KR100518888B1 - Unit pixel of CMOS image sensor - Google Patents

Abstract

Disclosed is a unit pixel of a CMOS image sensor which increases the reset efficiency of a photodiode and prevents image distortion by preventing the photodiode and the floating diffusion region from being directly connected in a reset path of the photodiode. A photodiode that senses light from the outside and generates photocharges; A floating diffusion region configured to receive and store charges generated from the photodiode through a transfer transistor; First and second reset transistors connected in series between a power supply voltage terminal and the floating diffusion region; And a connection wiring connecting the connection node of the first reset transistor and the second reset transistor and the photodiode, wherein the floating diffusion region is reset when both the first and second reset transistors are turned on. The photodiode provides a unit pixel of the CMOS image sensor, wherein the second reset transistor is turned off and the first reset transistor is turned on and reset.

Description

Unit pixel of CMOS image sensor

The present invention relates to a CMOS image sensor, and more particularly to a unit pixel of a CMOS image sensor for improving the reset efficiency of the unit pixel.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Among them, a charge coupled device (CCD) includes individual metal-oxide-silicon (MOS) capacitors. A device in which charge carriers are stored and transported in a capacitor while being in close proximity, and a CMOS (Complementary MOS) image sensor uses CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. A device employing a switching scheme that creates MOS transistors as many as pixels and sequentially detects outputs using the MOS transistors.

CCD (charge coupled device) has many disadvantages such as complicated driving method, high power consumption, high number of mask process steps, complicated process, and difficult to implement signal processing circuit in CCD chip. In recent years, CMOS image sensors using sub-micron CMOS manufacturing techniques have been developed to overcome such drawbacks.

The CMOS image sensor forms an image by forming photodiodes and MOS transistors in a unit pixel and sequentially detects signals in a switching method, and implements images by using CMOS manufacturing technology, which consumes less power and has about 20 masks. Compared to CCD process that requires 30 to 40 masks, the process is very simple, and various signal processing circuits and one-chips are possible.

FIG. 1A is a circuit diagram illustrating a unit pixel composed of one photodiode PD and four NMOS transistors in a conventional CMOS image sensor, the photodiode PD receiving light to generate photocharges, The transfer transistor Tx for transporting the photocharges collected from the photodiode PD to the floating diffusion region FD, and resets the floating diffusion region FD by setting the potential of the floating diffusion region to a desired value and discharging the electric charge. A reset transistor (Rx), a drive transistor (Dx) that acts as a source follower buffer amplifier, and a select transistor (Sx) that allows addressing (switching). It is composed. Outside the unit pixel, a load transistor is formed to read an output signal.

FIG. 1B illustrates a layout of unit pixels, in which an isolation defining an active region in which a photodiode and a diffusion region are to be formed and polysilicon constituting a gate of each transistor are illustrated. Referring to this, the photodiode 101 has a square shape, and the gate polysilicon 102 of the transfer transistor is formed in contact with one side of the photodiode 101.

The floating diffusion region 103 is laid out in contact with the other side of the gate polysilicon 102 of the transfer transistor by being turned 90 ° in the X-axis direction in the Y-axis direction, and in contact with one side of the gate polysilicon 104 of the reset transistor.

The other side of the gate polysilicon 104 of the reset transistor is formed in contact with the drain region 105, and the drain region 105 is formed at an angle of 90 ° from the X-axis direction to the Y-axis direction, and then the gate polysilicon 106 of the drive transistor. It comes in contact with.

Subsequently, the gate polysilicon 108 of the select transistor is formed in the same direction, between the other side of the gate polysilicon 106 of the drive transistor and the gate polysilicon 108 of the select transistor, and the other side of the gate polysilicon 108 of the select transistor. Source / drain regions 107 and 109 are formed in the substrate.

In the layout of a conventional unit pixel configured as described above, the floating diffusion region 103 is formed in an active region between the transfer transistor 102 and the reset transistor 104 and the gate polysilicon of the floating diffusion region 103 and the drive transistor. 106 is electrically connected through a contact and a connection wiring.

Operation of the image sensor unit pixel as described above is performed as follows.

(A) The photodiode PD is reset by turning on the transfer transistor Tx and the reset transistor Rx.

(B) Turn off the transfer transistor Tx, the reset transistor Rx and the select transistor Sx. At this time, the photodiode PD is in a fully depletion state.

(C) Photogenerated charges are collected on low voltage photodiode (PD).

(D) After a proper integration time, the reset transistor Rx is turned on to reset the floating diffusion region FD.

(E) Turn on the select transistor Sx to address the desired unit pixel.

(F) The output voltage V1 of the drive transistor Dx, which is a source follower buffer, is measured. This value merely represents a DC level shift of the floating diffusion region FD. .

(G) Turn on the Transistor (Tx). As a result, all the photocharges accumulated in the photodiode are transported to the floating diffusion region FD.

(H) Turn off the transfer transistor (Tx).

(I) Measure the output voltage V2 of the drive transistor Dx.

(D) Obtain output voltage difference. The output signals V1-V2 are the result of the photocharge transport resulting from the difference between V1 and V2, which are pure signal values without noise.

In order to operate the CMOS image sensor, it is necessary to first measure the voltage (V1) due to noise, measure the voltage (V2) in which noise components and image information are combined, and then subtract the voltage due to noise. Get the output for. This is called correlated double sampling (CDS) and is a technique commonly applied to CMOS image sensors.

Before performing the operation of reading the photocharge stored in the photodiode using the CDS, as described above, the operation of resetting the photodiode using the reset transistor or the like is performed.

That is, a photodiode reset operation is performed to turn on the reset transistor and the transfer transistor in order to remove residual electrons present in the photodiode, in order to remove noise components and obtain more accurate image values.

In the conventional structure, the unit pixel shown in Figure 1a the force to pull the remaining electronics present in the photodiode at the time of such reset operation there is dependent on the supply voltage (V _DD), a predetermined voltage from the power supply voltage (V _DD) The subtracted voltage is finally applied to the photodiode and used to remove the electrons.

That is, the voltage V _FD finally applied to the photodiode is

V _PD = V _DD- (V _TX + V _FD + V _RX ) = 3.3-(i × R _TX + i × R _FD + i × R _RX ).

Where R _TX is the resistance of the well in which the transfer transistor is formed, R _FD is the resistance of the floating diffusion region, R _RX is the resistance of the well in which the reset transistor is formed, and i is caused by electrons present in the photodiode. Indicate the generated current.

As a voltage used to remove electrons present in the photodiode, the power supply voltage is not used up and a voltage reduced by (V _TX + V _FD + V _RX ) in the power supply voltage removes electrons present in the photodiode. It is used to

That is, in the conventional unit circuit structure, as the voltage used to remove the residual electrons remaining in the photodiode is significantly lower than the power supply voltage, the residual electrons are not completely removed and the possibility of a dark signal due to the residual electrons is increased. there is a problem.

In addition, in the conventional unit pixel structure, since the floating diffusion region FD exists in the reset path of the photodiode, image distortion of the CMOS image sensor may occur.

The present invention has been proposed to solve the above-mentioned problems of the prior art, and an object thereof is to provide a unit pixel of a CMOS image sensor that increases the reset efficiency of a photodiode.

Another object of the present invention is to provide a unit pixel of a CMOS image sensor that prevents image distortion by preventing the photodiode and the floating diffusion region from being directly connected in the reset path of the photodiode.

The present invention according to one aspect for achieving the above object, a photodiode for generating a photocharge by sensing light from the outside; A floating diffusion region configured to receive and store charges generated from the photodiode through a transfer transistor; First and second reset transistors connected in series between a power supply voltage terminal and the floating diffusion region; And a connection wiring connecting the connection node of the first reset transistor and the second reset transistor and the photodiode, wherein the floating diffusion region is reset when both the first and second reset transistors are turned on. The photodiode provides a unit pixel of the CMOS image sensor, wherein the second reset transistor is turned off and the first reset transistor is turned on and reset.

In addition, the present invention according to another aspect is a photodiode for generating a photocharge by sensing light from the outside; A floating diffusion region configured to receive and store charges generated from the photodiode through a transfer transistor; First and second reset transistors connected in series between a power supply voltage terminal and the photodiode; And a connection wiring connecting the connection node of the first reset transistor and the second reset transistor and the floating diffusion region, wherein the photodiode is reset by turning on both the first and second reset transistors. The floating diffusion region provides a unit pixel of the CMOS image sensor, wherein the second reset transistor is turned off and the first reset transistor is turned on and reset.

Hereinafter, the most preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the technical idea of the present invention.

(First embodiment)

2A is a circuit diagram showing the configuration of unit pixels in a CMOS image sensor according to a first embodiment of the present invention.

Referring to FIG. 2A, a unit pixel of a CMOS image sensor according to an exemplary embodiment of the present invention may include a photodiode PD for generating light and accumulating photocharges, and a photocharge generated at the photodiode PD. It is connected in series between the transfer transistor Tx for transport, the floating diffusion region FD for receiving and storing the photocharge through the transfer transistor Tx, and the power supply voltage terminal VDD and the floating diffusion region FD. A connection wiring connecting the first and second reset transistors Rx1 and Rx2 and the connection node N and the photodiode PD of the first reset transistor Rx1 and the second reset transistor Rx2; A drive transistor Dx connected to the power supply voltage terminal VDD to serve as a source follower buffer amplifier for detecting an electrical signal from the floating diffusion region FD, and the other side of the drive transistor; One side connected And the other end is constituted by the select transistor Sx serving as the output terminal OUT of the unit pixel.

Importantly, the reset of the floating diffusion region FD is performed when both the first and second reset transistors are turned on, and the surplus electrons in the floating diffusion region are applied to the floating diffusion region FD by applying a power supply voltage. The reset is performed by removing.

In addition, the photodiode PD is reset by turning off the second reset transistor and turning on the first reset transistor.

FIG. 2B illustrates a preferred embodiment of a layout for implementing the unit pixel circuit of FIG. 2A.

Referring to FIG. 2B, there is shown an isolation defining an active region in which a photodiode and a diffusion region are to be formed, and polysilicon constituting the gate of each transistor.

A difference from the prior art of FIG. 1B is that the gate lines constituting the first and second reset transistors are formed of two polysilicon 204A and 204B separated from each other, and a connection node (N in FIG. 2A) is formed as a first node. And a common diffusion region 210 of the second reset transistor, and a connection wiring is formed through the contact C3 formed in the common diffusion region 210 and the contact C1 formed in the photodiode 201. .

The layout of such a configuration is very preferable because it minimizes the existing layout deformation and can shorten the development time of the CMOS image sensor of the present invention.

2B, the photodiode 201 is square and the gate polysilicon 202 of the transfer transistor is in contact with one side of the photodiode 201.

The floating diffusion region 203 is laid out by contacting the other side of the gate polysilicon 202 of the transfer transistor by 90 ° in the X-axis direction in the Y-axis direction, and in contact with the other side of the gate polysilicon 204B of the second reset transistor. .

A common diffusion region constituting one diffusion region of the first reset transistor and one diffusion region of the second reset transistor,

One side of the gate polysilicon 204B of the second reset transistor Rx2 is formed in contact with the common diffusion region 210, and the common diffusion region 210 is formed with the gate polysilicon 204A of the first reset transistor Rx1. It is constructed in contact. In other words, the common diffusion region 210 becomes the connection node N. A diffusion region 205 is formed on the other side of the gate polysilicon 204A of the first reset transistor Rx1 by 90 ° in the Y-axis direction from the X-axis direction, and a VDD contact is formed in the diffusion region 205.

The diffusion region 205 is in contact with the gate polysilicon 106 of the drive transistor, and the other side of the gate polysilicon 106 of the drive transistor is interposed between the source / drain regions 207 with the gate polysilicon 208 of the select transistor interposed therebetween. , 209 is formed.

The other diffusion region and the floating diffusion region 203 of the second reset transistor are configured in common, and the floating diffusion region 203 and the gate polysilicon 206 of the drive transistor are connected to the contacts C2 and C4, respectively. It is connected via wiring.

As described above, in the CMOS image sensor according to the first embodiment of the present invention, there is no transfer transistor on the reset path of the photodiode, unlike the conventional art, and thus a power supply voltage used to remove surplus electrons remaining in the photodiode Less loss than before. As a result, it is possible to solve the problem of the prior art that the residual electrons are not completely removed and the likelihood of causing a dark signal due to the residual electrons is increased.

In addition, the reset path of the photodiode of the present invention does not go through the floating diffusion region. As a result, image distortion of the CMOS image sensor can be prevented.

The operation of the CMOS image sensor unit pixel according to the present invention as described above is performed as follows.

(A) The photodiode PD is reset by turning on only the first reset transistor Rx1 while the transfer transistor Tx is turned off.

(B) The transfer transistor Tx, the first and second reset transistors Rx1 and Rx2 and the select transistor Sx are turned off. At this time, the photodiode PD is in a fully depletion state.

(C) Photogenerated charges are collected on low voltage photodiode (PD).

(D) After the appropriate integration time, the floating diffusion region FD is reset by turning on the first and second reset transistors Rx1 and Rx2.

(E) Turn on the select transistor Sx to address the desired unit pixel.

(F) The output voltage V1 of the drive transistor Dx, which is a source follower buffer, is measured. This value merely represents a DC level shift of the floating diffusion region FD. .

(G) Turn on the Transistor (Tx). As a result, all the photocharges accumulated in the photodiode PD are transported to the floating diffusion region FD.

(H) Turn off the transfer transistor (Tx).

(I) Measure the output voltage V2 of the drive transistor Dx.

(D) Obtain output voltage difference. The output signals V1-V2 are the result of the photocharge transport resulting from the difference between V1 and V2, which are pure signal values without noise.

As described above, the unit pixel of the CMOS image sensor of the present invention may slightly change the control of the first and second reset transistors Rx1 and Rx2 to obtain image data by a conventional correlated double sampling (CDS) method.

Second Embodiment

3A is a circuit diagram illustrating a configuration of unit pixels in a CMOS image sensor according to a second exemplary embodiment of the present invention.

Referring to FIG. 3A, a unit pixel of a CMOS image sensor according to a second exemplary embodiment of the present invention may include a photodiode PD for generating light and accumulating photocharges, and a photocharge generated at the photodiode PD. Is connected in series between a transfer transistor Tx for transporting the light source, a floating diffusion region FD for receiving and storing photocharges through the transfer transistor Tx, and a power supply voltage terminal VDD and a photodiode PD. A connection wiring connecting the first and second reset transistors Rx1 and Rx2 to the connection node N and the floating diffusion region FD of the first reset transistor Rx1 and the second reset transistor Rx2; One side of the drive transistor Dx, which is connected to the power supply voltage terminal VDD and serves as a source follower buffer amplifier for detecting an electrical signal from the floating diffusion region FD, and the other side of the drive transistor. One side of it And the other end is composed of a select transistor (Sx) that is an output terminal (OUT) of the unit pixel.

Importantly, the photodiode PD is reset when both the first and second reset transistors are turned on, and in the floating diffusion region FD, the second reset transistor is turned off and the first reset transistor is turned on. The reset transistor is turned on to reset.

As described above, the CMOS image sensor according to the second embodiment of the present invention does not have a floating diffusion region on the reset path of the photodiode PD. As a result, since the photodiode PD is reset without passing through the floating diffusion region FD, image distortion of the CMOS image sensor may be prevented.

FIG. 3B shows a preferred embodiment of a layout for implementing the unit pixel circuit of FIG. 3A.

Importantly, the connection node includes a common diffusion region 310 of the first reset transistor Rx1 and the second reset transistor Rx2, and the connection wiring is connected to the common diffusion region 310 and the floating diffusion region FD. It is formed through the contacts C34 and C32 respectively formed.

Another contact C31 is formed in the floating diffusion region FD, and a connection wiring is formed through the contact and the contact formed in the gate of the drive transistor Dx to be electrically connected to each other.

The embodiments described above are described in the conventional four-transistor component unit consisting of a transfer transistor, a reset transistor, a drive transistor, and a celet transistor, and in addition, all the elements having a photodiode, a floating diffusion region, and a transfer gate formed therebetween. It can work in the unit pixel of the structure.

As such, the present invention is not limited to the above-described embodiments and the accompanying drawings, and various changes, modifications, and changes are possible in the art without departing from the spirit of the present invention. It will be apparent to those who have knowledge.

The present invention increases the removal efficiency of residual electrons in the reset operation of the photodiode, thereby reducing the dark current, so that more accurate image reproduction is possible, the dynamic range is increased, and there is no floating diffusion region on the reset path of the photodiode. It has the effect of preventing image distortion.

1A is a circuit diagram showing the configuration of a unit pixel in a CMOS image sensor according to the prior art;

1B is a diagram illustrating a layout of unit pixels in a CMOS image sensor according to the related art;

2A is a circuit diagram showing the configuration of unit pixels in a CMOS image sensor according to a first embodiment of the present invention;

2B is a view showing the layout of unit pixels in a CMOS image sensor according to a first embodiment of the present invention;

3A is a circuit diagram showing the configuration of unit pixels in a CMOS image sensor according to a second embodiment of the present invention;

3B is a diagram illustrating a layout of unit pixels in a CMOS image sensor according to a second embodiment of the present invention.

Explanation of symbols on main parts of drawing

PD: Photodiode

Tx: transfer transistor

FD: Floating Diffusion Area

Rx1: First reset transistor

Rx2: 2nd reset transistor

Dx: drive transistor

Sx: Select Transistor

Claims (8)

A photodiode that senses light from the outside and generates photocharges; A floating diffusion region configured to receive and store charges generated from the photodiode through a transfer transistor; First and second reset transistors connected in series between a power supply voltage terminal and the floating diffusion region; And A connection wiring connecting the connection node of the first reset transistor and the second reset transistor and the photodiode, The floating diffusion region is reset when both the first and second reset transistors are turned on, and the photodiode is reset when the second reset transistor is turned off and the first reset transistor is turned on. Unit pixel of the CMOS image sensor, characterized in that. The method of claim 1, A drive transistor having one side connected to the power supply voltage for detecting an electrical signal from the floating diffusion region; And And a select transistor having one side connected to the other side of the drive transistor and the other end being an output terminal of the unit pixel. The method according to claim 1 or 2, The connection node includes a common diffusion region that constitutes one side diffusion region of the first reset transistor and one side diffusion region of the second reset transistor, and the connection wiring is connected to the common diffusion region and the photodiode, respectively. Unit pixel of the CMOS image sensor, characterized in that. The method of claim 2, The other diffusion region and the floating diffusion region of the second reset transistor are configured in common, and the floating diffusion region and the gate of the drive transistor are connected to each other through a connection wiring contacted to the unit pixel of the CMOS image sensor. . A photodiode that senses light from the outside and generates photocharges; A floating diffusion region configured to receive and store charges generated from the photodiode through a transfer transistor; First and second reset transistors connected in series between a power supply voltage terminal and the photodiode; And A connection wiring connecting the connection node of the first reset transistor and the second reset transistor and the floating diffusion region; The photodiode is reset when both the first and second reset transistors are turned on, and the floating diffusion region is reset when the second reset transistor is turned off and the first reset transistor is turned on. Unit pixel of the CMOS image sensor, characterized in that. The method of claim 5, A drive transistor having one side connected to the power supply voltage for detecting an electrical signal from the floating diffusion region; And And a select transistor having one side connected to the other side of the drive transistor and the other end being an output terminal of the unit pixel. The method according to claim 5 or 6, The connection node includes a common diffusion region that constitutes one side diffusion region of the first reset transistor and one side diffusion region of the second reset transistor, and the connection wiring is connected to the common diffusion region and the floating diffusion region, respectively. Unit pixel of the CMOS image sensor, characterized in that. The method of claim 2, And the floating diffusion region and the gate of the drive transistor are connected to each other via a connection wiring contacted to the floating diffusion region and the drive transistor.