Reduced dark current for CMOS image sensors

Details Reduced dark current for CMOS image sensors Reduced dark current for CMOS image sensors

2003-01-10

2004-05-04

Pyo, Kevin (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S214100, C257S292000

Reexamination Certificate

active

06730899

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the field of CMOS image sensors and, more particularly, to such image sensors having transfer gates having regions of enhanced conductivity in the substrate as defined by the area of the transfer gate.
BACKGROUND OF THE INVENTION
Referring to
FIG. 1
, prior art CMOS image sensors
10
are solid state imagers built in a substrate
5
wherein each pixel typically contains a photo-sensing region
20
, a reset transistor
30
, a charge transfer region
40
, a transfer gate electrode
45
separated from the substrate
5
by an insulating layer
42
, a charge to voltage conversion region
50
, and an amplifier
60
. In these CMOS sensors, dark current is generated in several regions of the device. Such prior art devices have addressed dark current reduction in the photo-sensing region
20
. Another region of dark current generation is the charge transfer region
40
.
With typical processing and design, and due to short-channel effects, the charge transfer region
40
of current day CMOS image sensors is basically the channel region of a “normally on”, or depletion-mode type NMOSFET. Unlike CCD based imaging systems, typical power supplies used in these CMOS systems are unipolar, (e.g.,+3.3V). As such, the “off” state voltage on the transfer gate
45
of the CMOS sensor
10
is limited to a minimum of 0V (ground). Therefore, since the transfer gate's surface is not “turned off”, a relatively high amount of dark current is generated in this region. This dark current, or a portion thereof, may leak into the photodiode during the integration period causing excess noise, thereby limiting the imaging performance of the device. To avoid this, it is desirable to provide a charge transfer region
40
whose surface region will be accumulated (with holes) in the off state (Vg=0V) so as to quench this excess dark-current generation.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a CMOS image sensor comprising (a) a substrate of a first conductivity type; (b) a photodctector for capturing incident light and converting it to a charge; (c) a transfer gate for passing the charge from the photodetector; and (d) a region of the first conductivity type of enhanced conductivity in the substrate which extends substantially along an entire length and width of the transfer gate.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
Advantageous Effect Of The Invention
The present invention has the following advantages of reducing dark current generated in the area of the transfer gate of a CMOS image sensor for reduced noise, increased dynamic range and overall improvement in image quality.


REFERENCES:
patent: 6100551 (2000-08-01), Lee et al.
patent: 6184055 (2001-02-01), Yang et al.

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