KR20030084340A - Unit Pixel with improved property in cmos image sensor - Google Patents

Abstract

PURPOSE: A unit pixel of a CMOS image sensor with improved characteristic is provided to replace a reset transistor with a reset input including a resistor to simplify the structure of the unit pixel and reduce leakage current. CONSTITUTION: A unit pixel of a CMOS image sensor includes a photo-diode for detecting external lights to generate optical charges, a floating diffusion region for receiving and storing the charges, and a transfer transistor that is connected between the photo-diode and the floating diffusion region to transfer the charges into the floating diffusion region. The unit pixel further includes a reset input and a drive transistor. The reset input is connected to one side of the transfer transistor and the floating diffusion region to reset the photo-diode according to an input signal and employs a resistor. The gate of the drive transistor is connected to the floating diffusion region to detect an electric signal from the floating diffusion region.

Description

Unit Pixel with improved property in cmos image sensor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CMOS image sensor, and more particularly, to simplify a circuit by removing a reset transistor in a unit pixel configuration and replacing it with a reset input having a resistor.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Among them, a charge coupled device (CCD) includes individual metal-oxide-silicon (MOS) capacitors. The charge carrier is stored and transported in a capacitor while being located in close proximity. The CMOS image sensor uses a CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. It is a device that adopts switching method to make MOS transistor and detect output by using it.

CCD (charge coupled device) has many disadvantages such as complicated driving method, high power consumption, high number of mask process steps, complicated process, and difficult to implement signal processing circuit in CCD chip. In order to overcome such drawbacks, the development of a CMOS image sensor using a sub-micron CMOS manufacturing technology has been studied in recent years. The CMOS image sensor forms an image by forming a photodiode and a MOS transistor in a unit pixel and sequentially detects signals in a switching method, and implements an image by using a CMOS manufacturing technology, which consumes less power and uses 30 to 40 masks as many as 20 masks. Compared to CCD process that requires two masks, the process is very simple, and it is possible to make various signal processing circuits and one chip, which is attracting attention as the next generation image sensor.

FIG. 1A is a conventional CMOS image sensor unit pixel circuit diagram using four transistors, which is composed of one photodiode (PD) and four NMOS transistors, and four NMOS transistors are collected from the photodiode (PD). A transfer transistor Tx for transporting the photocharges to the floating diffusion region FD, and a reset transistor Rx for setting the potential of the floating diffusion region to a desired value and discharging electric charges to reset the floating diffusion region FD. And a drive transistor Dx serving as a source follower buffer amplifier, and a select transistor Sx capable of addressing as a switching role. Outside the unit pixel, a load transistor is formed to read an output signal.

FIG. 1B is a unit pixel circuit diagram of an image sensor constructed by using one photodiode and three transistors, in which a transfer transistor Tx is removed from a unit pixel composed of four transistors.

That is, the photocharge generated by the photodiode PD is input directly to the gate terminal of the drive transistor Dx without passing through the transfer transistor Tx to drive the drive transistor Dx.

The reset transistor Rx and the select transistor Sx shown in FIG. 1B perform operations similar to those of the reset transistor Rx and the select transistor Sx shown in FIG. 1A. The reset transistor Rx initializes the photodiode PD according to the signal input to the gate terminal, and the select transistor Sx moves out of the unit pixel according to the switching signal input to the gate. It is responsible for passing output.

Since such a conventional unit pixel structure is composed of four or three transistors and one photodiode, the area is large and has acted as an obstacle to high integration of the device.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems. The present invention has been made by removing a reset transistor from a unit pixel of a CMOS image sensor and replacing it with a reset input having a resistor to increase the device integration density and reduce leakage current characteristics. It is an object to provide a unit pixel of an image sensor.

1A to 1B are circuit diagrams showing the configuration of unit pixels in a CMOS image sensor according to the prior art;

2A to 2B are circuit diagrams showing the configuration of unit pixels in the CMOS image sensor according to the present invention;

The present invention for achieving the above object, a photodiode for generating a photocharge by sensing light from the outside; A floating diffusion region configured to receive and store charge generated from the photodiode; A transfer transistor connected between the photodiode and the floating diffusion region to transfer charges generated from the photodiode to the floating diffusion region; A reset input having a resistor connected to one side end of the transfer transistor and the floating diffusion region to reset the photodiode according to a signal input thereto; And a drive transistor connected to the floating diffusion region to detect an electrical signal from the floating diffusion region.

The present invention also provides a photodiode for generating photocharges by sensing light from the outside; A drive transistor having a gate terminal connected to the photodiode; A reset input connected to one side of the photodiode to reset the photodiode but having a resistor; And a select transistor connected between a power supply voltage terminal and the drive transistor to perform a switching role.

The present invention reduces the size of the unit pixel and reduces the leakage current by removing the reset transistor and replacing it with a reset input having a resistor.

Hereinafter, the most preferred embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art can easily implement the technical idea of the present invention.

FIG. 2A is a circuit diagram showing a unit pixel composed of three transistors, one photodiode, and one resistor according to the present invention. Referring to this, a reset transistor is removed and replaced with a reset input having a resistor. The other part is similar to the prior art.

That is, the unit pixel of the CMOS image sensor according to an embodiment of the present invention composed of one photodiode, three transistors, and one resistor receives light to generate and accumulate photocharges and use the photodiode (PD) for image reproduction. ), A transfer transistor (Tx) for transporting the photocharges generated by the photodiode to the floating diffusion region, a potential of the photodiode and the floating diffusion region are set to a desired value, and the electrons of the photodiode and floating diffusion region are discharged A reset input stage for performing a reset operation, a drive transistor (Dx) serving as a source follower buffer amplifier, and a select transistor (Sx) for addressing (switching). It is composed. Outside the unit pixel, a load resistor is formed to read an output signal.

The image sensor is composed of a pixel array unit in which tens of thousands to hundreds of thousands of unit pixels are gathered, and a peripheral circuit unit for controlling the output of the pixel array unit. In the prior art, a reset operation for removing electrons remaining in a photodiode in a unit pixel is performed. The reset transistor was turned on to remove the electrons remaining in the photodiode. That is, the reset transistor is provided for each unit pixel and used in the reset operation. In one embodiment of the present invention, the area of the unit pixel can be reduced by removing the reset transistor and replacing it with a reset input having a resistor.

In the unit pixel of the CMOS image sensor according to an embodiment of the present invention having a reset input having a resistor, a positive voltage is input to the reset input during the reset operation to remove electrons remaining in the photodiode.

The signal input to the reset input is a pulse signal, and a positive voltage is applied during the reset operation, and in a floating state other than the reset operation, thereby replacing the role of the conventional reset transistor.

In one embodiment of the present invention, the leakage current is reduced by connecting a high resistance to the reset input. The leakage current problem that may occur when the reset input is in a floating state is solved by connecting the high resistance to the reset input. It was.

The high resistance of the reset input may be formed of polysilicon, and the high resistance of the polysilicon may be formed into a stack structure with the existing transistors (Tx, Rx, and Dx) to prevent an increase in the unit pixel area. can do.

That is, when a high resistance is added to the reset input to reduce the leakage current, since the high resistance made of polysilicon occupies a relatively large area, the area of the unit pixel may increase, but such high resistance If formed as a stack on an existing transistor, it is possible to prevent an increase in the unit pixel area.

2B is a diagram illustrating a unit pixel composed of two transistors, one photodiode, and one resistor by replacing the reset transistor with a reset input terminal having a resistor according to an embodiment of the present invention. The reset transistor is removed and replaced by a reset input with a resistor, the other parts being similar to the prior art.

That is, in the structure in which the photocharge generated by the photodiode PD is input directly to the gate terminal of the drive transistor Dx without passing through the transfer transistor Tx to drive the drive transistor D, It is replaced by a reset input terminal provided, and a load resistor is formed outside the unit pixel so that an output signal can be read.

The drive transistor receives the charge accumulated in the photodiode and outputs the image information generated by the photocharge, and the select transistor Sx moves out of the unit pixel according to the switching signal input to the gate end. It is responsible for passing output.

Like the unit pixel shown in FIG. 2A, the signal input to the reset input in the unit pixel shown in FIG. 2B is a pulse signal. In the reset operation, a positive voltage is applied to remove electrons remaining in the photodiode. Otherwise, the floating state can replace the role of the conventional reset transistor.

In the unit pixel shown in FIG. 2B, as in FIG. 2A, the leakage current is reduced by providing a high resistance at the reset input, and the reset resistance is higher than the leakage current that may occur when the reset input terminal is in a floating state. Solved by connecting to.

The high resistance of the reset input may be formed of polysilicon, and the high resistance of the polysilicon may be formed into a stack structure with the existing transistors (Tx, Rx, and Dx) to prevent an increase in the unit pixel area. can do.

In an embodiment of the present invention, the reset transistor is removed and a pulse signal is input instead, but a reset input having a resistance is used to reduce the unit pixel area, thereby simplifying the unit pixel configuration and making the device highly integrated. At the same time, by reducing the leakage current, by forming a high resistance for reducing the leakage current in the form of a stack, the fill factor, which is the area ratio occupied by the photodiode in the unit pixel, can be increased without increasing the area of the unit pixel. In addition, if the high integration of the device is made according to the present invention can reduce the manufacturing cost.

As described above, the present invention is not limited to the above-described embodiments and the accompanying drawings, and the present invention may be variously substituted, modified, and changed without departing from the spirit of the present invention. It will be apparent to those of ordinary skill in the art.

When the present invention is applied to the CMOS image sensor, since the reset transistor is replaced by a reset input having a resistance, the unit pixel configuration is simplified, which is advantageous for high integration of the device, and also has an effect of reducing leakage current.

Claims (9)

In the unit pixel of the CMOS image sensor, A photodiode that senses light from the outside and generates photocharges; A floating diffusion region configured to receive and store charge generated from the photodiode; A transfer transistor connected between the photodiode and the floating diffusion region to transfer charges generated from the photodiode to the floating diffusion region; A reset input having a resistor connected to one side end of the transfer transistor and the floating diffusion region to reset the photodiode according to a signal input thereto; And A drive transistor connected to the floating diffusion region to detect an electrical signal from the floating diffusion region Unit pixel of the CMOS image sensor comprising a. The method of claim 1, The reset input having the resistor is a unit pixel of the CMOS image sensor, characterized in that for receiving a pulse signal. The method according to claim 1 or 3, The unit pixel of the CMOS image sensor further comprises a select transistor connected to the source terminal of the drive transistor to serve as a switching. The method of claim 1, Resistor provided to the reset input unit pixel of the CMOS image sensor, characterized in that made of polysilicon. The method of claim 4, wherein Resistor made of polysilicon unit pixel of the CMOS image sensor, characterized in that formed in a stacked form. In the unit pixel of the CMOS image sensor, A photodiode that senses light from the outside and generates photocharges; A drive transistor having a gate terminal connected to the photodiode; A reset input connected to one side of the photodiode to reset the photodiode but having a resistor; And A select transistor connected between a power supply voltage terminal and the drive transistor to perform a switching role. Unit pixel of the CMOS image sensor comprising a. The method of claim 6. The reset input including the resistor is a unit pixel of the CMOS image sensor, characterized in that the input as a pulse signal. The method of claim 6. The resistor of the reset input unit pixel of the CMOS image sensor, characterized in that made of polysilicon. The method of claim 8. Resistor made of polysilicon unit pixel of the CMOS image sensor, characterized in that formed in a stacked form.