KR101029618B1 - Cmos image sensor - Google Patents

Abstract

PURPOSE: A CMOS image sensor is provided to increase the current amplification in a pixel by obtaining a high efficiency signal through a low voltage. CONSTITUTION: A photoelectric conversion element(410) receives light from the outside and generates photocharge. A transmission transistor(420) transmits the generated photocharge to a floating diffusion node according to a transmission control signal. A reset transistor(430) reset the floating diffusion node according to a reset control signal supplied to a gate terminal. A signal transmission circuit(440) outputs an electric signal according to the voltage of the floating diffusion node in response to a selection signal. A pixel pull-up amp circuit(450) changes an output signal of the signal transmission circuit into a voltage inverse proportion to the voltage of the floating diffusion node.

Description

CMOS image sensor {CMOS IMAGE SENSOR}

TECHNICAL FIELD The present invention relates to a CMOS image sensor, and more particularly, to a CMOS image sensor that operates so that the voltage of the floating diffusion node is inversely proportional to the output voltage.

An image sensor refers to a device for capturing an image using a property of a semiconductor that reacts to light. Each part of each subject in the natural world has different brightness and wavelength of light, and shows different electrical values in each pixel of the sensing device. This makes the electrical values at a level capable of signal processing. That's what sensors do.

To this end, an image sensor can consist of a pixel array consisting of tens of thousands to hundreds of thousands of unit pixels, a device that converts the analog voltage detected by thousands of pixels into a digital voltage, and hundreds to thousands of storage devices.

On the other hand, CMOS (Complementary Metal Oxide Semiconductor) image sensor further includes processes such as color filter and micro lens compared to general processes, and transmits light because it receives light. It has a large effect on the characteristics of, and is sensitive to process variations.

1 is a block diagram of a unit pixel of a general CMOS image sensor.

As shown in FIG. 1, the unit pixel 100 may include one photoelectric conversion element and four transistors. The four transistors transfer the phototransistors generated in the photoelectric conversion element 110 to the floating diffusion node, and discharge the charge stored in the floating diffusion node for the next signal detection. Source for driving a sample and hold to interface between a reset transistor 130 for reading a signal and an analog-to-digital converter receiving an output signal from the pixel without distorting the signal generated at the pixel. The drive transistor 141 connected to the follower and the select transistor 142 may be configured to address the voltage of the pixel in units of rows.

2 is an operation timing diagram of a unit pixel illustrated in FIG. 1.

1 and 2, after the floating diffusion node FD is reset to the high voltage by the reset (RST) signal, the transmission (TG) signal is activated to be high and the photodiode 110 is activated. ) Is transmitted to the floating diffusion node FD, amplified by the drive transistor 141, and output to the column signal Vout.

3A to 3D are conceptual diagrams for describing an operation of a unit pixel illustrated in FIG. 1.

3A illustrates a step of integrating charges due to light in a photodiode. In this step, since the transfer transistor 320 and the reset transistor 330 are turned off, charges by light may be integrated in the photodiode 310.

3B, the reset transistor 330 is turned on. In this step, the reset transistor may be turned on to discharge the charge stored in the floating diffusion node FD and increase the voltage of the floating diffusion node to be close to the power supply voltage Vdd.

3C, the reset transistor 330 is turned off. In this step, the voltage of the floating diffusion node FD may maintain the voltage sampled in the previous step.

FIG. 3D illustrates a step in which the transfer transistor 320 is turned on. In this step, charges integrated in the photodiode 310 may flow into the floating diffusion node FD.

However, according to the related art, when the voltage of the floating diffusion node is large, the output voltage of the unit pixel is also large. Therefore, there is a limit in driving a low-voltage high efficiency drive transistor.

In order to solve the above problems, it is possible to drive a high efficiency drive transistor even at low voltage by configuring the voltage of the floating diffusion node and the output voltage in inverse relationship when the CMOS image sensor is operated, and to provide a small size CMOS image sensor. .

An embodiment of the present invention provides a photoelectric conversion element that receives light from an external source and generates photocharges, a transfer transistor that transfers the photocharges generated by the photoelectric conversion element to a floating diffusion node according to a transmission control signal, and a power supply voltage at one end thereof. A reset transistor which is applied to reset the floating diffusion node connected to the other end according to a reset control signal, and a signal transfer circuit unit which is applied with a ground voltage at one end to output an electrical signal according to the voltage of the floating diffusion node to the other end according to a selection signal. And a pixel pull-up amplifier circuit unit connected to the signal transfer circuit unit to convert an output signal of the signal transfer circuit unit into a voltage inversely proportional to the voltage of the floating diffusion node.

Furthermore, another embodiment of the present invention includes a pixel array having a plurality of pixels arranged in rows and columns and a pixel pull-up amplifier circuit connected to the column signal lines of the pixel array, wherein each pixel in the pixel array is A photoelectric conversion element configured to receive light from an outside to generate photocharges; a transfer transistor configured to transfer the photocharges generated by the photoelectric conversion element to a floating diffusion node according to a transmission control signal; A reset transistor for resetting the floating diffusion node according to a reset control signal, wherein a ground voltage is applied to one end and the column signal line is connected to the other end so that an electrical signal according to the voltage of the floating diffusion node is connected to the other end according to a selection signal A signal transfer circuit part output to a signal line, wherein the pixel pull-up circuit The furnace unit may provide a CMOS image sensor that converts an output signal of the signal transfer circuit unit into a voltage inversely proportional to the voltage of the floating diffusion node.

According to the present invention, a signal of high efficiency can be obtained even at a low voltage, thereby increasing the current amplification degree of the pixel. In addition, the size of the unit pixel can be reduced by such high efficiency operation.

Hereinafter, each embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, this specification and drawings are for describing each embodiment of the present invention, the scope of the present invention is not limited to each embodiment only.

4A and 4B show a configuration diagram showing a detailed configuration of a CMOS image sensor as one embodiment of the present invention. 4C is a timing diagram illustrating a signal form over time of a pulse signal operating in the structure of FIG. 4B.

The CMOS image sensor 400 according to an exemplary embodiment of the present invention illustrated in FIG. 4A includes a photoelectric conversion element 410, a transfer transistor 420, a reset transistor 430, a signal transfer circuit unit 440, and a pixel pull-up amplifier circuit unit ( 450).

The photoelectric conversion element 410 is a device that generates a charge (photocharge) when light is introduced from the outside, a photodiode may be used in a preferred embodiment. One end (source / drain terminal) of the photoelectric conversion element 410 may be grounded, and the other end (source / drain terminal) may be connected to one end (source / drain terminal) of the transfer transistor 420.

The transfer transistor 420 may perform a switching role of transferring charges generated from the photoelectric conversion element 410 to the floating diffusion FD node by a transfer control signal TG.

That is, the transfer transistor 420 has a line for transmitting the transfer control signal connected to a gate terminal, and when the transfer transistor 420 operates (turns on) with a transfer control signal received at the gate terminal, Charge generated and accumulated in the photoelectric conversion element 410 may be moved to the floating diffusion node.

The reset transistor 430 may reset the floating diffusion node according to a reset control signal RST applied to a gate terminal. The gate terminal of the reset transistor 430 may be connected to a reset control signal RST line. The floating diffusion node may be reset by a power supply voltage applied to one end (source / drain terminal).

In one embodiment, a constant power supply voltage source Vdd as shown in FIG. 4A may be connected to the reset transistor. In this case, the reset transistor 430 may perform a reset operation by switching between the floating diffusion FD nodes using the power supply voltage source Vdd according to the reset control signal RST.

In another embodiment, the reset transistor 430 may be connected to the pulse signal line RSTP as shown in FIG. 4B. The pulse signal line may apply a pulse signal as shown in FIG. 4C to the reset transistor. In this case, the pulse signal line RSTP is supplied to one end (source / drain terminal) of the reset transistor in a predetermined period t1 to t3 including a period t2 in which the reset transistor 430 is reset. By applying a voltage signal, the same effects as those connected to the power supply voltage source Vdd can be provided. When the actual pulse signal is applied, the additionally wider sections t1 and t3 than the t2 section, which is the reset section, are provided to prevent deterioration of the reliability of the sensor due to a minute error. The reset transistor 430 may switch between the pulse signal line RSTP and the floating diffusion node.

The signal transmission circuit unit 440 may be applied with ground power at one end, and outputs a signal according to the voltage of the floating diffusion node to the other end (output terminal). As shown in FIG. 4A, one end of the signal transmission circuit unit 440 may be connected to a ground power supply terminal.

Alternatively, as shown in FIG. 4B, another end may be connected to the pulse signal line RSTP so that a ground signal is applied when an output operation of the signal transmission circuit 440 is performed. In this case, as shown in FIG. 4C, it can be seen that the pulse signal line applies the ground signal in the period t4 to which the ground signal is to be applied.

Preferably, as illustrated in FIGS. 4A and 4B, the signal transfer circuit 440 may include a drive transistor 441 for outputting an electric signal according to the voltage of the floating diffusion node, and a select transistor 442 connected in series with the drive transistor. ) May be included. The select transistor 442 may switch the output of the signal from the drive transistor 441 according to the selection signal ROW applied to the gate terminal.

One end (source / drain terminal) of the drive transistor 441 may receive a ground voltage during an output operation. This is because when the drain-source path (channel) is formed according to the voltage of the floating diffusion node input to the gate terminal, the direction of the current flowing through the drive transistor 441 is moved from the pixel pull-up amplifier circuit part 450 to the ground direction. To flow. The direction of the current is related to the operation of the pixel pull-up amplifier circuit 450, which will be described later.

In the select transistor 442, a gate terminal may be connected to a select signal ROW line, one end (source / drain terminal) of the drive transistor 441 and the other end (source / drain terminal) of the pixel pull-up amplifier circuit part. And may be connected to 450. The select transistor 442 may serve as a switch to transfer the voltage of the floating diffusion node transmitted through the drive transistor 441 to the pixel pull-up amplifier circuit 450 in response to the selection signal ROW.

In the present embodiment, the pixel pull-up amplifier circuit 450 may be connected to the signal transfer circuit 440, preferably the select transistor 442. The output of the pixel pull-up amplifier circuit part 450 may be input to a correlated double sampling (CDS) circuit part and used for dual sampling and image sensing.

In this embodiment, the pixel pull-up amplifier circuit 450 includes a clamp element 451, a current source 452, and an independent voltage source (hereinafter referred to as a 'pixel pull-up voltage source') 453. Can be.

The clamp device 451 may be a transistor implemented with PMOS or NNOS, and a separate gate power source (not shown) may be connected to the gate terminal of the clamp device 451. In addition, one end (source / drain terminal) of the clamp element 451 may be connected to the signal transfer circuit 440, preferably the select transistor 442, and the other end (source / drain terminal) may be connected to the current source ( 452). The current source 452 has a structure connected to the pixel pull-up amplifier voltage source 453.

The clamp element 451 maintains a constant voltage of one end (source / drain terminal) connected to the signal transmission circuit unit 440 (the maximum voltage value is the voltage value of the pixel pull-up amplifier voltage source 453) to provide ground power. It may serve to allow a current to flow in the direction of the terminal of the signal transmission circuit unit 440 is applied.

5A and 5B show a configuration diagram showing a detailed configuration of a CMOS image sensor including a pixel array structure which is another embodiment of the present invention.

As shown in FIG. 5A, a plurality of unit pixels 501, 502, 503, and 504 are arranged in rows and columns to form a pixel array 500. Pixel pull-up amplifier circuits 511 and 512 may be connected to each column signal line of the unit pixel. The pixel pull-up amplifier circuits 511 and 512 convert an output signal output from the signal transfer circuit of the selected pixel among the pixels connected to the column signal line to which the pixel pull-up amplifier circuits are inversely proportional to the voltage of the floating diffusion node of the selected pixel. You can print

Each of the unit pixels 501, 502, 503, and 504 has the same operation as the photoelectric conversion element 410, the transfer transistor 420, the reset transistor 430, and the signal transfer circuit 440 shown in FIGS. 4A and 4B. It may include. However, as shown in FIG. 4A, one end (source / drain terminal) of the reset transistors of the unit pixels 501, 502, 503, and 504 is connected to a power supply voltage source, and one end of the signal transfer circuit part is connected to a ground voltage source. 5B, a structure in which the reset transistors and the signal transfer circuits of the unit pixels 501, 502, 503, and 504 are all connected to the pulse signal line RSTP is illustrated in FIG. 5B.

Hereinafter, another embodiment of the present invention will be described in detail with reference to FIGS. 6 and 7.

6 and 7 respectively show an embodiment in which an additional photoelectric conversion element-transistor transistor is provided in a unit pixel of the present invention. For each pixel, in addition to the photoelectric conversion element 1011 and the transfer transistor 1021, one or more additional photoelectric conversion elements 1012 to 1014 and one or more additional transfer transistors 1022 to one connected to each additional photoelectric conversion element. 1024) is used. The photoelectric conversion elements 1011 to 1014 and the transistors 1021 to 1024 in one pixel commonly use one floating diffusion node FD.

FIG. 6 shows a power supply voltage source Vdd connected to one end (source / drain terminal) of the reset transistor 1030 in the embodiment of the present invention, and one end (source / drain terminal) of the drive transistor 1041 in the signal transmission circuit portion. The structure connected to the ground terminal is shown.

7 illustrates a structure in which a pulse signal line RSTP is connected to one end (source / drain terminal) of the reset transistor 1030 and the drive transistor 1041 in the embodiment of the present invention.

The structure of the unit pixel to which the additional photoelectric conversion element and the additional transfer transistor shown in FIGS. 6 and 7 may include a plurality of photoelectric conversion elements in one unit pixel may optimize the area efficiency of the unit pixel. .

The other structure and principle of operation are substantially the same as the structure and principle of operation of the unit pixel described above. In the present embodiment, four photoelectric conversion elements and a transfer transistor are connected, but the number may be adjusted according to circumstances.

8 and 9, the operating principle of the CMOS image sensor according to each embodiment of the present invention will be described.

Reset operation

In the structure of the image sensor according to an embodiment of the present invention as shown in FIG. 8, when the voltage of the reset control signal RST rises to operate the reset transistor 430 (turn-on), the voltage of the floating diffusion (FD) node is It may rise close to the unit pixel power supply voltage Vdd. At this time, the voltage of the floating diffusion (FD) node is transmitted to the pixel pull-up amplifier circuit 450 by the drive transistor 441 and the select transistor 442 to be first sampled, and thus the voltage of the output terminal of the pixel pull-up amplifier circuit section. This reference voltage can be.

Operation of photocharge generation and transfer transistor

Meanwhile, when light received from the outside enters the photoelectric conversion element 410 during the integration period, charges are accumulated in the photoelectric conversion element 410 in proportion thereto.

At this time, when the voltage of the transfer control signal TG rises, a channel is formed in the transfer transistor 420, and the charge accumulated in the photoelectric conversion element 410 is transferred to the floating diffusion FD node. The voltage of the floating diffusion node may change in proportion to the amount of signal charge transferred from the photoelectric conversion element 410 to the floating diffusion node.

Operation of Drive Transistors and Pixel Pullup Amplifier Circuits

In response to the voltage of the floating diffusion node applied to the gate terminal, the drive transistor 441 may form a drain-source path to allow current to flow.

In this case, the clamp element 451 of the pixel pull-up amplifier circuit portion 450 maintains the voltage of the terminal connected to the signal transfer circuit portion 440 constant (in FIG. 8, the maximum voltage of Vdd 'which is the voltage of the pixel pull-up voltage source). Voltage value) can provide a driving force to flow the current to the ground terminal. In addition, the current source 452 can supply an Isense current.

Output by the pixel pull-up amplifier circuit and light sensing using the same

In the structure of the image sensor according to the embodiment of the present invention, the voltage of the floating diffusion node and the output voltage of the pixel pull-up amplifier circuit may have an inverse relationship with each other as shown in FIG. 9. This is because the voltage difference between both ends of the drive transistor 441 is kept constant, and the magnitude of the Isense current flowing to the ground terminal is changed as the voltage of the floating diffusion node applied to the gate terminal is changed.

For example, if the voltage of the floating diffusion node is increased and the magnitude of the Isense current is increased, the voltage output by the pixel pull-up amplifier circuit is relatively low. Conversely, as the voltage at the floating diffusion node becomes smaller and the Isense current becomes smaller, the voltage output from the pixel pull-up amplifier becomes higher. Therefore, the voltage of the output terminal of the pixel pull-up amplifier circuit may be inversely related to the voltage of the floating diffusion node.

9 shows an inverse relationship between the voltage of such a floating diffusion node and the voltage of the output terminal of the pixel pull-up amplifier circuit. As shown in FIG. 9, when the voltages V ₁ and V ₂ of the floating diffusion node are transferred to the drive transistor 441 while the select transistor 442 is turned on, the voltage at the output terminal of the pixel pull-up amplifier circuit is V _1. 'And V ₂ ' may be output outside the unit pixel. As such, when the voltage of the floating diffusion node increases, the voltage at the source of the pixel pull-up amplifier circuit unit is reduced. On the contrary, when the voltage of the floating diffusion node decreases, the voltage of the output terminal of the pixel pull-up amplifier circuit appears high. Light sensing may be performed by the output V ₁ ′ and V ₂ ′.

As shown in FIG. 9, even when a low voltage signal is applied to the floating diffusion node, the voltage signal output through the pixel pull-up amplifier circuit part may be very high. Therefore, a CMOS image sensor having a high efficiency output signal can be realized even at a low voltage, and a high current amplification degree can be obtained. Further, due to the low voltage high efficiency, sufficient current amplification efficiency can be obtained even by connecting additional photoelectric conversion elements and additional transfer transistors to each pixel. In addition, since the current amplification degree of the pixel increases, the size of the drive transistor having an amplifying function can be reduced, and thus the size of the pixel can be reduced.

In each embodiment of the present invention, a case has been described in which the transistors are all implemented with NMOS. However, the transistors may also be implemented in the form of PMOS, in which case the connection of the drain and source electrodes may vary.

It is intended that the invention not be limited by the foregoing embodiments and the accompanying drawings, but rather by the claims appended hereto. It will be apparent to those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. something to do.

1 is a block diagram of a unit pixel of a general CMOS image sensor.

2 is an operation timing diagram of a unit pixel of a general CMOS image sensor.

3A to 3D are conceptual views for explaining the operation of a unit pixel of a general CMOS image sensor.

It is a block diagram which shows the detailed structure of the image sensor which is embodiment of this invention.

It is a block diagram which shows the detailed structure of the image sensor connected with the pulse signal line of other embodiment of this invention.

4C is a graph schematically illustrating a form of applying a pulse signal with time in an image sensor to which a pulse signal line is connected.

5A is a block diagram showing a detailed configuration of an image sensor including a pixel array that is another embodiment of the present invention.

5B is a block diagram illustrating a detailed configuration of an image sensor including a pixel array, which is another embodiment of the present invention, to which a pulse signal line is connected.

6 is a configuration diagram showing an operating principle of a unit pixel of the first embodiment of the present invention.

7 is a graph showing an inverse relationship between the voltage of the floating diffusion node and the voltage of the output terminal of the pixel pull-up amplifier circuit.

8 is a configuration diagram showing an embodiment of an image sensor to which a plurality of additional photoelectric conversion elements and additional transfer transistors are added.

9 is a configuration diagram illustrating an embodiment of an image sensor to which a plurality of additional photoelectric conversion elements and additional transmission transistors are added and to which a pulse signal line is connected.

Claims (12)

Photoelectric conversion element for generating light by receiving light from the outside; A transfer transistor configured to transfer the photocharge generated by the photoelectric conversion element to a floating diffusion node according to a transfer control signal; A reset transistor configured to reset the floating diffusion node connected to the other end by applying a power supply voltage to one end according to a reset control signal applied to a gate end; A signal transfer circuit unit having a ground voltage applied to one end thereof and outputting an electrical signal corresponding to the voltage of the floating diffusion node to the other end according to a selection signal; And And a pixel pull-up amplifier circuit connected to the signal transfer circuit to convert an output signal of the signal transfer circuit to a voltage in inverse proportion to the voltage of the floating diffusion node. The method of claim 1, wherein the pixel pull-up amplifier circuit, A current source coupled to the pixel pull-up amplifier voltage source; And And a clamp element connected between the signal transfer circuit unit and the current source, the clamp element maintaining a constant voltage at the connection end with the signal transfer circuit unit. The method of claim 1, wherein the signal transmission circuit unit, A drive transistor configured to output an electrical signal according to a voltage of the floating diffusion node applied to a gate terminal; And And a select transistor connected in series with the drive transistor to switch an output of a signal output from the drive transistor according to a selection signal applied to a gate terminal. The CMOS image sensor according to claim 1 or 2, wherein one end of the reset transistor is connected to a power supply voltage source, and one end of the signal transfer circuit part is connected to a ground power supply terminal. The terminal of claim 1 or 2, wherein one end of the reset transistor and one end of the signal transfer circuit are connected to a pulse signal line. In a reset operation of the reset transistor, a power supply voltage signal is applied through the pulse signal line, And a ground signal is applied through the pulse signal line during an output operation of the signal transfer circuit unit. The method of claim 1, wherein the CMOS image sensor, One or more additional photoelectric conversion elements that receive light from the outside to generate photocharges; And And at least one additional transfer transistor configured to transfer photocharges generated by connecting the at least one additional photoelectric conversion element to the floating diffusion node according to a transmission control signal. A pixel array having a plurality of pixels arranged in rows and columns; And A pixel pull-up amplifier circuit connected to the column signal lines of the pixel array; Each pixel in the pixel array is Photoelectric conversion element for generating light by receiving light from the outside; A transfer transistor configured to transfer the photocharge generated by the photoelectric conversion element to a floating diffusion node according to a transfer control signal; A reset transistor configured to reset the floating diffusion node connected to the other end by applying a power supply voltage to one end according to a reset control signal applied to a gate end; A signal transfer circuit unit having a ground voltage applied at one end thereof and connected to the column signal line at the other end thereof to output an electrical signal according to the voltage of the floating diffusion node to the column signal line connected to the other end according to a selection signal, And the pixel pull-up circuit unit converts an output signal of the signal transfer circuit unit into a voltage inversely proportional to the voltage of the floating diffusion node and outputs the converted signal. The method of claim 7, wherein the pixel pull-up amplifier circuit unit, A current source coupled to the pixel pull-up amplifier voltage source; And And a clamp element connected between the signal transfer circuit unit and the current source, the clamp element maintaining a constant voltage at the connection end with the signal transfer circuit unit. The method of claim 7, wherein the signal transmission circuit unit, A drive transistor for outputting an electrical signal according to the voltage of the floating diffusion node; And And a select transistor connected in series with the drive transistor to switch an output of a signal output from the drive transistor according to a selection signal applied to a gate terminal. 9. The CMOS image sensor of claim 7 or 8, wherein one end of the reset transistor is connected to a power supply voltage source, and one end of the signal transfer circuit part is connected to a ground power supply terminal. The terminal of claim 7 or 8, wherein one end of the reset transistor and one end of the signal transfer circuit are connected to a pulse signal line. In a reset operation of the reset transistor, a power supply voltage signal is applied through the pulse signal line, And a ground signal is applied through the pulse signal line during an output operation of the signal transfer circuit unit. The method of claim 7, wherein each pixel in the pixel array, One or more additional photoelectric conversion elements that receive light from the outside to generate photocharges; And And at least one additional transfer transistor configured to transmit photocharges generated by connecting each of the at least one additional photoelectric conversion element to the floating diffusion node according to a transmission control signal.

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