KR20110078974A - Driving Method of Image Sensor - Google Patents

Abstract

A method of driving an image sensor is provided. A driving method of an image sensor includes a photodiode formed on a semiconductor substrate to generate charges and accumulate therein according to an incident light amount, a transfer gate formed on the semiconductor substrate to transfer charges accumulated on the photodiode, and read as a signal. Turning off the transfer gate in an image sensor comprising a floating diffusion region that retains charge transferred by the transfer gate, and applying a positive voltage to the semiconductor substrate to cause the photodiode and floating diffusion to And a reflash step of discharging the charge retained in the region.

Description

A method of operating a image sensor device

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to semiconductor devices, and more particularly, to a relash method of an image sensor.

An image sensor refers to a semiconductor device that converts an optical image into an electrical signal, and includes a CCD (Charge Coupled Device) device and a CMOS (Complementary Metal-Oxide-Silicon) device. The image sensor is composed of a light receiving area including a photodiode for detecting light and a logic area for processing the detected light into an electrical signal to make data. The unit pixel of the image sensor may be composed of a photodiode and one or more transistors.

1 illustrates a layout 100 of unit pixels of an image sensor having a general 4-TR structure. Referring to FIG. 1, the layout 100 of the unit pixel includes one photodiode PD, 20, a transfer transistor (Tx) 18, a reset transistor (Rx) 30, and a driving transistor. (Drive transistor, Dx; 40), and Select transistor (Sx) 50.

FIG. 2 is a circuit diagram of a unit pixel illustrated in FIG. 1. Referring to FIG. 2, a method of converting photocharges accumulated in the photodiode 20 into an electrical signal is as follows.

The transfer transistor 18 and the reset transistor 30 are turned on to perform a refresh operation on the photodiode 20 and the floating diffusion region 25. That is, the re-flash operation herein refers to an operation of extracting electrons stored in the photodiode 20 and the floating diffusion region 25.

After the refresh operation, the transfer transistor 18 and the reset transistor 30 are turned off and the potential of the floating diffusion region 25 (hereinafter referred to as "first potential") is read. After that, the transfer transistor 18 is turned on again to read the potential of the floating diffusion region 25 (hereinafter referred to as “second potential”). The difference between the two potentials, that is, the first potential and the second potential, is represented by an image signal.

As described above, in order to perform the refresh operation, the transfer transistor 18 and the reset transistor 30 must be turned on or turned off, and the transfer transistor 18 and the reset transistor 30 are turned on or off. KTC noise may be present.

An object of the present invention is to provide a refresh method of an image sensor that can reduce KTC noise.

The driving method of an image sensor according to an embodiment of the present invention for achieving the above object is a photodiode formed on a semiconductor substrate to generate a charge according to the amount of incident light and to accumulate therein, and transfers the charge accumulated in the photodiode. 10. A method of relashing an image sensor comprising: a transfer gate formed on a semiconductor substrate, and a floating diffusion region that retains charge transferred by said transfer gate for reading as a signal; And a re-flash step of applying a positive voltage to the semiconductor substrate to discharge charges held in the photodiode and the floating diffusion region.

In the driving method of the image sensor according to the exemplary embodiment of the present invention, since the photodiode and the floating diffusion region are refreshed by applying a positive voltage to the semiconductor substrate while the transfer gate is turned off, the transfer transistor and the reset transistor are turned on or It is effective to remove KTC noise due to turn off.

Hereinafter, the technical objects and features of the present invention will be apparent from the description of the accompanying drawings and the embodiments. Looking at the present invention in detail.

FIG. 3 is a circuit diagram of a unit pixel of an image sensor according to an exemplary embodiment of the present invention, and FIG. 4 is a conceptual diagram illustrating a reflash operation of the unit pixel illustrated in FIG. 3.

3 and 4, a unit pixel of an image sensor according to an exemplary embodiment of the present invention may include a photodiode 320, a transfer gate 318, a floating diffusion region 325, and a driving transistor 340. And selection transistor 350.

The transfer gate 318 is connected to the photodiode 320 and the floating diffusion region 325, and transfers photocharges accumulated in the photodiode 320 to the floating diffusion region 325 in response to the transmission pulse TG. .

For example, the transfer gate 318 is formed on the second conductivity type (eg P-type) well 420 of the first conductivity type (eg N-type) semiconductor substrate 410. The transfer gate 318 is formed by stacking a gate oxide film and a gate electrode. In addition, the photodiode 320 is formed in the second conductivity type well 420 on one side of the transfer gate 318, and the floating diffusion region 325 is the second conductivity type well 420 on the other side of the transfer gate 318. Is formed within.

In the driving transistor 340, a drain is connected to the first power supply VDD and a gate electrode is connected to the floating diffusion region 325 to read the voltage of the floating diffusion region 325.

The selection transistor 350 is connected between the driving transistor 340 and the signal line 300 and selects a unit pixel to read a signal in response to the selection pulse SEL input to the gate. For example, the drain of the select transistor 350 is connected to the source of the driving transistor 340, the source of the select transistor 350 is connected to the signal line 300, and the select pulse is applied to the gate of the select transistor 250. (SEL) is input.

The unit pixel of the image sensor shown in FIGS. 3 and 4 does not include a reset transistor for resetting the floating diffusion region 325.

Instead of omitting the reset transistor, a refresh operation for the unit pixel of the image sensor is performed as follows.

Charge and floating diffusion regions accumulated in the photodiode 320 by applying a positive voltage Vb to the semiconductor substrate 410 during the refresh period for the unit pixel with the transfer gate 318 turned off. Empty the charge held at 325.

For example, by applying a positive high voltage (Vb, for example, 2.5 to 4V) to the semiconductor substrate 410, charges accumulated in the photodiode 320 and charges held in the floating diffusion region 325 are transferred to the semiconductor substrate 410. The reflash operation is performed by discharging. When the re-flash operation is completed, the ground voltage GND is applied to the semiconductor substrate 410. After completion of the refresh operation, the first potential of the floating diffusion region 325 is read into the signal line 300.

Next, the photodiode 320 is exposed to accumulate photocharges in the photodiode 320. When the exposure ends, the transfer gate 318 is turned on to transfer the photocharges accumulated in the photodiode 320 to the floating diffusion region 325.

The transfer gate 318 is turned off to maintain the photocharge transferred to the floating diffusion region 325. The driving transistor 340 is driven according to the photocharge maintained, and the second potential determined by the driving of the driving transistor 340 is read into the signal line 300.

The difference between the read first and second potentials is represented by an image signal.

The driving method of an image sensor according to an exemplary embodiment of the present invention does not reflash the photodiode and the floating diffusion region by turning on the transfer transistor and the reset transistor, but applies a positive voltage to the semiconductor substrate while the transfer gate is turned off. Since the photodiode and the floating diffusion region are applied to each other, the KTC noise due to the turning on or turning off of the transfer transistor and the reset transistor can be removed. In addition, since no reset transistor is required, the pixel factor can be improved by sufficiently securing the fill factor size of the photodiode in the unit pixel.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Will be clear to those who have knowledge of. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

1 illustrates a layout of unit pixels of an image sensor having a general 4-TR structure.

FIG. 2 is a circuit diagram of a unit pixel illustrated in FIG. 1.

3 is a circuit diagram of a unit pixel of an image sensor according to an exemplary embodiment.

FIG. 4 is a conceptual diagram illustrating a reflash operation of a unit pixel illustrated in FIG. 3.

Claims (3)

A photodiode formed on the semiconductor substrate to generate charges and accumulate therein according to the amount of incident light, a transfer gate formed on the semiconductor substrate to transfer charges accumulated on the photodiode, and a transfer gate to read as a signal In the driving method of an image sensor comprising a floating diffusion region for holding a charge transferred, Turning off the transfer gate; And And applying a positive voltage to the semiconductor substrate to discharge the charge accumulated in the photodiode. The method of claim 1, wherein the relash step, And discharging the charge accumulated in the photodiode and simultaneously discharging the charge held in the floating diffusion region. The method of claim 1, wherein the driving method of the image sensor comprises: And applying a ground voltage to the semiconductor substrate after the refreshing step is completed.

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