KR20110139477A - Image sensor - Google Patents

Abstract

PURPOSE: An image sensor is provided to increase the sensitivity of a lower luminance. CONSTITUTION: A first transmission switch(108) transmits the stored photocharge of PD(Photo Diode) to a first floating diffusion node. According to the second transmission signal, a second transmission switch(110) transmits the photocharge which is stored in the PD to a second floating diffusion node. According to the sensitivity signal, a sensitivity control switch(100) selectively controls the connection between the first floating diffusion node and the second floating diffusion node. A driving transistor(104) converts an electrical charge which is sanctioned to the second diffusion node into an electrical signal.

Description

Image sensor {IMAGE SENSOR}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor, and more particularly to a complementary metal-oxide-semiconductor (CMOS) image sensor for converting an external optical image signal into an electrical image signal. Technology.

In general, an image sensor is a device that converts an external optical image signal into an electrical image signal. Image sensors are largely divided into CMOS image sensors using complementary-MOS (CMOS) technology and CCD image sensors using Charge Coupled Device (CCD) technology, all of which are manufactured using semiconductor technology.

In particular, CMOS image sensors are image sensors fabricated using CMOS fabrication techniques. Each pixel in the CMOS image sensor converts the light signal radiated from the corresponding part of the subject into electrons using a photodiode and stores it, and then converts the amount of charge that appears in proportion to the number of accumulated electrons into a voltage signal and outputs the voltage signal. Use

Such a CMOS image sensor is a device widely used in various electronic products, for example, a mobile phone, a camera for a personal computer (PC), a video camera, a digital camera, and the like.

The CMOS image sensor is simpler to drive than the CCD used as an image sensor, and it is possible to integrate a signal processing circuit into one chip so that a system on chip (SoC) can be used to make the module smaller. Let's do it. In addition, since the conventional set-up (CMOS) technology can be used interchangeably, there are many advantages such as lowering the manufacturing cost, so the demand is increasing rapidly.

1 is a circuit diagram illustrating four transistor unit pixels used in a general image sensor.

The unit pixel includes a photodiode 2, a transfer transistor 4, a floating diffusion node 6, a reset transistor 8, a driving transistor 10, and a selection transistor 12. This conventional unit pixel includes four transistors 4, 8, 10, and 12 and one photodiode PD.

Here, the photodiode 2 generates an optical charge corresponding to the optical image of the subject. The transfer transistor 4 transfers the optical charge stored in the photodiode 2 to the floating diffusion node 6 in accordance with the transmission signal TX.

In addition, the reset transistor 8 emits the photocharge stored in the photodiode 2 and the floating diffusion node 6 through the power supply voltage terminal VDD in response to the reset signal RX, thereby floating the node 6 and the photodiode 2. Reset

In addition, the driving transistor 10 serves as a source follower for amplifying a current corresponding to the optical charge stored in the floating diffusion node 6. The select transistor 12 selects a row line of the pixel array in accordance with the select signal LX.

In this conventional image sensor, the floating diffusion node 6 is used as a charge sensing node in order to sense a signal charge amount.

On the other hand, the frequency of the fluorescent lamp is set to 50 Hz or 60 Hz depending on the country. When fluorescent lights are turned on in an indoor environment, a situation is created in which 100 watts or 120 watts of AC light are illuminated. As a result, even when the same exposure time is applied, an image having an inconsistent brightness is obtained as shown in FIG.

In order to solve this problem, if the pixel is reset to the same exposure amount as the fluorescent lamp cycle, flicker less may obtain an image as shown in FIG. 3.

By the way, even if the exposure period is set to the same exposure amount as that of the fluorescent lamp, if the photodiode is very large and the sensitivity is very good, the images all become saturated. As a result, as shown in FIG. 4, only a saturated image remains, and thus a dynamic range is hardly obtained.

In order to solve this problem, it is possible to arbitrarily reduce the sensitivity to obtain an image as shown in FIG. 3 under fluorescent lighting, but when the sensitivity is small, the low light characteristic has a fatal disadvantage.

The present invention includes a dual transmission switch and a dual floating diffusion node to improve sensitivity at low light to improve low light characteristics and flicker less range at high light. The purpose is to increase the range so that a wide dynamic range can be obtained.

An image sensor of the present invention for achieving the above object, the photodiode for storing the optical charge corresponding to the optical image of the subject; A first transfer switch transferring the optical charge stored in the photodiode to the first floating diffusion node according to the first transfer signal; A second transfer switch transferring the optical charge stored in the photodiode to the second floating diffusion node according to the second transfer signal; A sensitivity adjustment switch for selectively controlling a connection between the first floating diffusion node and the second floating diffusion node according to the transmission signal; A driving transistor converting a charge applied to the second floating diffusion node into an electrical signal and outputting the electrical signal; A first capacitor that stores a charge of the first floating diffusion node; And a second capacitor that stores the charge of the second floating diffusion node.

The present invention has the following effects.

First, it includes a dual transfer switch and a dual floating diffusion node to increase sensitivity at low light to improve low light characteristics, and to reduce sensitivity at high light to reduce the flicker less range for large pixel sizes. To be extended.

Second, by implementing a sensor that extends the flicker less range, a wide dynamic range can be obtained.

Third, the dual transmission switch provides an effect of improving transmission efficiency.

It will be apparent to those skilled in the art that various modifications, additions, and substitutions are possible, and that various modifications, additions and substitutions are possible, within the spirit and scope of the appended claims. As shown in Fig.

1 is a circuit diagram illustrating a unit pixel of a general image sensor.
2 to 4 are diagrams for explaining a problem of a general image sensor.
5 is a circuit diagram illustrating a unit pixel of an image sensor according to an exemplary embodiment of the present invention.
6 and 7 are views for explaining the operation of the image sensor in low light in the present invention.
8 to 10 are views for explaining the operation of the image sensor at high illumination in the present invention.
11 is a view for explaining the output of the pixel according to the incident light amount in the present invention.

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

5 is a circuit diagram of an image sensor according to the present invention.

The present invention provides a photodiode PD, a sensitivity adjustment switch 100, a reset transistor 102, a drive transistor 104, a select transistor 106, a first transfer switch 108, a second transfer switch 110 and a capacitor CFDL. And CFDH.

Here, the photodiode PD generates an optical charge corresponding to the optical image of the subject. The photodiode PD is connected in a forward direction between the ground voltage terminal and node A.

The sensitivity adjustment switch 100 may adjust sensitivity by separating or combining the capacitors CFDL and CFDH according to the sensitivity adjustment signal DFDG. The sensitivity adjustment switch 100 is connected between the floating diffusion node FD2 and the floating diffusion node FD1 to apply the sensitivity adjustment signal DFDG through the gate terminal.

The reset transistor 102 emits the photocharge stored in the photodiode PD and the photocharges applied to the two floating diffusion nodes FD1 and FD2 through the power supply voltage terminal VDD according to the reset signal RX. Reset FD1, FD2. The reset transistor 102 is connected between the power supply voltage VDD applying terminal and the floating diffusion node FD2 to apply the reset signal RX through the gate terminal.

In addition, the driving transistor 104 serves as a source follower for amplifying a current corresponding to an optical charge stored in the floating diffusion node. The light transistor is converted into an electrical signal by the driving transistor 104. The driving transistor 104 is connected between the power supply voltage terminal VDD and the selection transistor 106 so that the gate terminal is connected to the floating diffusion node FD2.

The select transistor 106 selects the row line of the pixel array in accordance with the select signal LX. The select transistor 106 is connected between the driving transistor 104 and the pixel output terminal and receives the select signal LX through the gate terminal.

The first transfer switch 108 is connected between the node A and the photodiode PD and the floating diffusion node FD1 to which the first transmit signal TX_LS is applied. The second transmission switch 110 is connected between the node A and the photodiode PD and the floating diffusion node FD2 to receive the second transmission signal TX_HS through the gate terminal. The two first transfer switches 108 and the second transfer switch 110 are connected to the photodiode PD.

The capacitor CFDH is connected between the floating diffusion node FD2 and the ground voltage terminal. Here, the capacitor CFDH is preferably designed to have the smallest possible capacitance within the range allowed by the process. The capacitor CFDL is connected between the floating diffusion node FD1 and the ground voltage terminal. Here, the capacitor CFDL is preferably designed by setting a design target value according to the pixel size. Usually, the value of the capacitor CFDL is designed to be larger than the capacitor CFDH.

In the image sensor of the present invention, two floating diffusion nodes FD1 and FD2 are used as charge sensing nodes to sense the amount of signal charge.

And two first transfer switches 108 and a second transfer switch 110 for transferring the photo charge of the photodiode PD to the floating diffusion nodes FD1 and FD2, respectively.

Here, the two first transfer switch 108 and the second transfer switch 110 may be implemented as NMOS transistors, and the size of the transistors may be designed identically or differently. However, in order to set the conversion efficiency of the optical charge differently, it is preferable to design the transistors differently.

For example, the second transfer switch 110 is designed to be smaller in size than the first transfer switch 108. Accordingly, the conversion efficiency can be further increased by reducing the overlap capacitance of the gate terminal of the floating diffusion node FD2 and the second transfer switch 110 to reduce the total input capacitance of the driving transistor 104.

Likewise, the capacitors CFDH and CFDL are included to set the conversion efficiency of the floating diffusion nodes FD1 and FD2 differently. These capacitors CFDH and CFDL are divided by the sensitivity adjustment switch 100 to distinguish the capacitances of the floating diffusion nodes FD1 and FD2.

In general, high dynamic range and high sensitivity are essential for surveillance and automotive sensors, which require large pixel sizes. However, since the sensitivity is increased when the pixel size is large, flicker occurs due to the difference in the amount of incident light at high illumination with an AC light source such as a fluorescent lamp. To solve this problem, the conversion gain is designed to be lowered arbitrarily.

Here, flicker is a phenomenon in which the intensity of light from the screen is not constant and changes periodically with time, so that flickering of the light is felt by the user. It is a changing phenomenon.

However, in this case, the sensitivity is low at low light, and thus the most important low light property in the security market cannot be obtained. In order to solve this problem, in the present invention, the flickerless range can be secured by reducing the sensitivity at high illumination, and the low illumination characteristic can be secured by increasing the sensitivity at low illumination. As a result, the present invention has the effect of ensuring the flickerless range and at the same time improve the low light characteristics.

Referring to the operation of the present invention having such a configuration in detail as follows.

6 illustrates a high sensitivity data sensing operation of the image sensor at low illumination.

First, in the low illuminance state, the photocharge of the photodiode PD becomes a low level as in (B). Then, the sensitivity adjustment signal DFDG is maintained at a low level. Accordingly, the signal is read through the photodiode PD while the sensitivity adjustment switch 100 is turned off.

When the sensitivity adjustment switch 100 is turned off, the connection between the floating diffusion node FD1 and the floating diffusion node FD2 is disconnected from each other. When the second transmission signal TX_HS is at the high level, the second transmission switch 110 is turned on as in (C). Accordingly, the photocharge stored in the photodiode PD is transferred to the floating diffusion node FD2.

At this time, the reset signal RX is at a low level so that the reset transistor 102 maintains a turn off state. Likewise, another first transfer switch 108 must remain turned off.

In this case, as in (D), the optical charge stored in the photodiode PD is stored in the capacitor CFDH having a relatively small size through the floating diffusion node FD2. Therefore, since the capacitor CFDL of the floating diffusion node FD1 is disconnected by the sensitivity adjustment switch 100, only the capacitor CFDH connected to the floating diffusion node FD2 contributes to the conversion efficiency, thereby obtaining a high conversion gain.

Here, the equation for converting the signal charge at the floating diffusion node FD2 into an electrical signal through the driving transistor 104 may be defined as follows.

[Equation 1]

V = Q / C _FDH

Where V is the voltage output from the driving transistor 104, Q is the signal charge generated by light, and C _FDH is the total capacitance of the floating diffusion node FD2.

In this case, the total capacitance C _FDH of the floating diffusion node FD2 is the junction capacitance of the floating diffusion node FD2, the overlap capacitance of the second transfer switch 110 and the floating diffusion node FD2, the reset transistor 102 and the floating diffusion. It includes both the overlap capacitance of node FD2 and the parasitic capacitance associated with the gate terminal of drive transistor 104.

Setting the Conversion Efficiency of Optical Charge Differently The second transfer switch 110 is designed to be smaller in size than the first transfer switch 108. Accordingly, the total capacitance of the floating diffusion node FD2 connected to the capacitor CFDH can be set small.

Therefore, in the low light state, only the capacitor CFDH operates to operate in a high sensitivity mode, thereby improving low light characteristics.

7 is an operation timing diagram of an image sensor at low light.

The pixel driving of the CMOS image sensor largely performs a pre-reset operation and a correlated double sampling (CDS) and analog digital converting (ADC) operation for proper post-exposure read out. Here, the pre-reset indicates a start point of exposure, and is a step of resetting all the signal charges acquired in the previous operation.

At this time, since the capacitors CFDH, CFDL, and the photodiode PD must all be reset, the first transfer switch 108, the second transfer switch 110, the sensitivity adjustment switch 100, and the reset transistor 102 must all be turned on. do. Accordingly, the first transmission signal TX_LS, the second transmission signal TX_HS, the sensitivity adjustment signal DFDG, and the reset signal RX are activated to a high level. On the other hand, the select signal LX is inactivated to a low level so that the select transistor 106 remains turned off.

After the pre-reset, the first transfer switch 108 and the second transfer switch 110 are turned off, and the photodiode PD receives light by an exposure time. The photodiode PD generates electrons as much as light and stores them in the photodiode PD. Thereafter, the stored signal charges perform CDS and ADC operations. The selection signal LX for selecting a corresponding row is activated to a high level, and the selection transistor 106 is turned on.

Subsequently, upon entering the reset reading step, the reset signal RX is inactivated to a low level so that the reset transistor 102 is turned off.

Subsequently, the reset level is read and the second transmission signal TX_HS is activated to a high level so that only the second transmission switch 110 is turned on to read the signal charge. At this time, while performing the CDS operation, the sensitivity adjustment signal DFDG and the first transmission signal TX_LS are set to the low level to turn off the sensitivity adjustment switch 100 and the first transmission switch 108.

Thereafter, when the CDS operation ends, the selection signal LX is deactivated to a low level so that the selection switch 106 is turned off.

Accordingly, in the low light state, only the capacitor CFDH operates to operate in the high sensitivity mode, thereby improving low light characteristics.

Subsequently, an analog signal is converted into a digital signal through an analog to digital conversion (ADC) operation to obtain an image signal.

8 illustrates low-sensitivity data sensing operation of the image sensor at high illumination.

First, in the case of a high illuminance state, the photocharge of the photodiode PD becomes a high level as in (E). Then, the sensitivity adjustment signal DFDG is in a high level state. Accordingly, the signal is read through the photodiode PD while the sensitivity adjustment switch 100 is turned on.

When the sensitivity adjustment switch 100 is turned on, the floating diffusion node FD1 and the floating diffusion node FD2 are connected to each other. When the first transmission signal TX_LS becomes high, the first transmission switch 108 is turned on as in (F). Accordingly, the photocharge stored in the photodiode PD is transferred to the floating diffusion nodes FD1 and FD2. At this time, the reset signal RX is at a low level so that the reset transistor 102 maintains a turn off state.

In this case, as in (G), the optical charge stored in the photodiode PD is simultaneously stored in the capacitors CFDL and CFDH through the floating diffusion nodes FD1 and FD2. Therefore, the capacitance of the floating diffusion nodes FD1 and FD2 becomes the capacitance of the capacitor CFDL + the capacitance of the capacitor CFDH, so that the value increases. Accordingly, in the high illuminance state, the total capacitance values of the floating diffusion nodes FD1 and FD2 are increased, so that the conversion gain is smaller than that of the high sensitivity sensing mode.

Here, since the sensitivity in the high illuminance state is inversely proportional to the size of the capacitor CFDL, it should be appropriately designed according to the size of the pixel.

Equation for converting the signal charges at the floating diffusion nodes FD1 and FD2 into the electrical signals through the driving transistor 104 may be defined as follows.

[Equation 2]

V = Q / (C _FDL + C _FDH )

Here, V is the voltage output from the driving transistor 104, Q is the signal charge generated by the light, C _FDL is the total capacitance of the floating diffusion node FD1, C _FDH is the total capacitance of the floating diffusion node FD2.

In this case, the total capacitance of the floating diffusion nodes FD1 and FD2 may be expressed as the sum of the capacitance C _FDL of the floating diffusion node FD1 and the capacitance C _FDH of the floating diffusion node FD2.

That is, the total capacitance of the floating diffusion node FD2 is the junction capacitance of the floating diffusion node FD2, the overlap capacitance of the second transfer switch 110 and the floating diffusion node FD2, the reset transistor 102 and the floating diffusion node FD2. And the parasitic capacitance associated with the gate terminal of the driving transistor 104.

The total capacitance of the floating spreading node FD1 includes the junction capacitance of the floating spreading node FD1, and the overlap capacitance between the first transmission switch 108 and the floating spreading node FD1.

Setting the Conversion Efficiency of Optical Charge Differently The second transfer switch 110 is designed to be smaller in size than the first transfer switch 108. Accordingly, the total capacitance of the floating diffusion node FD2 connected to the capacitor CFDH can be set small.

In the present invention, the capacitors CFDH and CFDL operate in the high illuminance state, thereby operating in the low sensitivity mode. When operating in the low sensitivity mode, the floating diffusion nodes FD1 and FD2 do not saturate because the sensitivity is small even during the minimum flicker period exposure time. Therefore, the present invention enables to secure a dynamic range in the low sensitivity mode.

9 is an operation timing diagram of an image sensor at high illumination. FIG. 9 is an operation timing diagram illustrating a case where only the first transfer switch 108 is turned on at high illuminance. The general description is similar to the operation timing diagram in the low illuminance state of FIG. 7, and only the portions in which the operation differs in the high illuminance will be described in detail.

First, when the CDS operation starts, the select signal LX is activated to a high level before the signal is read, and the select transistor 106 is turned on. Then, the sensitivity adjustment signal DFDG is activated to a high level so that the sensitivity adjustment switch 100 is turned on.

Thereafter, when the first transmission signal TX_LS is activated to a high level to perform the CDS operation, only the first transmission switch 108 of the first transmission switch 108 and the second transmission switch 110 is turned on to take up the signal charge. Lead.

At this time, while performing the CDS operation, the sensitivity adjustment signal DFDG and the first transmission signal TX_LS are turned to a high level to turn on the sensitivity adjustment switch 100 and the first transmission switch 108.

Thereafter, when the CDS operation ends, the selection signal LX is deactivated to a low level so that the selection switch 106 is turned off. Next, the first transmission signal TX_LS is deactivated to a low level so that the first transmission switch 108 is turned off.

Subsequently, an analog signal is converted into a digital signal through an analog to digital conversion (ADC) operation to obtain an image signal. After the ADC operation is completed, the sensitivity adjustment signal DFDG is activated again to a high level, so that the sensitivity adjustment switch 100 is turned on.

10 is another embodiment illustrating an operation timing diagram of an image sensor at high illumination. FIG. 10 is an operation timing diagram illustrating a case in which both the first transmission switch 108 and the second transmission switch 110 are turned on at high illumination. The general description is similar to the operation timing diagram in the high illuminance state of FIG. 9, and only the portions where the operation differs in the high illuminance will be described in the embodiment of FIG. 10.

First, when the CDS operation starts, the select signal LX is activated to a high level before the signal is read, and the select transistor 106 is turned on. Then, the sensitivity adjustment signal DFDG is activated to a high level so that the sensitivity adjustment switch 100 is turned on.

Subsequently, when both the first transmission signal TX_LS and the second transmission signal TX_HS are activated to a high level to perform the CDS operation, the first transmission switch 108 and the second transmission switch 110 are both turned on to take up the signal charge. Lead.

At this time, during the CDS operation, the sensitivity adjustment signal DFDG, the first transmission signal TX_LS, and the second transmission signal TX_HS are at a high level so that the sensitivity adjustment switch 100, the first transmission switch 108, and the second transmission switch ( Turn all 110) on.

FIG. 11 is a diagram illustrating a pixel output (PIXEL OUTPUT) in each mode according to the amount of incident light to which the photodiode PD is exposed.

First, (H) represents a low illuminance state in which the incident light amount is low. In the low illumination high sensitivity mode, the sensitivity adjustment signal DFDG is deactivated to a low level so that the sensitivity adjustment switch 100 is turned off. In this case, the output value of the pixel, that is, the value of the above-described voltage V becomes high.

That is, in low light, the exposure time is long and the low light characteristics can be improved by operating in the high sensitivity mode.

On the other hand, (I) represents a high illuminance state in which the incident light amount is high. In the high illuminance low sensitivity mode, the sensitivity adjustment signal DFDG is activated to a high level so that the sensitivity adjustment switch 100 is turned on. In this case, the output value of the pixel, that is, the value of the above-described voltage V becomes relatively low.

That is, at high light intensity, it operates in a low sensitivity mode, thereby broadening the flicker less range, thereby ensuring the overall dynamic range.

Then, when the incident light amount is at a level between the low light condition and the high light condition as in (J), the output of the pixel is at a constant voltage level. Therefore, in the present invention, the flicker less range can be secured widely so that the dynamic range can be extended as in (J).

Claims (16)

A photo diode for storing an optical charge corresponding to the optical image of the subject;
A first transfer switch transferring the optical charge stored in the photodiode to a first floating diffusion node according to a first transfer signal;
A second transfer switch transferring the optical charge stored in the photodiode to a second floating diffusion node according to a second transfer signal;
A sensitivity adjustment switch for selectively controlling a connection between the first floating diffusion node and the second floating diffusion node according to a sensitivity adjustment signal;
A driving transistor for converting a charge applied to the second floating diffusion node into an electrical signal and outputting the electrical signal;
A first capacitor for storing charge of the first floating diffusion node; And
And a second capacitor for storing charge of the second floating diffusion node. The method of claim 1,
A reset transistor for resetting the second floating diffusion node according to a reset signal; And
And a selection transistor connected to an output terminal of the driving transistor to select a row line of the pixel array according to a selection signal. The image sensor of claim 1, wherein the sensitivity adjustment signal is inactivated when the photodiode is in a low light state, and the sensitivity adjustment switch is turned off. The method of claim 1, wherein when the photodiode is in a low light state, the second transmission signal is activated and the second transmission switch is turned on to store the charge of the second floating diffusion node in the second capacitor. Image sensor, characterized in that. The image sensor according to claim 1 or 3, wherein the total capacitance of the second floating diffusion node is set smaller than the first capacitor by the second capacitor when the photodiode is in a low light condition. The method of claim 1, wherein when the photodiode is in a high illumination state, the sensitivity adjustment signal is activated to turn on the sensitivity adjustment switch so that the first floating diffusion node and the second floating diffusion node are connected to each other. Image sensor. The method of claim 6, wherein when the photodiode is in a high illumination state, the first transmission signal is activated, the second transmission signal is deactivated, the first transmission switch is turned on, and the second transmission switch is turned off. And the charge of the first floating diffusion node is simultaneously stored in the first capacitor and the second capacitor. The method of claim 7, wherein the output voltage (V) of the driving transistor is Q / (C1 + C2), wherein Q is the charge of the first floating diffusion node, the second floating diffusion node, C1 is the first floating The capacitance of the spreading node, C2, represents the capacitance of the second floating spreading node). 7. The image sensor of claim 6, wherein the total capacitance of the second floating diffusion node is set to be larger than the second capacitor by the first capacitor when the photodiode is in a high illuminance state. The first floating diffusion node of claim 6, wherein when the photodiode is in a high illumination state, the first transmission signal and the second transmission signal are activated, and the first transmission switch and the second transmission switch are turned on. And the charge of the second floating diffusion node is simultaneously stored in the first capacitor and the second capacitor. The image sensor of claim 1, wherein the first transfer switch and the second transfer switch have different transistor sizes. 12. The image sensor of claim 1 or 11, wherein the second transfer switch has a smaller transistor size than the first transfer switch. The image sensor of claim 1, wherein the first capacitor and the second capacitor have different capacitances. The image sensor of claim 1, wherein the second capacitor has a smaller capacitance value than the first capacitor. The image sensor of claim 1, wherein the first transfer switch, the second transfer switch, and the sensitivity adjustment switch are all turned on during the pre-reset operation to reset all the signal charges acquired in the previous operation. The method of claim 1, wherein in the low light condition, the first capacitor and the second capacitor are operated in a high sensitivity mode to operate only the second capacitor with less capacitance than the first capacitor, and in a high light condition to operate in the low sensitivity mode. Image sensor, characterized in that simultaneously operating.

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