KR20140004944A - Cmos image sensor and operation method thereof - Google Patents

Abstract

A CMOS image sensor and an operating method thereof are disclosed. The CMOS image sensor operating method according to the present invention is related to a method for adjusting a gate voltage of a transfer transistor which transfers accumulated charge by absorbing light using a light detecting device. According to an embodiment of the present invention, while the light detecting device is accumulating charge by absorbing light, the gate voltage of the transfer transistor can be a first voltage and a second voltage, and while the light detecting device finishes absorbing light and transfers the accumulated charge, the gate voltage of the transfer transistor can be a third voltage. When the third voltage is applied to the gate of the transfer transistor, the transfer transistor makes the light detecting device depleted incompletely, the light detecting device can hold photoelectrons according to the threshold voltage of the transfer transistor. As the first voltage and the third voltage are applied to the gate of the transfer transistor, the influence of the threshold voltage of the transfer transistor on the amount of photoelectrons can be canceled out. [Reference numerals] (AA) Voltage; (BB,CC) Time; (DD) T_INT1 section; (EE) T_INT2 section; (FF) T_RD section

Description

[0001] CMOS image sensor and operation method thereof [0002]

The present invention relates to a CMOS image sensor and a method of operating the CMOS image sensor, and more particularly, to a CMOS image sensor and a method of operating the CMOS image sensor for correcting the threshold voltage distribution of the transistor included in the CMOS image sensor.

Generally, an image sensor is an electronic device that converts an image into an electrical signal. Image sensors are typically CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) image sensors. CMOS image sensors are fabricated based on CMOS LSI (large scale integration) fabrication processes, while CCD image sensors are fabricated based on specially developed fabrication processes. Because of this, CMOS image sensors can integrate various functional circuits together to better perform the performance of CCD as well as the performance of general CMOS image sensor.

CMOS image sensors are used in cameras mounted in automobiles, security devices, and robots, as well as consumer electronics such as digital cameras, cell phone cameras and portable camcorders. Many of these products require better performance CMOS image sensors, one of which is a wide dynamic range.

Dynamic range is one of the performance factors of a CMOS image sensor and refers to a brightness range of an image that the CMOS image sensor can acquire without loss. Various methods have been proposed to extend the dynamic range of such CMOS image sensors. As one of them, there is a double sampling image sensor which uses a photosensitive device or a floating diffusion (FD) inside a pixel as a charge storage, so that a separate memory is unnecessary. Such a double sampling image sensor has a problem in that true-CDS (correlated double sampling) is not possible, a large fixed pattern noise (FPN) occurs, or a burden of sampling times exist, and there is a need for improvement.

The present invention relates to a CMOS image sensor and a method of operating the same, and more particularly, to provide a CMOS image sensor and a method of operating the same, which reduce the FPN of a dual-capture image sensor improved from a double sampling image sensor. It is done.

The present invention relates to a CMOS image sensor and a method of operating the same, and more particularly, to provide a CMOS image sensor and a method of operating the same, which reduce the FPN of a dual-capture image sensor improved from a double sampling image sensor. It is done.

In order to achieve the above object, in the method of operating a complementary meta-oxide semiconductor (CMOS) image sensor according to an embodiment of the present invention, the CMOS image sensor is a photosensitive device and the photosensitive device and the floating diffusion ( and a transfer transistor configured to transfer or block charges between floating diffusions, and sequentially accumulating charges in the photosensitive device by sequentially applying first and second voltages having different levels to gates of the transfer transistors. Transferring a portion of the charge accumulated in the photosensitive device to the floating diffusion so as to incomplete depletion by applying a third voltage to the gate of the transfer transistor; and sensing a potential of the floating diffusion. It features.

On the other hand, the CMOS image sensor according to an embodiment of the present invention is a transfer transistor for transferring or blocking the charge between the photosensitive device, the photosensitive device and the floating diffusion and a different level while the photosensitive device accumulates the charge. Outputting first and second voltages to the gate of the transfer transistor, and transferring a third voltage to the floating diffusion so that some of the charge accumulated in the photosensitive device is transferred to the floating diffusion so that the photosensitive device is incompletely depleted. And a control unit for outputting the gate.

According to the CMOS image sensor and its operation method as described above, it is possible to reduce the FPN by changing the operation method of the photosensitive device, without changing the conventional pixel structure.

1 is a block diagram showing the structure of a CMOS image sensor according to an embodiment of the present invention.
2 is a diagram illustrating an embodiment of a pixel circuit according to an embodiment of the present invention.
FIG. 3 is a diagram illustrating an embodiment of extending a dynamic range in the pixel circuit of FIG. 2.
4 is a graph illustrating a characteristic change of a pixel circuit according to an exemplary embodiment of FIG. 3.
5 is a diagram illustrating a relationship between light intensity and output voltage of a pixel circuit according to a threshold voltage distribution of the transfer transistor of FIG. 2.
6 is a diagram illustrating a method of driving the pixel circuit of FIG. 2 according to an exemplary embodiment of the present invention.
7 is a diagram illustrating operating characteristics of a pixel circuit according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings without intending to intend to provide a thorough understanding of the present invention to a person having ordinary skill in the art to which the present invention belongs.

1 is a block diagram showing the structure of a CMOS image sensor according to an embodiment of the present invention. The CMOS image sensor 100 may include a pixel array 1000, a controller 2000, a row driver 3000, and a read circuit 4000. The pixel array 1000 includes a plurality of pixels. A pixel refers to a unit in which a CMOS image sensor recognizes and stores an image, and each pixel includes a pixel circuit. The pixel circuit may include a photo sensing element and one or more transistors. The photo-sensing device can absorb light and store the charge, and the accumulated charge can be output to the outside through the transistor. The photo-sensing device may be a photodiode, a photogate, a phototransistor, or the like. Hereinafter, it is assumed that the photo-sensing device is a photodiode. The photodiode can sense light in each pixel circuit and convert the sensed light into an electrical signal. In particular, photodiodes can accumulate electrons in response to absorption of light. The quality of the image, which is the result of a CMOS image sensor, can be influenced by the performance of such a pixel circuit.

The control unit 2000 outputs a control signal for the pixel circuits and controls the operation of the CMOS image sensor 100. [ The row driver 3000 may output a signal for selecting a row including a plurality of pixel circuits belonging to the pixel array 1000 and the read circuit 4000 may output signals for selecting pixels included in the row selected by the row driver 3000 The circuits can receive the converted electrical signals. The electrical signal output by each pixel circuit may be an analog signal, and the read circuit may convert the analog signal to a digital signal through an analog-to-digital converter (ADC). The read circuit 4000 may sequentially output digital signals corresponding to the plurality of pixel circuits included in one row.

2 is a diagram illustrating an embodiment of a pixel circuit according to an embodiment of the present invention. Each pixel circuit included in a pixel array of a CMOS image sensor may include a component for amplifying an electrical signal converted by a photodiode. Such a pixel circuit is called an active pixel sensor (APS). For example, as illustrated in FIG. 2, one pixel may include a photodiode PD, a transfer transistor M_TG, a reset transistor M_RS, a source-follower transistor M_SF, and a selection transistor M_SEL. . Photodiode (PD) is a kind of photosensitive device that is characterized by the linear increase in the photocurrent (current) according to the intensity of the input light in the reverse bias state, the photodiode (PD) is exposed to light and electrically connected to the outside When floating, electrons can accumulate. As the electrons accumulate, the cathode voltage of the photodiode PD may decrease, and the intensity of light absorbed by the photodiode PD may be detected by measuring the reduced voltage. This accumulation of electrons can also be explained as the discharge process of the charged capacitor by the generated photocurrent.

The transfer transistor M_TG may connect or block the photodiode PD with the floating diffusion FD according to the gate voltage. While the photodiode PD accumulates electrons in response to light, a voltage is applied to the gate of the transfer transistor to turn off the transfer transistor to electrically block the photodiode PD and the floating diffusion. You can. When the photodiode PD finishes absorbing light, the transfer transistor may be turned on to output a voltage change caused by the electrons accumulated in the photodiode PD, and thus the photodiode The changed voltage at the cathode of PD) may be transferred to the floating diffusion FD.

Before the voltage of the photodiode PD is transferred to the floating diffusion FD, the floating diffusion FD is reset by the turn-on reset transistor M_RS. The reset voltage of the floating diffusion FD is amplified through the source-follower transistor M_SF and output to the outside when the selection transistor M_SEL is turned on. The read circuit receives an analog voltage corresponding to the reset voltage of the floating diffusion FD output to the outside.

When the output of the reset voltage of the floating diffusion FD is completed, the reset transistor M_RS is turned off and the transfer transistor M_TG is turned on, so that the voltage according to the electrons accumulated by the photodiode PD is floating. Delivered to diffusion (FD). Like the reset voltage of the floating diffusion FD, the changed voltage of the floating diffusion FD is output to the outside through the source-follower transistor M_SF and the selection transistor M_SEL. The read circuit receives the analog voltage corresponding to the voltage change of the floating diffusion FD output to the outside.

The read circuit receives the reset voltage of the floating diffusion FD and the voltage due to the photodiode PD and calculates the amount of light sensed by the photodiode PD through the difference of the positive voltage. This operation is called correlated double sampling (CDS), and the order of receiving the reset voltage and the voltage due to the photodiode PD may be changed. Also, in Fig. 2, the pixel circuit is shown as including an NMOS transistor, but this is illustrative, and the pixel circuit may comprise a PMOS transistor.

FIG. 3 is a diagram illustrating an embodiment of extending a dynamic range in the pixel circuit of FIG. 2. As mentioned earlier, various methods can be used to extend the dynamic range of the CMOS image sensor. For example, FIG. 2 illustrates a dual-capture method, in which a dynamic range of a CMOS image sensor can be extended by changing a voltage TX applied to a gate of a transfer transistor M_TG. The method is shown.

FIG. 3A illustrates a change in the gate voltage TX of the transfer transistor M_TG and the gate voltage RST of the reset transistor M_RS while the pixel circuit of FIG. 2 recognizes an image according to the dual-capture method. A graph representing. The voltage TX applied to the gate of the transfer transistor M_TG may be changed according to the double-capture method. For example, in FIG. 3A, the gate voltage TX of the transfer transistor M_TG is the power supply voltage VDD. ), An example of the first voltage V_TX1 and the second voltage V_TX2. The photodiode PD may accumulate electrons due to light absorbed for the T_INT1 and T_INT2 times, and the gate voltage TX of the transfer transistor M_TG is the first voltage V_TX1 and the first in the T_IN1 and T_INT2 sections, respectively. It may be two voltages V_TX2. The pixel circuit may output a voltage change according to accumulated electrons of the photodiode PD to the outside during the T_RD period.

3B illustrates a photodiode PD belonging to the pixel circuit of FIG. 2, a gate lower portion of the transfer transistor M_TG, a floating diffusion FD, and a gate lower portion of the reset transistor M_RS in the periods T_INT1, T_INT2, and T_RD. And the potential of the power supply voltage VDD, respectively. In the T_INT1 period, the gate voltage TX of the transfer transistor M_TG is a first voltage V_TX1, and the first voltage V_TX1 is lower than the power supply voltage VDD and higher than the second voltage V_TX2. Can be. Therefore, when strong light is absorbed by the photodiode PD in the T_INT1 section, as shown in FIG. 3B, the photodiode PD is driven by the transfer transistor M_TG to which the first voltage V_TX1 is applied. It is not possible to accumulate a certain amount of electrons (or charges). In other words, the electrons accumulated over a certain amount may move to the floating diffusion FD having a low potential through the transfer transistor M_TG, and may flow to the power supply voltage VDD node through the reset transistor M_RS.

When entering the T_INT2 period, the gate voltage TX of the transfer transistor M_TG may be changed to the second voltage V_TX2. The second voltage V_TX2 may have a lower value than the first voltage V_TX1 (eg, the second voltage V_TX2 may be a ground voltage), and thus the gate of the transfer transistor M_TG. When the first voltage V_TX1 is applied, the potential barrier under the gate of the transfer transistor M_TG may be increased. In the T_IN2 period, the photodiode PD may further accumulate electrons in response to the intensity of light absorbed as the height of the potential barrier under the gate of the transfer transistor M_TG increases. As shown in (a) of FIG. 3, since the T_INT2 section is shorter than the T_INT1 section, the electrons accumulated by the photodiode PD in the T_INT2 section as compared to the amount of electrons accumulated in the photodiode PD in the T_INT1 section. The amount of can be small.

When entering the T_RD period, the gate voltage TX of the transfer transistor M_TG may be the power supply voltage VDD, and the gate voltage RST of the reset transistor M_RS is reset before and after the time when the T_RD period enters the T_RD period. The transistor M_RS may be a voltage at which the transistor M_RS is turned off (eg, a ground voltage). As shown in FIG. 3B, as the potential barrier under the gate of the transfer transistor M_RS is lowered, the potential of the floating diffusion FD is changed to the photodiode PD according to the electrons accumulated by the photodiode PD. ) And the potential of the photodiode PD is lowered. As a result, a voltage corresponding to the raised potential of the floating diffusion FD may be output to the outside through the source-follower transistor and the selection transistor of FIG. 2.

FIG. 3B illustrates a case where the light intensity absorbed by the photodiode PD is relatively strong, but the case where the light intensity absorbed by the photodiode PD is relatively weak will be described below. When the intensity of light absorbed by the photodiode PD is weak, the potential according to the electrons accumulated in the photodiode PD in the T_INT1 section may not cross the potential barrier under the gate of the transfer transistor M_TG. Accordingly, the photodiode may continuously accumulate electrons by light absorbing regardless of the change in the gate voltage TX of the transfer transistor M_TG in the T_INT1 and T_INT2 sections, respectively. Therefore, due to the double trapping method, in the case of weak light, the potential change of the photodiode PD that changes linearly with the light intensity can be used without affecting the gate voltage TX of the transfer transistor M_TG. On the other hand, in the case of strong light, the final potential change amount of the photodiode PD may be adjusted under the influence of the gate voltage TX.

4 is a graph illustrating a change in characteristics of a pixel circuit according to an exemplary embodiment of FIG. 3. FIG. 4A is a graph showing the potential change of the photodiode when the photodiode absorbs strong light as shown in FIG. 3B. As mentioned in FIG. 3, since the gate voltage TX of the transfer transistor is the first voltage V_TX1 in the T_INT1 period, the potential of the photodiode increases linearly and constantly exceeds the potential barrier under the gate of the transfer transistor. It may be maintained and may be kept constant until the end of the T_INT1 section. When entering the T_INT2 period, the gate voltage TX of the transfer transistor is changed to the second voltage V_TX2 and the potential barrier under the gate of the transfer transistor becomes high. Therefore, the photodiode may further accumulate electrons in response to the light to be absorbed. At this time, the rate at which the photodiode potential rises is the same in the T_INT1 and T_INT2 sections, but since the T_INT2 section is shorter than the T_INT1 section, the change in the photodiode potential in the T_INT2 section is smaller than the change in the photodiode potential in the T_INT1 section. Can be.

FIG. 4B is a graph showing an output voltage according to light intensity in a pixel circuit operating according to an embodiment of FIG. 3. As shown in (b) of FIG. 4, when the light intensity is equal to or less than a predetermined value INT_X, the output voltage may be transmitted to the outside in proportion to the light intensity linearly. On the other hand, when the intensity of the light is greater than a certain value INT_X, the output voltage may be transmitted to the outside in a linear proportion to the intensity of light with a different slope than that of the case where the intensity of light is weak. The point where the slope is changed is called an inflection point or knee point.

Since the potential of the photodiode increases as the photodiode absorbs light and accumulates electrons, the slope of the graph shown in FIG. 4B may be proportional to the time for the photodiode accumulation of electrons. That is, the slope may be changed according to the length of the T_INT1 section and the T_INT2 section. For example, as each interval becomes longer, the slope of the graph increases. Therefore, as shown in (b) of FIG. 4, in order to extend the dynamic range of the pixel circuit, the slope of the graph when the intensity of the light is greater than or equal to the constant value INT_X should be smaller than that of the T_INT2 interval. May be shorter than the length of the T_INT1 interval.

5 is a diagram illustrating a relationship between light intensity and output voltage of a pixel circuit according to a threshold voltage distribution of the transfer transistor of FIG. 2. The pixel array of FIG. 2 includes as many photodiodes and transfer transistors as the total number of pixels. Unfortunately, the transistors may have different threshold voltages for various reasons. Such a threshold voltage variation may cause an error in the potential change of the photodiode with respect to the same light intensity in the T_INT1 section of FIG. 3.

FIG. 5A illustrates the photodiode PD of FIG. 2, the lower gate of the transfer transistor M_TG, the floating diffusion FD, and the lower gate of the reset transistor M_RS according to the threshold voltage distribution of the transfer transistor M_TG. And the potential of the power supply voltage VDD, respectively. When the first voltage V_TX1 is applied to the gate of the transfer transistor M_TG, the potential when the photodiode PD is completely depleted due to the threshold voltage distribution of the transfer transistor M_TG and the gate of the transfer transistor M_TG. The potential difference between the heights of the lower potential barriers may be different for each pixel circuit belonging to one image sensor. Accordingly, when the first voltage V_TX1 is applied to the gate of the transfer transistor M_TG, a potential change according to electrons accumulated in the transfer transistor M_TG for relatively strong light may be different for each of the fill cell circuits. . For example, as illustrated in FIG. 5A, when the threshold voltage of the transfer transistor M_TG included in a pixel circuit is relatively low, the potential change POT_1 of the photodiode PD is relatively small. When the threshold voltage of the transistor M_TG included in another pixel circuit is relatively high, the potential change POT_2 of the photodiode PD may be relatively large.

FIG. 5B illustrates the relationship between the light intensity and the output voltage according to the threshold voltage distribution of the transfer transistor M_TG in the pixel circuit. As mentioned above, as the dispersion occurs in the threshold voltage of the transfer transistor M_TG included in each pixel circuit, the dispersion occurs at a position of an inflection point in the light intensity and output voltage graph. For example, as shown in FIG. 5B, when the threshold voltage of the transfer transistor M_TG is relatively low, an inflection point of the graph may be formed in light of relatively low intensity. On the contrary, when the threshold voltage of the transfer transistor M_TG is relatively high, an inflection point of the graph may be formed at relatively high intensity light. This distribution of inflection point positions may affect the characteristics of each pixel circuit included in one image sensor. For example, as shown in FIG. 5B, when two pixel circuits have different inflection points for light INT_Y having a specific intensity, different output voltages VOUT_1 and VOUT_2 may be output.

As such, when each pixel circuit included in one image sensor has different characteristics of output voltage according to light intensity, FPN (fixed pattern noise) may be caused to deteriorate an image quality. Hereinafter, embodiments of the present invention can improve the quality of an image by reducing the threshold voltage distribution of the transfer transistors M_TG included in each of the pixel circuits.

6 is a diagram illustrating a method of driving the pixel circuit of FIG. 2 according to an exemplary embodiment of the present invention. As mentioned above, in order to extend the dynamic range of the pixel circuit, the first voltage V_TX1 and the gate of the transfer transistor M_TG and the photodiode PD absorb light and accumulate electrons. The second voltage V_TX2 may be applied. According to an embodiment of the present invention, when the potential of the photodiode PD is transferred to the floating diffusion FD, the gate voltage of the transfer transistor M_TG is maintained so that the photodiode PD remains incompletely depleted. TX may be the third voltage V_TX3. The third voltage V_TX3 may be lower than the power supply voltage VDD and higher than the first voltage V_TX1.

FIG. 6A illustrates the gate voltage TX of the transfer transistor M_TG and the gate voltage RST of the reset transistor M_RS while the pixel circuit of FIG. 2 recognizes an image according to an exemplary embodiment of the present invention. A graph showing the change in. As shown in (a) of FIG. 6, while the photodiode PD absorbs light, the gate voltage TX of the transfer transistor M_TG is the first voltage V_TX1 and the first voltage in the T_INT1 and T_INT2 sections, respectively. It may be two voltages V_TX2. When the T_INT2 section ends and the T_RD section starts, the gate voltage TX of the transfer transistor M_TG may be a lower third voltage V_TX3 instead of the power supply voltage VDD.

FIG. 6B illustrates a photodiode PD, a gate lower portion of the transfer transistor M_TG, and a floating diffusion FD included in the pixel circuit of FIG. 2 in the T_INT1, T_INT2, and T_RD periods according to an embodiment of the present invention. And the potential of the gate lower portion of the reset transistor M_RS and the power supply voltage VDD, respectively. As mentioned above, the threshold voltage distribution of the transfer transistor M_TG can be reflected in the height of the potential barrier under the gate of the transfer transistor M_TG, and the transfer transistor M_TG shown in FIG. Assume that the threshold voltage is higher by a constant value VAR at the reference value. In the T_INT1 period, the first voltage V_TX1 is applied to the gate of the transfer transistor M_TG, and thus the potential barrier under the gate of the transfer transistor M_TG rises. However, as shown in FIG. 6B, when the photodiode PD absorbs strong light, the potential of the photodiode PD is limited to the potential barrier under the gate of the transfer transistor M_TG. Can't climb anymore. In the T_INT2 period, the second voltage V_TX2 is applied to the gate of the transfer transistor M_TG, and thus the potential of the photodiode PD may rise again. According to the exemplary embodiment of the present invention, since the T_INT2 section is shorter than the T_INT1 section, the potential change amount of the photodiode PD in the T_INT2 section may be smaller than the potential change amount of the photodiode PD in the T_INT1 section.

Meanwhile, according to an exemplary embodiment, the gate voltage of the transfer transistor M_TG may be the third voltage V_TX3 at the end of the T_INT2 period. The third voltage V_TX3 may be lower than the power supply voltage VDD and higher than the first voltage V_TX1. As the third voltage V_TX3 is applied, the height of the potential barrier under the gate of the transfer transistor M_TG may be lower than that in the T_INT2 period, and as shown in FIG. 6B, the photodiode PD is accumulated. As the electrons are transferred to the floating diffusion FD, the potential of the photodiode PD may drop. The potential of the photodiode PD may be lowered by the potential barrier under the gate of the transfer transistor M_TG determined by the third voltage V_TX3, and the photodiode PD does not exhaust electrons and retains part of the charge. I can hold it. That is, when transferring electrons of the photodiode PD to the floating diffusion FD, instead of completely depleting the photodiode PD, a third voltage V_TX3 is applied to the gate of the transfer transistor M_TG. The photodiode PD is kept in an incomplete depletion state.

As the photodiode PD maintains a potential, the amount of electrons transferred from the photodiode PD to the floating diffusion FD is not affected by the threshold voltage of the transfer transistor M_TG, and the photodiode PD absorbs the photodiode PD. It can be proportional to one light intensity. That is, the maximum value of the potential of the photodiode PD in the T_INT1 period is influenced by the threshold voltage of the transfer transistor M_TG, and the potential according to the charge remaining in the photodiode PD in the T_RD period is also transferred in the transfer transistor M_TG. The influence of the threshold voltage of the transfer transistor M_TG may cancel each other because of the influence of the threshold voltage of the transfer transistor M_TG.

7 is a diagram illustrating operating characteristics of a pixel circuit according to an exemplary embodiment of the present invention. As mentioned above, according to one embodiment of the present invention, the influence of the threshold voltage of the transfer transistor on the amount of charge transferred to the floating diffusion may be cancelled.

FIG. 7A is a graph showing the maximum potential of the photodiode PD according to the light intensity in the pixel circuit according to the exemplary embodiment of the present invention. There may be a dispersion in the threshold voltage of the transfer transistor belonging to each pixel circuit. For example, as shown in FIG. 7A, for three transfer transistors having different threshold voltages, the maximum potential of the photodiode PD according to the light intensity of the pixel circuit including the same has a constant error. Can form parallel graphs. When the charge is transferred to the floating diffusion and the photodiode PD begins to absorb light again (at the point where the light intensity is zero), the potential of the photodiode PD is determined by the pixel voltage distribution of the transfer transistors for each pixel circuit. May differ from Therefore, the influence of the threshold voltage of the transfer transistor according to the first voltage applied to the gate of the transfer transistor while the photodiode PD absorbs light can be canceled out.

FIG. 7B is a graph showing an output voltage according to light intensity in a pixel circuit according to an exemplary embodiment of the present invention. As shown in FIG. 7A, the maximum potential of the photodiode PD has a different value for each pixel circuit due to the threshold voltage distribution of the transfer transistor, but the amount of electrons transferred to the floating diffusion is equal to the threshold voltage. The influences cancel each other out and may be proportional to the intensity of light absorbed by the photodiode PD. Thus, unlike the characteristics shown in FIG. 5B, the pixel circuit may reduce the influence of the threshold voltage distribution of the transfer transistor so as to equalize the characteristics of the output voltage according to the light intensity of each pixel circuit.

The foregoing description of the embodiments is merely illustrative of the present invention with reference to the drawings for a more thorough understanding of the present invention, and thus should not be construed as limiting the present invention. It will be apparent to those skilled in the art that various changes and modifications may be made without departing from the basic principles of the present invention.

Claims (11)

In the method of operating a complementary meta-oxide semiconductor (CMOS) image sensor,
The CMOS image sensor includes a photosensitive device; And a transfer transistor configured to transfer or block an electric charge between the photosensitive device and the floating diffusion,
Accumulating charge in the photosensitive device by sequentially applying first and second voltages having different levels to the gate of the transfer transistor;
Applying a third voltage to a gate of the transfer transistor to transfer a portion of the charge accumulated in the photosensitive device to the floating diffusion so as to incomplete depletion; And
Sensing a potential of the floating diffusion; and operating the CMOS image sensor. The method of claim 1, wherein the second voltage is
And the height of the potential barrier between the photosensitive device and the floating diffusion is set to be higher than when the first voltage is applied to the gate of the transfer transistor. The method of claim 2, wherein the third voltage,
The height of the potential barrier between the photosensitive device and the floating diffusion is set to be lower than that when the first voltage is applied to the gate of the transfer transistor and higher than the potential of the photosensitive device in a fully depleted state. CMOS image sensor operation method. The method of claim 3,
And the time of applying the second voltage to the gate of the transfer transistor is shorter than the time of applying the first voltage. The method of claim 1, wherein the sensing of the potential of the floating diffusion comprises:
Sensing a reset potential of the floating diffusion; And
Sensing a potential of the floating diffusion in which charge is transferred through the transfer transistor. Photo sensing device;
A transfer transistor configured to transfer or block an electric charge between the photosensitive device and floating diffusion; And
While the photosensitive device accumulates electric charges, the first and second voltages having different levels are output to the gate of the transfer transistor, and the charge accumulated in the photosensitive device so that the photosensitive device is incomplete depletion. And a control unit for outputting a third voltage to a gate of the transfer transistor so that a part of the floating diffusion is transferred to the floating diffusion. 7. The apparatus of claim 6, wherein the control unit
And outputting the second voltage such that the height of the potential barrier between the photosensitive device and the floating diffusion is higher than when the first voltage is applied to the gate of the transfer transistor. 8. The apparatus of claim 7, wherein the control unit
The height of the potential barrier between the photosensitive device and the floating diffusion is lower than that when the first voltage is applied to the gate of the transfer transistor and is higher than the potential of the photosensitive device in a fully depleted state. CMOS image sensor, characterized in that the output. The method of claim 8, wherein the control unit
And a time for outputting the second voltage is shorter than a time for outputting the first voltage. The method according to claim 6,
The CMOS image sensor further includes a reset transistor connected to the floating diffusion and a power supply voltage.
And the controller controls the gate voltage of the reset transistor to turn off the reset transistor when the photo-sensing element finishes accumulating charge and to turn on the reset transistor at the start. . The method of claim 10, wherein the CMOS image sensor
A source-follower transistor outputting a voltage in response to the potential of the floating diffusion; And a selection transistor connected to the source-follower transistor and the output line.
The control unit controls the gate voltage of the selection transistor to transfer the output voltages of the source-follower transistor corresponding to the reset potential of the floating diffusion and the potential of the transferred state to the output line, respectively; Image sensor.

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