KR100889928B1 - Complementary metal oxide semiconductor image sensor - Google Patents

Abstract

A complementary metal oxide semiconductor image sensor for improving color sensitivity and picture quality by clearing gray level according to an inputted code is provided to manufacture image sensor and increase a dynamic range by adding a transistor without increasing the space of photo diode. A photo diode(111) produces optical electric charge. A reset transistor(113) performs a reset function by connecting between the floating diffusion area and power supply voltage. A drive transistor(114) performs a source follower buffer amplifier by granting photo charge. It is connected between the floating diffusion area and power supply voltage and if photo-charge is not generated, the subsidiary transistor(120) delivers the power supply voltage to the gate of the drive transistor.

Description

CMOS Image Sensor {Complementary Metal Oxide Semiconductor Image Sensor}

Embodiments relate to a CMOS image sensor.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal, in which charge carriers are stored and transferred to a capacitor while individual metal oxide-silicon (MOS) capacitors are located in close proximity to each other. By using CMOS technology, which uses charge coupled device (CCD), control circuit and signal processing circuit as peripheral circuits, MOS transistors are made and used as many as the number of pixels. There is a CMOS (complementary MOS) image sensor that employs a switching method that sequentially detects the output.

The CMOS image sensor, which converts the information of the subject into an electrical signal, is composed of signal processing chips containing a photodiode. An amplifier, an analog / digital converter, and an internal chip are included in one chip. Internal voltage generators, timing generators, and digital logic can also be combined, which greatly reduces space, power, and cost.

On the other hand, CMOS image sensors are classified into 3T type, 4T type, and 5T type according to the number of transistors. The 3T type consists of one photodiode and three transistors, and the 4T type consists of one photodiode and four transistors.

Herein, the layout of the unit pixels of the 4T-type CMOS image sensor will be described.

1 is an equivalent circuit diagram of a general 4T CMOS image sensor.

As shown in FIG. 1, the unit pixel of the CMOS image sensor includes a photo diode (PD) 11 as a photoelectric converter and four transistors. Each of the four transistors is a transfer transistor 12, a reset transistor 13, a drive transistor 14 and a select transistor 15.

Here, reference numeral FD is a floating diffusion node, Tx is a gate voltage of the select transistor 15, Rx is a gate voltage of the reset transistor 13, Dx is a gate voltage of the drive transistor 14, Sx is It is the gate voltage of the select transistor 15.

A typical 4T CMOS image sensor turns on the reset transistor 13 when there is no light to draw out all the leaking electrons of the floating diffusion node FD to VDD so that the voltage of the floating diffusion node FD becomes coincident with the VDD voltage 3.0. It becomes V.

When the reset transistor 13 is turned off, electrons randomly move in the channel of the reset transistor 13, and noise is generated by the electrons returning to the floating diffusion node, and the noise decreases from 3.0V to 2.9V.

As such, when the voltage of the floating diffusion node decreases due to the noise generated by the reset transistor 13, the dynamic range of the pixel decreases and the color sensitivity decreases.

The embodiment provides a CMOS image sensor that can increase the dynamic range by adding transistors without increasing the area of the photodiode.

According to an embodiment, a CMOS image sensor may include a photodiode for generating a photocharge, a floating diffusion region for storing the photocharge, a reset transistor connected between the floating diffusion region and a power supply voltage and performing a reset function, wherein the photocharge is performed. A driving transistor acting as a source follower buffer amplifier and an auxiliary transistor connected between the floating diffusion region and the power supply voltage to transfer the power supply voltage to a gate of the drive transistor when the photocharge is not generated.

According to an embodiment, a CMOS image sensor may include a photodiode for generating a photocharge, a floating diffusion region for storing the photocharge, a reset transistor connected between the floating diffusion region and a power supply voltage, and performing a reset function. A drive transistor applied as a source follower buffer amplifier, a first auxiliary transistor connecting the floating diffusion region and a drain terminal of the drive transistor, a second auxiliary transistor connecting the floating diffusion region and the power supply voltage, and the power supply voltage And a third auxiliary transistor connecting the drain terminal of the drive transistor.

The embodiment can increase the dynamic range by adding a transistor without increasing the area of the photodiode, thereby making it possible to manufacture a miniature image sensor.

In addition, the embodiment is resistant to noise and the contrast difference for each gray level with respect to the input code is sure, thereby improving color sensitivity and improving image quality.

The CMOS image sensor according to the embodiment has an effect of improving the dynamic range by further forming an auxiliary transistor in a unit pixel.

Hereinafter, a CMOS image sensor according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.

2 is a circuit diagram illustrating an image sensor according to a first embodiment.

Hereinafter, to facilitate understanding of the embodiment, it is assumed that the dynamic range of the photodiode PD is 1.0V, the threshold voltage Vth of the drive transistor 114 is 0.5V, and the power supply voltage VDD is 3.0V. Although described, this is only an example.

As shown in FIG. 2, the unit pixel Px of the image sensor includes a photodiode 111 for generating photocharges and four transistors. Each of the four transistors is a transfer transistor 112, a reset transistor 113, a drive transistor 114, and a select transistor 115.

The reset transistor 113 is connected between a floating diffusion region (herein referred to as a floating diffusion node) and a power supply voltage for storing the photocharges to perform a reset function, and the drive transistor 114 receives the photocharges. It acts as a source follower buffer amplifier.

The select transistor 115 is connected to the drive transistor 114 and selects the corresponding unit pixel Px.

The image sensor may include an auxiliary transistor 120 that is not formed in the unit pixel Px but is formed in a logic circuit region to control the unit pixel Px.

The auxiliary transistor 120 is connected in parallel with the reset transistor 113. That is, the auxiliary transistor 120 is connected to VDD and the floating diffusion node FD.

Here, reference numeral FD is a floating diffusion node, Tx is a gate voltage of the select transistor 115, Rx is a gate voltage of the reset transistor 113, Dx is a gate voltage of the drive transistor 114, Sx is It is the gate voltage of the select transistor 115. M is a gate voltage input to the auxiliary transistor 120.

When the select transistor 115 of the image sensor is turned on, the corresponding unit pixel Px is selected. At this time, light enters the photodiode 111 to generate signal electrons, and the signal electrons are output as an output voltage.

When there is no light entering the photodiode 111, the auxiliary transistor 120 is turned on so that the VDD 3.0V is connected to the drain terminal of the drive transistor 114 and the VDD 3.0V is the gate terminal of the drive transistor 114. Since the reference output voltage Vout1 is reduced to 0.5V at 3.0V, the threshold voltage Vth of the drive transistor 114 is reduced to 2.5V.

According to the embodiment, when the light does not enter the photodiode 111, the control is performed by using the auxiliary transistor 120, and thus electrons randomly move to the channel of the reset transistor 113 to return to the floating diffusion node FD. No noise is generated by them, so there is no voltage reduction.

Since the auxiliary transistor 120 is formed in the logic circuit region, the auxiliary transistor 120 is stronger in noise than the reset transistor 113 formed in the pixel and does not need to increase the area of the unit pixel Px.

Therefore, the embodiment can increase the dynamic range by adding a transistor without increasing the area of the photodiode 111, thereby making it possible to manufacture a miniature image sensor.

Thereafter, the maximum amount of light enters and the photodiode 111 generates the maximum signal electrons. Then, the transistor 110 is turned on so that the signal electrons having the 1.0V capacitance generated in the photodiode 111 are floated. In step 1), the voltage of the floating diffusion node FD decreases to 1.9V.

In this case, the auxiliary transistor 120 is turned off so that the voltage applied to the floating diffusion node FD is connected to the gate of the drive transistor 114 to decrease the voltage by the threshold voltage Vth of the drive transistor 114. The output voltage Vout2 is output at 1.4V.

That is, the reference output voltage Vout1 when no light enters the photodiode 111 and the output voltage Vout2 when maximum light enters the photodiode 111 are output at 1.4V.

Therefore, the voltage difference between the reference output voltage Vout1 when there is no light and the output voltage Vout2 when the light is maximum becomes the maximum capacity of the photodiode PD, that is, the dynamic range, and the dynamic range is the two output voltages. The difference between Vout1 and Vout2 is 1.1V, which increases the dynamic range than when affected by the noise of the reset transistor 113.

Dividing the dynamic range 1.1V into 8 bits (256 codes) results in approximately 4.3 mV per code. The one code is a gray voltage (one gradation) of each color. When the dynamic range is 1 V, the level is about 3.9 mV per code. Therefore, in the embodiment, the voltage per code is high, thus the noise is high, and the voltage difference between the codes increases. Contrast by gray improves the color sensitivity.

3 is a circuit diagram illustrating an image sensor according to a second embodiment.

Hereinafter, the operation of the photodiode PD is assumed to be 1.0V and the threshold voltage Vth of the drive transistor is 0.5V for the purpose of understanding the present invention. However, this is only an example.

As shown in FIG. 3, the image sensor includes a photo diode 211 as a photoelectric conversion unit and four transistors in the unit pixel Px. Each of the four transistors is a transfer transistor 212, a reset transistor 213, a drive transistor 214 and a select transistor 215.

The reset transistor 213 is connected between a floating diffusion region (herein referred to as a floating diffusion node) and a power supply voltage VDD to store the photocharges, and performs a reset function. It acts as a source follower buffer amplifier.

The select transistor 215 is connected to the drive transistor 214 and selects the unit pixel Px.

The image sensor may include a first auxiliary transistor 221 connecting the floating diffusion node FD and a drain terminal of the drive transistor 214, and a second auxiliary transistor connecting the floating diffusion node FD and the VDD. 222 and a third auxiliary transistor 223 connecting the VDD and the drain terminal of the drive transistor 214.

The second auxiliary transistor 222 may be connected in parallel with the reset transistor 213.

Here, reference numeral FD is a floating diffusion node, Tx is a gate voltage of the select transistor 215, Rx is a gate voltage of the reset transistor 213, Dx is a gate voltage of the drive transistor 214, Sx is It is the gate voltage of the select transistor 215. M1 is a gate voltage input to the first auxiliary transistor 221, M2 is a gate voltage input to the second auxiliary transistor 222, and M3 is a gate voltage input to the third auxiliary transistor 223. to be.

When the select transistor 215 of the image sensor is turned on, the corresponding unit pixel Px is selected. At this time, light enters the photodiode 211 to generate signal electrons, and the signal electrons are output as an output voltage.

When there is no light entering the photodiode 211, the third auxiliary transistor 223 is turned off and the first and second auxiliary transistors 221 and 222 are turned on so that the voltage of the floating diffusion node FD is turned on. Is connected to the drain terminal of the drive transistor 214 and VDD 3.0V is connected to the gate terminal of the drive transistor 214 so that the threshold voltage Vth of the drive transistor 214 is 0.5V when the reference output voltage Vout1 is 3.0V. Reduced to 2.5V output.

According to the exemplary embodiment, when light does not enter the photodiode 211, the first to third auxiliary transistors 221, 222, and 223 are controlled by using electrons in the channel of the reset transistor 213 to float. The electrons returning to the diffusion node FD do not generate noise and thus no voltage decrease.

Since the second and third auxiliary transistors 222 and 223 are formed in the logic circuit, the second and third auxiliary transistors 222 and 223 are stronger in noise than the reset transistor 213 formed in the pixel, and only the first auxiliary transistor 221 is formed in the unit pixel Px. do.

Therefore, the embodiment can increase the dynamic range by using the auxiliary transistor, thereby making it possible to manufacture a miniature image sensor.

Thereafter, the maximum amount of light enters the photodiode 211 to generate the maximum signal electrons, and then the transfer transistor 212 is turned on so that the signal electrons having the 1.0V capacitance generated in the photodiode 211 are floated. In step 1), the voltage of the floating diffusion node FD decreases to 1.9V.

The third auxiliary transistor 223 is turned on and the first and second auxiliary transistors 221 and 222 are turned off so that a voltage applied to the floating diffusion node FD is connected to the gate of the drive transistor 214. The voltage is reduced by the threshold voltage Vth of the drive transistor 214 so that the output voltage Vout2 is output as 1.4V.

That is, the reference output voltage Vout1 when no light enters the photodiode 211 and the output voltage Vout2 when the maximum light enters the photodiode 2 are output at 1.4V.

Therefore, the voltage difference between the reference output voltage Vout1 when there is no light and the output voltage Vout2 when the light is maximum becomes the maximum capacity of the photodiode 211, that is, the dynamic range, where the dynamic range is the two output voltages. The difference between Vout1 and Vout2 is 1.1V, which increases the dynamic range than when affected by the noise of the reset transistor 213.

Dividing the dynamic range 1.1V into 8 bits (256 codes) results in approximately 4.3 mV per code. The one code is a gray voltage (one gradation) of each color. When the dynamic range is 1 V, the level is about 3.9 mV per code. Therefore, in the embodiment, the voltage per code is high, thus the noise is high, and the voltage difference between the codes increases. Contrast by gray improves the color sensitivity.

Although described above with reference to the embodiments, which are merely examples and are not intended to limit the present invention. Those skilled in the art to which the present invention pertains are not exemplified above without departing from the essential characteristics of the present invention. It will be appreciated that many variations and applications are possible. For example, each component specifically shown in the embodiment of the present invention can be modified. And differences relating to such modifications and applications will have to be construed as being included in the scope of the invention defined in the appended claims.

1 is an equivalent circuit diagram of a general 4T CMOS image sensor.

2 is a circuit diagram illustrating an image sensor according to a first embodiment.

3 is a circuit diagram illustrating an image sensor according to a second embodiment.

Claims (6)

Photodiodes for generating photocharges; A floating diffusion region for storing the photocharges; A reset transistor connected between the floating diffusion region and a power supply voltage to perform a reset function; A drive transistor configured to receive the photocharge and serve as a source follower buffer amplifier; And And an auxiliary transistor connected between the floating diffusion region and the power supply voltage to transfer the power supply voltage to a gate of the drive transistor when the photocharge is not generated. The method of claim 1, And the auxiliary transistor is formed in a logic circuit region. The method of claim 1, And the auxiliary transistor transfers a voltage of the floating diffusion region to a gate of the drive transistor when the photocharge is generated. Photodiodes for generating photocharges; A floating diffusion region for storing the photocharges; A reset transistor connected between the floating diffusion region and a power supply voltage to perform a reset function; A drive transistor configured to receive the photocharge and serve as a source follower buffer amplifier; A first auxiliary transistor connecting the floating diffusion region and a drain terminal of the drive transistor; A second auxiliary transistor connecting the floating diffusion region and the power supply voltage; And And a third auxiliary transistor connecting the power supply voltage and a drain terminal of the drive transistor. The method of claim 4, wherein If the photocharge is not generated, the third auxiliary transistor is turned off and the first and second auxiliary transistors are turned on so that the voltage of the floating diffusion region is connected to the drain terminal of the drive transistor, and the power supply voltage is the drive transistor. CMOS image sensor, characterized in that connected to the gate terminal. The method of claim 4, wherein And when the photocharge is generated, the third auxiliary transistor is turned on and the first and second auxiliary transistors are turned off so that a voltage across the floating diffusion region is connected to a gate of the drive transistor. .

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