High dynamic range pixel having a plurality of amplifier transistors

A pixel cell for use in a high dynamic range image sensor includes a photodiode disposed in semiconductor material to accumulate charge in response to light incident upon the photodiode. A transfer transistor is disposed in the semiconductor material and is coupled between a floating diffusion and the photodiode. A first amplifier transistor is disposed in the semiconductor material having a gate terminal coupled to the floating diffusion and a source terminal coupled to generate a first output signal of the pixel cell. A second amplifier transistor is disposed in the semiconductor material having a gate terminal coupled to the floating diffusion and a source terminal coupled to generate a second output signal of the pixel cell.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention is generally related to image sensors, and more specifically, the present invention is directed to high dynamic range image sensors.

An image capture device includes an image sensor and an imaging lens. The imaging lens focuses light onto the image sensor to form an image, and the image sensor converts the light into electric signals. The electric signals are output from the image capture device to other components of a host electronic system. The electronic system may be, for example, a mobile phone, a computer, a digital camera or a medical device.

The demands on the image sensor to perform over a large range of lighting conditions, varying from low light conditions to bright light conditions are becoming more difficult to achieve as pixel cells become smaller. This performance capability is generally referred to as having high dynamic range imaging (HDRI or alternatively just HDR). In conventional image capture devices, pixel cells require multiple successive exposures to achieve HDR.

During operation of pixel cell100, transfer transistor120receives a transfer signal TX, which transfers charge accumulated in photosensitive element110to floating diffusion FD180. Reset transistor130is coupled between power supply VDD and floating diffusion FD180to reset the pixel cell (e.g., to discharge or charge floating diffusion FD180and/or photosensitive element110to a preset voltage) under control of reset signal RST. FD180is also coupled to control the gate of SF transistor140. SF transistor140is coupled between power supply VDD and row select transistor150. SF transistor140operates as a source follower providing a high impedance connection to floating diffusion FD180. Under control of a select signal SEL, row select transistor150selectively provides an output of the pixel cell to a readout column line, or bit line170.

Capacitor165and dual conversion gain transistor160are coupled in series between power supply VDD and floating diffusion FD180, with dual conversion gain transistor160coupled to FD180and capacitor165coupled to power supply VDD. The capacitance of capacitor165may be added to FD180by asserting dual conversion gain signal, DCG, thereby decreasing the conversion gain of the pixel cell100.

Photosensitive element110and FD180are reset by temporarily asserting the reset signal RST and the transfer signal TX. An image accumulation window (e.g., an exposure period) is commenced by de-asserting the transfer signal TX and permitting incident light to photogenerate electrons in photosensitive element110. As photogenerated electrons accumulate in photosensitive element110, the voltage on photosensitive element110decreases. The voltage or charge on photosensitive element110is indicative of the intensity of the light incident on photosensitive element110during the exposure period. At the end of the exposure period, the reset signal RST is de-asserted to isolate FD180and the transfer signal TX is asserted to allow an exchange of charge between photosensitive element110and FD180, and hence the gate of SF transistor140. The charge transfer causes the voltage of FD180to change by an amount that is proportional to photogenerated electrons accumulated on photosensitive element110during the exposure period. This second voltage biases SF transistor140, which in combination with the select signal SEL being asserted, drives a signal from row select transistor150to the bit line170. Data is then readout from the pixel cell100through bit line170as an analog signal.

By changing the conversion gain of the pixel cell100between successive image captures, the HDR of the resultant image can be increased. However, this would increase amount of time required to capture and readout one HDR image and affect the performance of the image capture device.

DETAILED DESCRIPTION

Examples in accordance with the teaching of the present invention describe an image sensor pixel cell for use in a high dynamic range (HDR) image sensor, including a plurality of amplifier transistors. In various examples, the amplifier transistors are coupled as source followers having different threshold voltages and gain characteristics. In the examples, the amplifier transistors are configured as dual source followers and the output signal from each amplifier transistors of the pixel cell is a component of an output amplification signal of the pixel cell. Operation of the dual source follower transistors may be based on the floating diffusion node transitioning to a voltage level corresponding to an amount of charge accumulated in a photodiode of the pixel cell. Each source follower transistor is coupled to their respective readout column line and readout circuit. In one example, an image sensor system with a pixel array comprising a plurality of pixel cells with this architecture includes two readout column lines per column of pixel cells.

In one example, under higher light intensity conditions, the voltage level at the floating diffusion node is low, since under these conditions, more photogenerated electrons resulting from the incident light are transferred to the floating diffusion node than under lower light intensity conditions. In such conditions, the source follower transistor with a lower threshold voltage will be active. Under lower light intensity conditions, the voltage level at the floating diffusion node will be high, since under these conditions fewer photogenerated electrons resulting from the incident light are transferred to the floating diffusion node than under higher light intensity conditions. In such conditions, both the source follower transistor with high threshold voltage and the source follower transistor with low threshold voltage will be active.

To illustrate,FIG. 2is a circuit diagram showing one example of circuitry of a pixel cell200having a plurality of amplifier transistors240A and240B in accordance with the teachings of the present invention. As shown in the depicted example, amplifier transistors240A and240B are configured as source followers. In the example shownFIG. 2, pixel cell200is arranged to provide two output signals from transistors240A and240B to two readout column signal lines270A and270B. In the example, pixel cell200includes a photosensitive element, shown as photodiode210, a transfer transistor220, a reset transistor230, a floating diffusion FD280, a first amplifier transistor240A, a second amplifier transistor240B, a first row select transistor250A and a second row select transistor250B disposed in semiconductor material. In other examples, it is appreciated that pixel cell200may include a variety of alternative pixel cell architectures that include two amplifier transistors, which are coupled to floating diffusion FD280in a configuration similar to that of first amplifier transistor240A and second amplifier transistor240B in accordance with the teachings of the present invention.

During operation of pixel cell200, charge accumulates in photodiode210in response to light incident upon photodiode210. In one example, the type of charge that is accumulated in photodiode210in response to the incident light includes electrons. Transfer transistor220may receive a transfer signal TX, which transfers charge accumulated in photodiode210to floating diffusion FD280. Reset transistor230may be coupled between a power supply VDD and floating diffusion FD280to reset the pixel cell200(e.g., to discharge or charge floating diffusion FD280and/or photodiode210to a preset voltage) under control of a reset signal RST.

As shown in the depicted example, floating diffusion FD280is coupled to control the gate of first amplifier transistor240A. First amplifier transistor240A may be coupled between power supply VDD and first row select transistor250A. First amplifier transistor240A may operate as a source follower providing a high impedance connection to floating diffusion FD280and amplify the voltage at floating diffusion FD280with a first gain. Second amplifier transistor240B may be coupled between power supply VDD and second row select transistor250B. Second amplifier transistor240B may operate as a source follower providing a high impedance connection to floating diffusion FD280and amplify the voltage at floating diffusion FD280with a second gain. In one example, the first gain of first amplifier transistor240A is different than the second gain of second amplifier transistor240B in accordance with the teachings of the present invention.

In one example, first amplifier transistor240A and second amplifier transistor240B each provide a respective output signal from their respective source terminals. In one example, the output signals generated by first amplifier transistor240A and second amplifier transistor240B may be component signals of an amplification signal representative of the intensity of the light incident upon photodiode210. As shown in the example depicted inFIG. 2, first row select transistor250A may, under control of a select signal RS_H, selectively provide the output signal from the source terminal of first amplifier transistor240A to readout column line BL_H270A. Similarly, as shown in the depicted example, second row select transistor250B may, under control of a select signal RS_L, selectively provide the output signal from the source terminal of second amplifier transistor240B to readout column line BL_L270B. In another example, pixel cell200does not include any row select transistors250A and250B such that the output signals from each of the first and second amplifier transistors240A and240B are directly connected to their respective first and second readout column lines270A and270B.

Referring back to the illustrated example, photodiode210and floating diffusion FD280may be reset by temporarily asserting the reset signal RST on reset transistor230and the transfer signal TX on transfer transistor220. In one example, photodiode210and floating diffusion FD280are reset prior to the acquisition of image data using pixel cell200. At the end of the reset period, the reset signal RST and transfer signal TX may be de-asserted. An image accumulation window (e.g., an exposure period) may then be commenced by permitting incident light to photogenerate charge in photodiode210. In one example, as photogenerated electrons accumulate on photodiode210, the voltage on photodiode210decreases from the reset voltage. The voltage or charge on photodiode210may be representative of the intensity of the light incident on photodiode210during the exposure period.

After the exposure period, the transfer signal TX may then be asserted to allow an exchange of charge between photodiode210and floating diffusion FD280, and hence to the respective gates of both first amplifier transistor240A and second amplifier transistor240B. The charge transfer between photodiode210and floating diffusion FD280causes the voltage of floating diffusion FD280to change by an amount representative of photogenerated electrons accumulated on photodiode210during the exposure period. As shown in the example depicted inFIG. 2, the voltage at floating diffusion FD280is coupled to the gate terminals of first and second amplifier transistors240A and240B, where the voltage at floating diffusion FD280is then amplified by first amplifier transistor240A and second amplifier transistor240B in accordance with the teachings of the present invention.

In one example, first row select transistor250A selectively couples the output signal from first amplifier transistor240A to first readout column line BL_H270A in response to first row select signal RS_H and second row select transistor250B selectively couples the output signal from second amplifier transistor240B to second readout column line BL_L270B in response to first row select signal RS_L. Thus, it is noted that example pixel200ofFIG. 2includes two readout column lines270A and270B for a single photodiode210in accordance with the teachings of the present invention.

In one example, first amplifier transistor240A has a first threshold voltage and the second amplifier transistor240B has a second threshold voltage. In the example, the first and second threshold voltages are different. Accordingly, in the example, the first and second amplifier transistors have different gain characteristics such that the first and second amplifier transistors240A and240B have different sensitivities to the intensity of the light incident on the photodiode210in accordance with the teachings of the present invention.

In one example, first amplifier transistor240A has a lower threshold voltage than second amplifier transistor240B. As will be discussed in greater detail below and inFIG. 3, under higher light intensity conditions, the voltage level at floating diffusion FD node280will be low due to the accumulation the electrons that are photogenerated in photodiode210as a result of the higher intensity incident light. Thus, in such higher light intensity conditions, the voltage level at floating diffusion FD node280will cause first amplifier transistor240A to remain substantially ON as second amplifier transistor240B turns substantially OFF. However, under lower light intensity conditions, the voltage level at floating diffusion FD node280will be higher, since under these conditions, fewer photogenerated electrons are transferred to floating diffusion FD node280than under higher light intensity conditions. In such lower light intensity conditions, the voltage level at floating diffusion FD node280will cause both first amplifier transistor240A, with a lower threshold voltage, and second source follower transistor240b, with a higher threshold voltage to remain substantially ON.

In one example, the different threshold voltages of first amplifier transistor240A and second amplifier transistor240B may be obtained by varying the doping concentrations and/or dopant type in the channel regions in the semiconductor material under their respective polysilicon gates accordingly. Thus, in this example, the doping concentration in the channel region of the first amplifier transistor240A is different than the doping concentration in the channel region of the second amplifier transistor240B. In one example, the threshold voltage of second amplifier transistor240B may be increased by doping the channel region of this transistor with p-type dopants.

In another example, the different threshold voltages of first amplifier transistor240A and second amplifier transistor240B may be obtained by doping the polysilicon gates of the two amplifier transistors with dopants having opposite polarity. For instance, in the example illustrated inFIG. 2, the polysilicon gate of first amplifier transistor240A may be doped with a p-type dopant, while the polysilicon gate of second amplifier transistor240B may be doped with an n-type dopant. In one example, the polysilicon gate of first amplifier transistor240A and second amplifier transistor240B may each have a dopant concentration of 1018to 1019ions/cm3.

FIG. 3is a diagram390illustrating an example relationship between examples of output signals340A and340B from the plurality of amplifier transistors having different gain characteristics and an example amplification signal345utilizing the output signals340A and340B as component signals in accordance with the teachings of the present invention. In one example, it is appreciated that output signal340A may be one example of an output signal from amplifier transistor240A ofFIG. 2and output signal340B may be one example of an output signal from amplifier transistor240B ofFIG. 2. Accordingly, in the depicted example, it is assumed that the amplifier transistor that generates output signal340A has a different gain characteristic and a lower threshold voltage than the amplifier transistor that generates output signal340B.

In the depicted example, the first and second output signals340A and340B are each component signals of amplification signal345, which is representative of the light incident on the photodiode of the pixel cell. As shown in the example ofFIG. 3, when generating amplification signal345, for a higher intensity of light incident on the photodiode, the first output signal340A has greater weight than the second output signal340B in the amplification signal. Indeed, as discussed above, as the intensity of light increases, the number of electrons accumulated in the photodiode increases, which lowers the voltage on the gates of the first and second amplifier transistors accordingly. Since the first amplifier transistor has a lower threshold voltage than the second amplifier transistor in the example, the first output signal340A tends to remain substantially ON for higher light intensities while the second output signal340B tends to turn substantially OFF.

On the other hand, as shown in the depicted example, the second output signal340B has a greater weight than the first output signal340A in the amplification signal345for a lower intensity of light incident on the photodiode. As discussed above, as the intensity of light decreases, the number of photogenerated electrons that are accumulated in the photodiode remains smaller, which allows the voltage on the gates of the first and second amplifier transistors to remain higher. Since the voltage on the gates of the first and second amplifier transistors remains higher, both the first output signal340A and second output signal340B remain substantially ON for lower intensities of incident light.

Therefore, by having different threshold voltages and gain characteristics as described above, the first and second amplifier transistors that generate first and second output signals340A and340B have different sensitivities to different intensities of light that is incident upon the photodiode of the pixel cell. By weighting the component contributions of the first and second output signals340A and340B based on the intensity of the incident light as discussed, amplification signal345provides HDR information having increased sensitivity over a higher dynamic range of light intensities from the pixel cell utilizing first and second output signals340A and340B in accordance with the teaching of the present invention.

FIG. 4is a block diagram illustrating an example imaging system400utilizing a pixel array405including a plurality of pixel cells in accordance with an embodiment of the invention. In particular, as shown in the depicted example, imaging system400includes pixel array405, readout circuitry410, function logic420and control circuitry430.

In the example, pixel array405is a two-dimensional (2D) array of imaging sensor cells or pixel cells (e.g., pixels P1, P2, . . . , Pn). In one example, each pixel cell is a complementary metal-oxide-semiconductor (CMOS) imaging pixel including first and second amplifier transistors in accordance with the teachings of the present invention. Pixel array405may be implemented as a front-side illuminated image sensor or a backside illuminated image sensor. As illustrated, each pixel cell is arranged into a row (e.g., rows R1 to Ry) and a column (e.g., column C1 to Cx) to acquire image data of a person, place or object, which can then be used to render an image of the person, place or object.

In particular, after each pixel cell has acquired its image data or image charge, the image data is read out by readout circuitry410and transferred to function logic420. Readout circuitry410comprises a plurality of column readout blocks415respectively. In the illustrated example, pixel cells arranged in the same column have their respective first and second output signals BL_H470A and BL_L470B coupled to be received by the same column readout block415in readout circuitry410. In one example, each column readout block415includes circuitry to generate corresponding amplification signals in response to the component first and second output signals470A and470B based on the intensity of the incident light as discussed in detail above in accordance with the teachings of the present invention.

In one example, readout circuitry410may include amplification circuitry, analog-to-digital (ADC) conversion circuitry or otherwise. Function logic420may 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 example, readout circuitry410may read out a row of image data at a time along readout column lines (illustrated as first and second output signal bit lines BL_H470A and BL_L470B inFIG. 4) or may read out the image data using a variety of other techniques (not illustrated), such as serial readout, column readout along readout row lines, or a full parallel readout of all pixels simultaneously.

In one example control circuitry430is coupled to pixel array405and includes logic for controlling operational characteristics of pixel array405. For example, reset RST, row select RS_H and RS_L signals and transfer signals TX may be generated by control circuitry430. Control circuitry430may also 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 array405to simultaneously capture their respective image data during a single acquisition window. In an alternative example, the shutter signal is a rolling shutter signal whereby each row, column, or group of pixels is sequentially enabled during consecutive acquisition windows.

In one example, imaging system400is subsystem included in an electronic system. Examples of such electronic systems include a mobile phone, a computer, a digital camera, a medical device, and may further include an operating unit comprising a computing or processing unit related to the electronic system. For instance, an example electronic system may be a mobile phone, and the operating unit may be a telephone module included in the mobile phone that handles the telephone operation of the electronic system.

FIG. 5Ais a diagram showing one example of an arrangement of pixel cells500in accordance with the teachings of the present invention. As shown in the depicted example, pixel cells500arranged in the same column may be coupled to the same first and second readout column lines BL_H570A and BL_L570B. In this example, each pair of readout column lines BL_H570A and BL_L570B is coupled to one of a plurality of column readout blocks515. A pixel array with X columns of pixel cells500may have readout circuitry which includes X column readout blocks.

FIG. 5Bis a diagram showing another example of an arrangement of pixel cells500in accordance with the teachings of the present invention. As shown in the depicted example, pixel cells500arranged in two adjacent columns may time-share one column readout block515. In this example, a pixel array with X columns of pixel cells500may have X/2 column readout blocks515. In yet another example, N adjacent columns of pixel cells500may time-share each column readout block515. In such an example, a pixel array with X columns may include X/N column readout blocks.