to the capacitive coupling effect of Ceq. Eventually, the voltage at circuit node FD will stabilize at a new equilibrium voltage that balances the current on bit line 315 with the current passing through AMP transistor T 7 . The voltage on circuit node FD will stabilize at a final voltage relatively close to the voltage at circuit node FD when the reference signal was sampled, but the voltage on bit line 315 will change by an amount approximately equal to the ratio of the charge transferred from the PD and the effective capacitance of Ceq.

Since the initial voltage (pre-transfer voltage) on circuit node FD was boosted, a large voltage swing is available at gate terminal of AMP transistor T 7 . As such, the image sensor has a larger dynamic range, can operate in brighter environments, and achieves this greater dynamic range with little or no tradeoff of conversion gain.

In a process block 645 , an image signal is acquired from the pixel cell to capture the steady state voltage value at node N 2 . To acquire the image signal, the voltage at circuit node N 2 is sampled and held by temporarily coupling capacitor Csig to bit line 315 via transistor T 11 under control of the signal HDSIG (time t 9 -t 10 ). Finally, in a process block 650 , the reference signal and the image signal are transferred out of sample and hold circuitry 310 under control and select signals SEL 1 and SEL 2 and compared. In one embodiment, the reference signal is used to apply an offset value to the image signal.

FIG. 8 is a circuit diagram illustrating configurable pixel circuitry 800 for selectively implementing a standard gain image sensor and a high conversion gain image sensor, in accordance with an embodiment of the invention. The illustrated embodiment of configurable pixel circuitry 800 includes a switched ring interconnect 805 , switches SW 1 -SW 6 , pixel cell 305 , sample and hold circuitry 310 , current source 320 , and a current source 810 . Although FIG. 8 illustrates configurable pixel circuitry 800 as including pixel cell 305 , other embodiments may substitute pixel cell 305 for pixel cell 505 and optionally include boost circuitry 401 .

Configurable pixel circuitry 800 can be reconfigured during regular or in-use operation by appropriate assertion of control signals S 0 and S 1 , to selectively alternate between a high conversion gain image sensor and a standard conversion gain image sensor. When S 1 is asserted and S 0 de-asserted, switches SW 1 -SW 6 configure configurable pixel circuitry 800 to implement high conversion gain image sensor 300 illustrated in FIG. 3 . When S 1 is de-asserted and S 0 is asserted, then configurable pixel circuitry 800 is configured as a standard conversion gain image sensor, as illustrated in FIG. 9 .

The processes explained above are described in terms of computer software and hardware. The techniques described may constitute machine-executable instructions embodied within a machine (e.g., computer) readable storage medium, that when executed by a machine will cause the machine to perform the operations described. Additionally, the processes may be embodied within hardware, such as an application specific integrated circuit (“ASIC”) or the like.

A machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a form accessible by a machine (e.g., a computer, network device, personal digital assistant, manufacturing tool, any device with a set of one or more processors, etc.). For example, a machine-readable storage medium includes recordable/non-recordable media (e.g., read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.).

The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.