KR20100059190A - Image sensor and driving method the same - Google Patents

Abstract

PURPOSE: An image sensor and a driving method of the same are provided to add barrier device at addition of electrical storage device and reset of floating diffusion area in overflow of electric charge, thereby reducing PD(Photo Diode) capacity. CONSTITUTION: The first electricity storage device(Cp) collect electric charge by light sensing of light receiving device. The second electricity storage device(Co) collects electric charge which is overflowed at the first electric charge device. The second switching element(Rx) transmits a signal which resets a floating diffusion area to supply voltage level. When the floating diffusion area is reset to supply voltage level, the third switching element(Bx) preserves the stored electric charge of the second electric storage device.

Description

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

The present invention relates to an image sensor, and more particularly, to an image sensor and a driving method thereof capable of maintaining a dynamic range (D / R) by sufficiently securing saturation illuminance even when the capacity of a photodiode is reduced.

As is well known, an image sensor is a semiconductor device that converts an optical image into an electrical signal, and includes a Charged Coupled Device (CCD) image sensor and a Complementary Metal Oxide Semiconductor (CMOS) image sensor. .

The CMOS image sensor is a device that converts an optical image into an electrical signal by using CMOS manufacturing technology, and employs a switching method in which MOS transistors are made by the number of pixels and the outputs are sequentially detected using the same. The CMOS image sensor is simpler to drive than the CD image sensor, which is widely used as a conventional image sensor, and can realize various scanning methods, and it is possible to miniaturize a product by integrating a signal processing circuit onto a single chip. In addition, the use of compatible CMOS technology can reduce the manufacturing cost and has the advantage of significantly lower power consumption.

1 is a circuit diagram of a CMOS image sensor implementing unit pixels using four transistors. FIG. 1 illustrates a unit pixel of a CMOS image sensor including a photodiode (PD) and four NMOS transistors. It is showing. Of the four NMOS transistors, the transfer transistor Tx transmits a signal for transferring the photocharge generated in the photodiode PD to the floating diffusion region FD, and the reset transistor Rx transmits the floating diffusion region FD. transmitting a signal to reset the supply voltage (V _DD) level, and the drive transistor (Dx) is, and serves as a source follower (source follower), the select transistor (Sx) is received is the pixel data enable (pixel data enable) signal It sends the pixel data signal to the output.

Operation of the image sensor unit pixel configured as described above is performed as follows. Initially, the unit pixel is reset by turning on the reset transistor Rx, the transfer transistor Tx, and the select transistor Sx. At this time, the photodiode PD starts to deplete, and carrier accumulation occurs, and the floating diffusion region FD is charged and stored up to the supply voltage V _DD . The transfer transistor Tx is turned off, the select transistor Sx is turned on, and the reset transistor Rx is turned off. In such an operating state, the output voltage V1 is read from the unit pixel output terminal Vout and stored in the buffer, and then the carrier transistor Tx is turned on to transfer the carriers of the capacitor Cp changed according to the light intensity to the capacitor Cf. After moving, the output voltage V2 is read from the unit pixel output terminal Vout, and the analog data for V1-V2 is converted into digital data to complete an operation cycle of one frame for the unit pixel.

On the other hand, according to the trend of miniaturization of the image sensor, the photodiode capacity of the unit pixel is inevitably smaller. As shown in FIG. 2, reducing the photodiode capacity effectively removes the image lag, and reducing the implant dose of the photodiode reduces the electric field, thus reducing the electric field, thereby reducing the PN. Junction leakage can be reduced, thereby improving the dark characteristics.

For this reason, there is a limit to increasing the photodiode capacity in the future.

As described above, when the photodiode capacity is reduced, the saturation illuminance is reduced, thereby reducing the operating range of the image sensor.

3 is a graph showing a change in the output signal according to the charging time for each saturation illuminance.

If the photodiode capacity (PDcapa) is reduced to I / 2 level and charged for one frame at low saturation illuminance I / 2, the saturation signal Vs is reached, and higher illuminance I is already saturated if charged for T / 2 hours. As a result, subsequent generations of electrons overflow, which makes them unable to contribute to the signal voltage. That is, since the saturation illuminance of the image sensor is I / 2, the range of illuminance that can be expressed by the image sensor is reduced, so that bright light cannot be expressed.

The present invention has been proposed to solve the above problems, and the image sensor that can maintain the saturation roughness without loss, while implementing the advantages such as miniaturization of the unit pixel, reduced image lag and improved dark characteristics by reducing the photodiode capacity It provides a driving method.

As a first aspect of the present invention, an image sensor includes: a light receiving element capable of sensing light, a first power storage element capable of accumulating charges by photo-sensing of the light receiving element, and an overflow in the first power storage element. a second power storage element capable of accumulating overflowed charges, a first switching element capable of transmitting a signal for transferring charge of the first power storage element to a floating diffusion region, and a charge transferred to the floating diffusion region A third power storage element capable of storing power, a second switching element capable of transmitting a signal for resetting the floating diffusion region to a supply voltage level, and the second power storage element when the floating diffusion region is reset to a supply voltage level A third switching element capable of preserving the electric charge of the battery, and a burr for amplifying and outputting the electric charges stored in the second power storage element and the third power storage element. And a fourth switching device capable of transmitting a pixel data signal of the signal amplifying device to an output.

Here, the light receiving element is a photodiode, at least one of the first to third power storage elements is a capacitor, and at least one of the first to fourth switching elements or the signal amplification element is a transistor. .

According to a second aspect of the present invention, there is provided a method of driving an image sensor, the method comprising: accumulating charges overflowed from the first power storage device to a second power storage device while accumulating charges by photo-sensing of the light receiving device to the first power storage device; Resetting a third power storage device capable of storing charge transferred to the floating diffusion region to a supply voltage level, reading a first output voltage from a unit pixel output terminal, and generating the second power storage device and the second power storage device. Amplifying and storing the electric charge stored in the three power storage elements, reading a second output voltage from the unit pixel output terminal, and converting analog data based on the first output voltage and the second output voltage into digital data; Steps.

According to the present invention, there is provided a barrier element that preserves the overflow capacitor charge when resetting the floating diffusion region while adding a capacitor to accumulate the charge when the charge overflows in the capacitor that accumulates charges by photo-sensing of the light receiving element. In addition, the photodiode capacity is reduced to allow for miniaturization of the unit pixel, reduce image lag, reduce PN junction leakage, and improve dark characteristics while maintaining saturation roughness without loss.

Therefore, there is an effect that can maintain a sufficient operating range of the image sensor.

Hereinafter, some embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, in describing the present invention, when it is determined that the detailed description of the related known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

4 is a circuit diagram of a CMOS image sensor implementing unit pixels using five transistors according to an embodiment of the present invention, and FIG. 5 is a unit pixel configuring a CMOS image sensor according to an embodiment of the present invention. As a cross-sectional view, the unit pixel of a CMOS image sensor composed of a photodiode (PD) as a light receiving element, four NMOS transistors, and three capacitors is shown.

As shown therein, the unit pixel of the CMOS image sensor includes a photodiode PD for sensing light, a first capacitor Cp for accumulating charge by photodetection of the photodiode PD, A second capacitor Co for accumulating charges overflowed from the first capacitor Cp, a transfer transistor Tx for transmitting a signal for transferring charge of the first capacitor Cp to the floating diffusion region FD, and A third capacitor Cf for storing electric charges transferred to the floating diffusion region FD, a reset transistor Rx for transmitting a signal for resetting the floating diffusion region FD to a supply voltage V _DD level, When the floating diffusion region FD is reset to the supply voltage V _DD level, the barrier transistor Bx which serves to preserve the storage charge of the second capacitor Co is electrically connected to the floating diffusion region FD. And the second capacitor Co and the third capacitor Cf Drive transistor (Dx) that acts as a source follower buffer amplifier to amplify and output the stored charge and the pixel data enable signal is applied to transmit the pixel data signal of the drive transistor (Dx) to the output (Vout) And a select transistor Sx.

As described above, the first capacitor Cp, the second capacitor Co, and the third capacitor Cf of the unit pixels of the CMOS image sensor are examples of power storage elements that store electric charges. The transfer transistor Tx, the reset transistor Rx, the barrier transistor Bx, the drive transistor Dx, and the select transistor Sx are examples of switching elements. The drive transistor Dx is an example of a signal amplifier.

6 to 8 are distribution diagrams of potential energy of electrons for each input illuminance of a unit pixel constituting the CMOS image sensor according to the present invention, and FIG. 6 is a case where the photodiode capacitance PDcapa is I / 2. When the light of I / 3 intensity enters the photodiode PD, FIG. 7 is a case where the light of 3I / 4 illumination enters the photodiode PD when the photodiode capacitance PDcapa is I / 2, FIG. 8 illustrates a case where I illumination of light enters the photodiode PD when the photodiode capacitance PDcapa is I / 2.

First, referring to FIGS. 4 and 6, the driving process of the unit pixels constituting the CMOS image sensor according to the present invention will be described.

When light of I / 3 illuminance enters the photodiode PD, the photodiode PD starts to deplete, charge storage of the first capacitor Cp occurs, and no overflow of charge occurs. The charge storage of Co) does not occur. At this time, the barrier transistor Bx maintains 2Vs / 3 (see FIG. 6A).

By a reset transistor (Rx) on (on) to reset the reset, that is, the unit pixel to the floating diffusion region (FD) to the supply voltage (V _DD) level (see (b) in Fig. 6).

In this operation state, the output voltage V1 is read from the unit pixel output terminal Vout and stored in the buffer.

After the reset transistor Rx is turned off and the transfer transistor Tx is turned on to move the charge of the first capacitor Cp to the second capacitor Co, the transfer transistor Tx is turned off. (See (c) of FIG. 6), the output voltage V2 of the unit pixel output terminal Vout is read, and the analog data for V1-V2 is converted into digital data to complete the operation cycle of one frame for the unit pixel. do.

By this driving process, the unit pixels constituting the CMOS image sensor output a signal voltage of Vs / 3.

Since the second capacitor Co must be discharged for the next frame, the barrier transistor Bx is sufficiently turned on and the reset transistor Rx is turned on to reset the supply voltage V _DD . (See FIG. 6 (d)).

Next, referring to FIGS. 4 and 7, the driving process of the unit pixels constituting the CMOS image sensor according to the present invention will be described.

When light of 3I / 4 illuminance enters the photodiode PD, the photodiode PD starts to deplete, and charge storage of the first capacitor Cp occurs, and an overflow of charge occurs as much as I / 4. The charge storage of the two capacitors Co occurs (see FIG. 7A).

By a reset transistor (Rx) on (on) to reset the reset, that is, the unit pixel to the floating diffusion region (FD) to the supply voltage (V _DD) level. At this time, the potential barrier of 2Vs / 3 by the barrier transistor Bx is blocked, and the electric charge stored in the second capacitor Co is preserved without loss (see FIG. 7B).

In this operation state, the output voltage V1 is read from the unit pixel output terminal Vout and stored in the buffer.

After the reset transistor Rx is turned off and the transfer transistor Tx is turned on to move the charge of the first capacitor Cp to the second capacitor Co and the third capacitor Cf, the transfer transistor After (Tx) is turned off (refer to (c) of FIG. 7), the output voltage V2 of the unit pixel output terminal Vout is read, and the analog data for V1-V2 is changed into digital data, so that The operation cycle of one frame is completed.

By the driving process, the unit pixel constituting the CMOS image sensor outputs a signal voltage of 3Vs / 4 by combining the overflowed I / 4 and I / 2 remaining in the photodiode PD.

Since the second capacitor Co must be discharged for the next frame, the barrier transistor Bx is sufficiently turned on and the reset transistor Rx is turned on to reset the supply voltage V _DD . (See FIG. 7D).

Finally, referring to FIGS. 4 and 8, the driving process of the unit pixels constituting the CMOS image sensor according to the present invention will be described.

When the light of I illuminance enters the photodiode PD, the photodiode PD starts to deplete and charge storage of the first capacitor Cp occurs, and an overflow of charge occurs as much as I / 2 so that the second capacitor Charge storage of Co occurs (see FIG. 8A).

By a reset transistor (Rx) on (on) to reset the reset, that is, the unit pixel to the floating diffusion region (FD) to the supply voltage (V _DD) level. At this time, the potential barrier of 2Vs / 3 by the barrier transistor Bx is blocked, and the electric charge stored in the second capacitor Co is preserved without loss (see FIG. 8B).

In this operation state, the output voltage V1 is read from the unit pixel output terminal Vout and stored in the buffer.

After the reset transistor Rx is turned off and the transfer transistor Tx is turned on to move the charge of the first capacitor Cp to the second capacitor Co and the third capacitor Cf, the transfer transistor After (Tx) is turned off (see (c) of FIG. 8), the output voltage V2 of the unit pixel output terminal Vout is read, and the analog data for V1-V2 is changed into digital data to convert the unit pixel to the unit pixel. The operation cycle of one frame is completed.

By such a driving process, the unit pixel constituting the CMOS image sensor outputs a signal voltage of Vs by combining the overflowed I / 2 and the I / 2 remaining in the photodiode PD.

Since the second capacitor Co must be discharged for the next frame, the barrier transistor Bx is sufficiently turned on and the reset transistor Rx is turned on to reset the supply voltage V _DD . (See FIG. 8 (d)).

As described above, according to the exemplary embodiment of the present invention, the photodiode capacitance PDcapa is reduced to I / 2, which enables the miniaturization of the unit pixel, reduces the image lag, and reduces the PN junction leakage, thereby improving the dark characteristics. The illuminance can be maintained at I, not I / 2.

So far, the present invention has been described based on some embodiments thereof. Those skilled in the art will appreciate that the present invention can be implemented in a modified form without departing from the essential features of the present invention. Therefore, the disclosed embodiments should be considered in descriptive sense only and not for purposes of limitation. The scope of the present invention is shown in the claims rather than the foregoing description, and all differences within the scope will be construed as being included in the present invention.

1 is a circuit diagram of a unit pixel constituting a CMOS image sensor according to the prior art;

2 is an energy distribution diagram showing a change in potential distribution according to a decrease in photodiode capacity;

3 is a graph showing a change in the output signal according to the charging time for each saturation illuminance,

4 is a circuit diagram of a unit pixel constituting a CMOS image sensor according to an embodiment of the present invention;

5 is a cross-sectional view of a unit pixel constituting a CMOS image sensor according to an embodiment of the present invention;

6 to 8 are distribution diagrams of potential energies of electrons for each input illuminance of a unit pixel constituting the CMOS image sensor according to the present invention, and FIG. 6 is I / 3 for a photodiode when the photodiode capacitance is I / 2. 7 is a case where light of illuminance is turned on, FIG. 7 is a case where 3I / 4 illuminance light enters the photodiode when the photodiode capacitance is I / 2, and FIG. 8 is a photodiode when the photodiode capacitance is I / 2. It shows the case that the light of I illuminance is on.

Claims (5)

A light receiving element capable of detecting light, A first power storage element capable of storing charges by photo-sensing of the light receiving element; A second power storage device capable of storing charges overflowed in the first power storage device; A first switching element capable of transmitting a signal for transferring charge of the first power storage element to a floating diffusion region; A third power storage element capable of storing charge transferred to the floating diffusion region; A second switching element capable of transmitting a signal for resetting the floating diffusion region to a supply voltage level; A third switching element capable of preserving the electric charge of the second power storage element when resetting the floating diffusion region to a supply voltage level; A signal amplifying device capable of acting as a buffer amplifier for amplifying and outputting the electric charges stored in the second power storage device and the third power storage device; A fourth switching element capable of transmitting the pixel data signal of the signal amplifying element to an output Image sensor comprising a. The method of claim 1, The light receiving element is a photodiode Image sensor. The method of claim 1, At least one of the first to third power storage elements is a capacitor Image sensor. The method of claim 1, At least one of the first to fourth switching elements or the signal amplification element is a transistor. Image sensor. Accumulating the charges overflowed in the first power storage device to the second power storage device while accumulating the charges by photo-sensing of the light receiving device to the first power storage device; Resetting to a supply voltage level a third power storage element capable of storing charge transferred to the floating diffusion region, Reading the first output voltage from the unit pixel output stage; Amplifying and outputting electric charges stored in the second power storage device and the third power storage device to read a second output voltage from the unit pixel output terminal; Changing analog data based on the first output voltage and the second output voltage into digital data; Driving method of the image sensor comprising a.

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