KR100984698B1 - Image sensor and its driving method - Google Patents

Abstract

The present invention relates to an image sensor and a driving method thereof, and for this purpose, the present invention provides a photodiode (PD), a transfer transistor (Tx), a diffusion node region (FD), a reset transistor (Rx), a drive transistor (Dx) and Unlike a conventional image sensor having a pixel structure including a selective transistor Sx, a first switching transistor and a second power supply that are selectively switched by connecting a diffusion node region FD and a drain terminal of a drive transistor Dx A second switching transistor that is selectively switched by connecting the gate terminal of the VDD2 and the drive transistor Dx, and a third switching transistor that is selectively switched by connecting the drain terminal of the first power supply VDD1 and the drive transistor Dx. By outputting the first output voltage and the second output voltage via selective switching, the dynamic being the difference between the first output voltage and the second output voltage. It is possible to increase the dynamic range.

Image sensor, photodiode (PD), transfer transistor (Tx), diffusion node region (FD), reset transistor (Rx), drive transistor (Dx), selective transistor (Sx)

Description

Image sensor and its driving method {IMAGE SENSOR AND ITS DRIVING METHOD}

The present invention relates to a driving technique of an image sensor, and more particularly, to an image sensor and a driving method thereof suitable for driving an image sensor having a four-transistor pixel structure of a CMOS image sensor.

As is well known, an image sensor can be classified into a CCD sensor, a CMOS image sensor, etc., and basically uses an electron-hole pair generated by light of energy larger than the silicon bandgap. Holes are used to estimate the amount of light emitted.

In particular, the CMOS image sensor implements a photo diode (PD) and a transistor similarly to a general CMOS device in each photosensitive pixel, and thus uses a conventional CMOS semiconductor manufacturing process almost as it is. Compared to a CCD image sensor that must have an image signal processor, the integrated circuit for image signal processing and detection can be integrated into an external block of pixels, and low voltage operation is possible and manufacturing cost is low.

The CMOS image sensor is divided into a 4-transistor pixel structure and a 3-transistor pixel structure by the number of transistors constituting a single photosensitive pixel, and the three-transistor pixel structure has advantages in terms of fill factor and manufacturing cost. In spite of having a light-receiving part and a detecting part, and a light-receiving part made of silicon bulk except for the surface, a 4-transistor pixel structure having high responsiveness to light, high sensitivity, and strong dark current and noise is generally used.

1 is a diagram illustrating a 4-transistor pixel structure of a CMOS image sensor according to the related art.

Referring to FIG. 1, the four-transistor pixel structure is composed of four transistors. The photo-sensing means includes a photodiode (PD, 104) and four NMOS transistors, which constitute one unit photosensitive pixel. The transfer transistor (Tx: Transfer TR, 106) transfers photocharge generated in the photodiode (PD) to the diffusion node region (FD: Floating Diffusion, 108), and reset transistor (Rx: Reset TR, 110). ) Discharges the charge stored in the diffusion node region (FD) or photodiode (PD) for signal detection, and the drive transistor (Dx: Driving TR) 112 serves as a source follower transistor. In addition, the selective transistor Sx (Selective TR) 114 is for switching / addressing.

In addition, the photodiode region PD and the capacitance present in parallel with each other form a light-receiving portion, and the transfer transistors Tx and 106 which transmit the received electrons may receive electrons generated by photons. It serves to transfer to the diffusion node region (FD) 108, and takes a method of applying a potential through the gate of the selective transistor (Sx, 114) for selecting one column to obtain a two-dimensional image. In particular, each photosensitive pixel is biased by a current source, which operates the drive transistors Dx and 112 and the selective transistors Sx and 114 to read the potential of the diffusion node region FD 108 into the output node. do. Here, the area A indicated by a dotted line means one pixel size of the image sensor.

For example, the selective transistors Sx and 114 are turned on to select a specific pixel, and light enters the photodiode PD 104 to generate signal electrons, which are output voltages V _out. When there is no light, the reset transistors Rx and 110 are turned on to apply all of the electrons in the diffusion node region FD 108 to VDD 102 so that the voltage of the diffusion node region FD 108 is reduced. This voltage becomes the same as the VDD voltage (for example, 3.0 V, etc.), and when the reset transistors Rx and 110 are turned off, electrons move randomly in the channel of the reset transistors Rx and 110, and the diffusion node among them. The electrons returning to region FD 108 reduce the voltage of diffusion node region FD 108 to approximately 2.9V. Here, a decreasing voltage of approximately 0.1 V means noise.

When the voltage 2.9 V of the diffusion node region FD 108 is applied to the gates of the drive transistors Dx and 112, the output voltage is reduced by about 0.5 V by the threshold voltage Vth of the drive transistors Dx and 112. The reference output voltage V _out 1 is output at approximately 2.4V. Here, since the gate voltage is about 2.9 V, even when 3 V is applied to the drain as VDD, the transferable voltage is completely transferred to the source terminal, so that the transferable voltage is reduced by about 0.5 V, which is about 0.5 V, which is the threshold voltage Vth. do.

Subsequently, when light comes in at maximum (i.e., the photodiode produces the maximum signal electrons), the transistors of the approximately 1.0 V capacitance generated in the photodiode PD, 104 are turned on by the transfer transistors Tx and 106. Are moved to the diffusion node region FD 108, the voltage of the diffusion node region FD 108 is reduced to approximately 1.9 V, and the reduced voltage of 1.9 V of the diffusion node region FD 108 is driven. When applied to the gates of the transistors Dx and 112, the output voltage is reduced by approximately 0.5 V by the threshold voltage Vth of the drive transistors Dx and 112, so that the output voltage V _out 2 when the light is turned on at the maximum is reduced. Output is approximately 1.4V.

Accordingly, the difference between the two output voltages (V _out 1-V _out 2) becomes approximately 1.0 V, and the difference between the reference output voltage Vout1 when there is no light and the output voltage Vout2 when the light is maximum. Denotes a maximum capacity of the photodiode PD, that is, a dynamic range. Here, the example as described above is when the dynamic range of the photodiode PD, 104 is 1.0V, the threshold voltage Vth of the drive transistors Dx, 112 is 0.5V, and the VDD 102 is 3.0V. It is assumed and explained.

However, in the conventional 4-transistor pixel structure, dividing the dynamic range of 1.0 V into 8 bits (256 codes) results in 3.9 mV per code, and 1 code means gray voltage (1 gradation) of each color. However, the higher the voltage per code, the better the noise, and the difference in the business cards of each gray appear clearly, which improves the color sensitivity. However, the pixel size is limited and the area of the photodiode (PD, 104) is limited. There is a limit to increase the.

Accordingly, the present invention provides an image sensor and a driving method thereof that can increase the dynamic range by selectively connecting to the terminal of the diffusion node region (FD), the gate terminal and the drain terminal of the drive transistor (Dx) using a switching transistor. To provide.

In addition, the present invention is to provide an image sensor and a driving method thereof that can increase the dynamic range by connecting an additional VDD to the image sensor connected to the switching transistor.

In one aspect, the present invention has a pixel structure including a photodiode (PD), a transfer transistor (Tx), a diffusion node region (FD), a reset transistor (Rx), a drive transistor (Dx) and a selective transistor (Sx). An image sensor comprising: a first switching transistor to be selectively switched by connecting the diffusion node region FD and a drain terminal of the drive transistor Dx, and a gate terminal of the second power supply VDD2 and the drive transistor Dx. And a second switching transistor to be selectively switched by connecting, and a third switching transistor to be selectively switched by connecting the first power supply VDD1 and the drain terminal of the drive transistor Dx. The image sensor may include a corresponding pixel. When not emitting light, when the first switching transistor and the second switching transistor are turned on and the third switching transistor is turned off Claim to provide an image sensor, characterized in that for outputting a first output voltage.

In another aspect, the present invention has a pixel structure including a photodiode (PD), a transfer transistor (Tx), a diffusion node region (FD), a reset transistor (Rx), a drive transistor (Dx) and a selective transistor (Sx). A method of driving an image sensor, comprising: configuring a first switching transistor, a second switching transistor, and a third switching transistor in the pixel structure, wherein the first switching transistor, the second switching transistor, and the third switching transistor are configured; Supplying a first power supply VDD1 and a second power supply VDD2 to a pixel structure, turning on the first switching transistor and the second switching transistor when the corresponding pixel having the pixel structure does not emit light; Turning off the third switching transistor to output a first output voltage, and the third switching upon maximum emission of the pixel And turning on the transistor, and turning off the first switching transistor and the second switching transistor to output a second output voltage.

The present invention is a conventional image having a pixel structure including a photodiode (PD), a transfer transistor (Tx), a diffusion node region (FD), a reset transistor (Rx), a drive transistor (Dx) and a selective transistor (Sx). Unlike the sensor, by using the selective switching of the first switching transistor, the second switching transistor and the third transistor, when there is no light emission of the pixel, the third switching transistor is turned on simultaneously with turning on the first switching transistor and the second switching transistor. By turning off the first output voltage, turning off the first switching transistor and the second switching transistor at the time of maximum light emission of the pixel, and turning on the third switching transistor to output the second output voltage. 2 You can increase the difference between the output voltages and increase the corresponding dynamic range. .

The technical idea of the present invention is to turn on the first transistor and the second transistor when there is no light, turn off the third transistor to apply a second power source to the gate terminal of the drive transistor, and apply the voltage of the diffusion node region of the drive transistor. The first output voltage is output by applying to the drain terminal, and when the maximum light comes in, the first transistor and the second transistor are turned off, and the third transistor is turned on to apply the voltage in the diffusion node region to the gate terminal of the drive transistor. It is to output the second output voltage, through the technical means can solve the problems in the prior art.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a diagram illustrating a four-transistor pixel structure of a CMOS image sensor according to an exemplary embodiment.

Referring to FIG. 2, when the first power supplies VDD1 and 202a are supplied with 3.0 V and the second power supplies VDD2 and 202b are supplied with 3.5 V, when there is no light, the third transistors M3 and 216c are not present. Is turned off, and the first transistors M1 and 216a and the second transistors M2 and 216b are turned on so that the voltage of the diffusion node region FD 208 of 2.9 V is the drive transistors Dx and 212. Is applied to the drain terminal of the second transistor (VDD2, 202b), 3.5V is applied to the gate terminal of the drive transistors (Dx, 212), so that all of the 2.9V of the drain terminal of the drive transistors (Dx, 212) are source terminals. Is transferred to the first output voltage V _out 1.

In this case, since the gate voltage of the drive transistors Dx and 212 is 3.5 V, the gate voltage of the drive transistors Dx and 212 may be transferred to approximately 3.0 V even when a decrease of 0.5 V, which is the threshold voltage Vth of the drive transistors Dx and 212 is considered. Here, the first transistors M1 and 216a are configured to connect the diffusion node region FD and the drain terminal of the drive transistors Dx and 212, and the second transistors M2 and 216b are connected to the second power source VDD2, 202b is configured to connect the gate terminals of the drive transistors Dx and 212, and the third transistor M3 is configured to connect the first power sources VDD1 and 202a to the drain terminals of the drive transistors Dx and 212. . The first transistors M1 and 216a, the second transistors M2 and 216b, and the third transistors M3 and 216c operate as switching transistors.

Afterwards, when light enters to the maximum, signal electrons having a 1.0 V capacitance are generated in the photodiodes PD and 204, and are transferred to the diffusion node regions FD and 208 through the transfer transistors Tx and 206. The voltage at FD 208 decreases to approximately 1.9V. At this time, the third transistors M3 and 216c are turned on, and the first transistors M1 and 216a and the second transistors M2 and 216b are turned off.

Here, the voltages of the diffusion node regions FD and 208 are applied to the gate terminals of the drive transistors Dx and 212, and are reduced by the threshold voltage Vth so that the second output voltage Vout2 is output at approximately 1.4V. .

Accordingly, the dynamic range is the difference between the first output voltage Vout1 and the second output voltage Vout2 (that is, Vout1-Vout2 = 2.9 V-1.4 V), which is approximately 1.5 V and is 0.5 V. Is increased. Here, since the second power sources VDD2 and 202b, the second transistors M2 and 216b, and the third transistors M3 and 216c are commonly used in connection with each pixel, the B region indicated by a dotted line (that is, , Pixel size) is not affected, and only one first transistor M1, 216a is increased in the pixel.

Meanwhile, although the number of transistors per pixel is increased by one, this may reduce the number of transistors per unit pixel when a plurality of pixels are bundled and shared. Here, when the pixels of the image sensor are configured in a common manner, a plurality of photodiodes PD and a plurality of transfer transistors Tx may be connected and applied to one diffusion node region FD.

Next, through the pixel structure having the above-described configuration, when there is no light, the first transistor and the second transistor are turned on, the third transistor is turned off to apply the second power to the gate terminal of the drive transistor, and diffused. The voltage of the node region is applied to the drain terminal of the drive transistor to output the first output voltage. When the light comes in at maximum, the first transistor and the second transistor are turned off, and the third transistor is turned on to reduce the voltage of the diffusion node region. A process of outputting the second output voltage by applying to the gate terminal of the drive transistor will be described.

3 is a flowchart illustrating a process of outputting an output voltage to increase a dynamic range by adding a switching transistor to a pixel of an image sensor according to an exemplary embodiment of the present invention.

Referring to FIG. 3, when the pixel does not emit light when the first power sources VDD1 and 202a are supplied with 3.0 V and the second power sources VDD2 and 202b are supplied with 3.5 V, the first transistors M1 and 216a which are switching transistors. And the second transistors M2 and 216b are turned on (step 302). Here, the first transistors M1 and 216a are configured to connect the diffusion node region FD and the drain terminal of the drive transistors Dx and 212, and the second transistors M2 and 216b are connected to the second power source VDD2, 202b and the gate terminal of the drive transistors Dx and 212.

At the same time, the third transistors M3 and 216c, which are switching transistors configured to connect the first power supplies VDD1 and 202a and the drain terminals of the drive transistors Dx and 212, are turned off (step 304).

Accordingly, the voltage of the diffusion node regions FD and 208 of 2.9 V is applied to the drain terminals of the drive transistors Dx and 212, and 3.5 V as the second power sources VDD2 and 202b is the drive transistors Dx and 212. Is applied to the gate terminal of the drive transistors Dx and 212, and all of the 2.9 Vs of the drain terminals of the drive transistors Dx and 212 are transferred to the source terminal and output as the first output voltage V _out 1 (step 306).

On the other hand, when the pixel emits maximum light, the first transistors M1 and 216a which are switching transistors and the second transistors M2 and 216b are turned off and the third transistors M3 and 216c which are switching transistors are turned on ( Steps 308 and 310).

Accordingly, the voltages of the diffusion node regions FD and 208 are applied to the gate terminals of the drive transistors Dx and 212, and are reduced by the threshold voltage Vth to output the second output voltage Vout2 at approximately 1.4 V. (Step 312).

Accordingly, the dynamic range can be increased by selectively applying the voltages of the first power source or the second power source and the diffusion node region to the gate terminal and the drain terminal using the switching transistor to output respective output voltages.

Meanwhile, in the above-described exemplary embodiment of the present invention, only the third transistor is included per unit pixel indicated by the region B as illustrated in FIG. 2, but the third transistor is included in the unit pixel. The number of transistors per sugar can be further reduced.

In the above description, the present invention has been described with reference to preferred embodiments, but the present invention is not necessarily limited thereto, and a person having ordinary skill in the art to which the present invention pertains may have various limitations without departing from the technical spirit of the present invention. It will be readily appreciated that various substitutions, modifications and variations are possible.

1 is a diagram illustrating a 4-transistor pixel structure of a CMOS image sensor according to the related art;

2 illustrates a 4-transistor pixel structure of a CMOS image sensor according to an embodiment of the present invention;

3 is a flowchart illustrating a process of outputting an output voltage to increase a dynamic range by adding a switching transistor to a pixel of an image sensor according to an exemplary embodiment of the present invention.

Claims (11)

delete An image sensor having a pixel structure including a photodiode (PD), a transfer transistor (Tx), a diffusion node region (FD), a reset transistor (Rx), a drive transistor (Dx), and a selective transistor (Sx). Selective switching by connecting the first switching transistor to be selectively switched by connecting the diffusion node region (FD) and the drain terminal of the drive transistor (Dx), and the second power supply (VDD2) and the gate terminal of the drive transistor (Dx) A second switching transistor, and a third switching transistor which is selectively switched by connecting the first power supply VDD1 and the drain terminal of the drive transistor Dx, And when the pixel does not emit light, the image sensor turns on the first switching transistor and the second switching transistor, and turns off the third switching transistor to output a first output voltage. The method of claim 2, And the image sensor outputs a second output voltage by turning on the third switching transistor and turning off the first and second switching transistors when the pixel emits light. The method of claim 3, wherein And said image sensor comprises said first switching transistor inside each pixel and said second switching transistor and said third switching transistor in a common manner. The method of claim 3, wherein And the image sensor comprises the first switching transistor, the second switching transistor and the third switching transistor in a common manner. delete Driving an image sensor having a pixel structure including a photodiode PD, a transfer transistor Tx, a diffusion node region FD, a reset transistor Rx, a drive transistor Dx, and a selective transistor Sx. As a method, Configuring a first switching transistor, a second switching transistor, and a third switching transistor in the pixel structure; Supplying a first power source VDD1 and a second power source VDD2 to the pixel structure including the first switching transistor, the second switching transistor, and the third switching transistor; When the corresponding pixel having the pixel structure does not emit light, turning on the first switching transistor and the second switching transistor, and turning off the third switching transistor to output a first output voltage; Outputting a second output voltage by turning on the third switching transistor and turning off the first switching transistor and the second switching transistor at the time of maximum light emission of the pixel; Driving method of the image sensor comprising a. The method of claim 7, wherein The driving method includes the first switching transistor connecting the diffusion node region FD and the drain terminal of the drive transistor Dx, the gate terminal of the second power supply VDD2 and the drive transistor Dx. And selectively switching using the second switching transistor to connect and the third switching transistor connecting the first power supply VDD1 and the drain terminal of the drive transistor Dx. Way. The method of claim 8, The driving method includes the first switching transistor configured inside each pixel, and the second switching transistor and the third switching transistor configured in a common manner to be selectively switched. The method of claim 8, And the driving method is configured to selectively switch the first switching transistor, the second switching transistor, and the third switching transistor in a common manner. delete

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