ASYMMETRIC RESET TRANSISTOR

Technical Field

[1] The present invention relates to an image sensor, and more particularly, to a unit pixel having a 3-transistor structure.

Background Art

[2] Generally, pixels used in CMOS (Complementary Metal-Oxide Semiconductor) image sensors have a 3-transistor structure or a 4-transistor structure.

[3] FIG. 1 illustrates a unit pixel having the 3-transistor structure.

[4] As illustrated in FIG. 1, the 3-transistor structure includes a photo diode sensing an image, a source follower converting the image into an electrical signal, a reset transistor resetting the photo diode, and a line selecting transistor LS selecting a line.

[5] In order to build an image sensor in a camera phone, the size of the image sensor should be reduced to a chip size and the size of a unit pixel should be accordingly reduced. The decrease in size of the unit pixel is more advantageous in the 3-transistor structure than in the 4-transistor structure. In the 3-transistor structure, since a source region of the reset transistor connecting the photo diode to the source follower is formed by implanting heavy ions into the silicon surface with a high dose, leakage current and dead pixels are caused.

[6] FIG. 2 illustrates a physical structure of the reset transistor of the unit pixel having a conventional 3-transistor structure.

[7] Since the drain region 210 and the source region 220 of the reset transistor RT are designed to have the same structure, noise is increased in the unit pixel due to implanted heavy ions, thereby deteriorating image quality remarkably.

[8] In other words, in the unit pixel structure having the 3-transistor structure, heavy ions with a high dose should be implanted for a connection between the photo diode and the source follower. Accordingly, leakage current due to implant damage is generated, thereby causing deterioration in image quality due to increase in dark current and the number of dead pixels. Disclosure of Invention

Technical Problem

[9] In order to solve the aforementioned problems, an object of the present invention is to provide a unit pixel of an image sensor having an asymmetric reset transistor in which deterioration in image quality due to increase in noise and implant damage can be prevented.

Technical Solution

[10] A ccording to an aspect of the present invention, there is provided a unit pixel of a

CMOS (Complementary Metal-Oxide Semiconductor) image sensor having an asymmetric reset transistor, the asymmetric reset transistor comprising: a drain region in which heavy ions with a first dose are implanted; and a source region in which heavy ions with a second dose lower than the first dose are implanted, wherein the source region and the drain region are asymmetric.

Description of Drawings

[11] FIG. 1 illustrates a pixel structure of an image sensor having a 3-transistor structure;

[12] FIG. 2 illustrates a physical structure of a conventional reset transistor;

[13] FIG. 3 illustrates a physical structure of a unit pixel of an complementary metal- oxide semiconductor (CMOS) image sensor having an asymmetric reset transistor according to an embodiment of the present invention; and

[14] FIG. 4 illustrates a physical structure of a unit pixel of a CMOS image sensor having an asymmetric reset transistor according to another embodiment of the present invention.

Best Mode

[15] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[16] FIG. 3 illustrates a physical structure of a unit pixel of a CMOS (Complementary

Metal-Oxide Semiconductor) image sensor having an asymmetric reset transistor according to an embodiment of the present invention.

[17] The drain region 310 of the reset transistor is formed by implanting heavy ions with a first dose N++ and the source region 320 thereof is formed by implanting heavy ions with a second dose N+ lower than the first dose N++.

[18] In other words, implant damage can be minimized by implanting ions with a lower dose N+ in the source region using a predetermined mask so that the source region has a structure different from that of the drain region. In addition, the surface noise occurring from a silicon surface below a side wall can be removed by not forming an LDD (Lightly Doped Drain).

[19] FIG. 4 illustrates a physical structure of a unit pixel of a CMOS image sensor having an asymmetric reset transistor according to another embodiment of the present invention.

[20] Impurity ions with a third dose N- lower than the second dose and with higher energy are implanted in the source region of FIG. 3A in which the heavy ions have been implanted with the second dose N+ lower than the first dose N++, thereby

reducing an electric field.

[21] Accordingly, since the source region 320 includes the N- region 325 in which impurity ions have been implanted with the lower dose, a photon efficiency can be increased to enhance sensitivity by enlarging a photo diode region PD to increase Qsat

[22] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Industrial Applicability

[23] According to the present invention, it is possible to reduce pixel pattern noise and thus to enhance the image quality by implanting impurity ions in the source region of the reset transistor with the dose lower than that of the drain region.