KR20030084341A - 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 simplify the composition of the unit pixel by replacing a reset transistor with a reset input of receiving a pulse signal. CONSTITUTION: A unit pixel of a CMOS image sensor includes a photo-diode, a floating diffusion region, a transfer transistor, a reset input, and a drive transistor. The photo-diode detects external lights to generate optical charges. The floating diffusion region receives and stores the charges. The transfer transistor is connected between the photo-diode and the floating diffusion region and transfers the charges generated by the photo-diode to the floating diffusion region. The reset input is connected to one side of the transfer transistor and the floating diffusion region and receives a pulse signal to reset the photo-diode. 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. In particular, the present invention simplifies a circuit by removing a reset transistor in a unit pixel configuration and replacing it with a reset input receiving a pulse signal.

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 not small, and thus 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-mentioned problems. The present invention provides a unit pixel of a CMOS image sensor that increases the integration of a device by removing a reset transistor from a unit pixel of a CMOS image sensor and replacing it with a reset input receiving a pulse signal. For that purpose.

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 connected to one side of the transfer transistor and the floating diffusion region, the reset input receiving a pulse signal to reset the photodiode; And a drive transistor connected to the floating diffusion region to detect an electrical signal from the floating diffusion region. In another aspect, the present invention provides a photodiode for generating photocharge 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 and receiving a pulse signal to reset the photodiode; 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 a unit pixel by removing the reset transistor and replacing it with a reset input receiving a pulse signal.

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 the configuration of a unit pixel composed of three transistors and one photodiode according to the present invention. Referring to this, the reset transistor is removed and replaced with a reset input having a pulse input. Is similar to the prior art.

In other words, the unit pixel of the CMOS image sensor according to an embodiment of the present invention composed of one photodiode and three transistors is a photodiode (PD) for generating and accumulating photocharges to be used for image reproduction and a photo; A transfer transistor (Tx) for transporting the photocharges generated by the diode to the floating diffusion region, the potential of the photodiode and the floating diffusion region are set to a desired value, and a reset operation is performed to discharge electrons in the photodiode and the floating diffusion region A reset input, a drive transistor Dx serving as a source follower buffer amplifier, and a select transistor Sx for addressing through a switching role. 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. The reset transistor was turned on to remove the electrons remaining in the photodiode. That is, a reset transistor is provided for each unit pixel and used in a reset operation. However, 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 receiving a pulse signal.

In the unit pixel of the CMOS image sensor according to an embodiment of the present invention, a positive voltage (for example, 5 volts or 3.3 volts is applied) is input as a reset input and remains in the photodiode during a reset operation. Remove the electrons.

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.

FIG. 2B is a diagram illustrating a unit pixel composed of two transistors and a photodiode by replacing a reset transistor with a reset input according to an embodiment of the present invention. Has been replaced with a reset input and the other parts are 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, the reset transistor is reset. The load resistor is formed to read an output signal outside the unit pixel.

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, and a positive voltage (for example, 5 volts or 3.3 volts is applied) during the reset operation. It is applied to remove the electrons remaining in the photodiode, and the floating state other than the reset operation can replace the role of the conventional reset transistor.

In one embodiment of the present invention, the reset transistor is removed and replaced with a reset input for inputting a pulse signal, thereby reducing the area of the unit pixel, thereby facilitating the simplification of the unit pixel configuration and the high integration of the device. By removing, the fill factor, which is an area ratio occupied by the photodiode in the unit pixel, increases, thereby improving image characteristics. In addition, if the high integration of the device is made according to the present invention can reduce the manufacturing cost. The circuit for generating the pulse signal can be simply implemented by using a plurality of inverters having different sizes.

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 the reset input, the unit pixel configuration is simplified, which is advantageous for high integration of the device, and the manufacturing cost is reduced.

Claims (3)

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 connected to one side of the transfer transistor and the floating diffusion region, the reset input receiving a pulse signal to reset the photodiode; 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 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. 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 and receiving a pulse signal to reset the photodiode; 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.