Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-02-02
2001-11-20
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S239000
Reexamination Certificate
active
06320175
ABSTRACT:
This application claims the benefit of Korean Applicationi No. 18888/1999 filed May 25, 1999, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CCD (Charge Coupled Device), and more particularly, to a structure of a signal detecting part for a CCD with an improved signal sensitivity.
2. Discussion of the Related Art
In general, a CCD (Charge Coupled Device) is an image sensor which converts an image signal into an electrical signal. It is also called a “CCD image sensor” as the CCD uses a charge coupling in scanning and reading the image signal. The CCD is provided with a photodiode (PD) for converting an image signal into an electrical signal, a VCCD (vertical CCD) for transferring a signal charge converted by the photodiode (PD) in a vertical direction, an HCCD (Horizontal CCD) for transferring the charge transferred from the VCCI) in a horizontal direction, and a sense amplifier (SA) for sensing the signal charge transferred from the HCCD.
In order to improve a sensitivity of the CCI), different methods are suggested. One example is a method for improving a conversion ratio, in which a parasitic capacitance of a signal detecting part is reduced to increase a voltage conversion ratio which can be obtained from a signal charge. Another example is a method for improving a filter factor using a microlens. Because the method for improving a conversion ratio is more fundamental than the method for improving a filter factor of the microlens and the like, the improvement of the conversion ratio should first be considered.
In order to increase a conversion ratio, a parasitic capacitance between a floating diffusion (FD) region and a gate of a transistor in the sense amplifier should be first reduced. Such a parasitic capacitance is dependent on (1) design criteria, such as an FD area and a size of the transistor in the sense amplifier, (2) layout factors, and (3) process conditions. Consequently, in order to minimize a parasitic capacitance of the signal detecting part, different methods for designing a layout are suggested to minimize the amount of overlap between the FD and a metal or polysilicon layer connecting the gate of the transistor in the sense amplifier.
A CCD according to a related art will be explained with reference to the attached drawings.
FIG. 1
illustrates an equivalent circuit of a signal detecting part for the CCD according to the related art.
As shown in
FIG. 1
, the signal detecting part for the CCD according to related art is provided with an N-type BCCD region
12
on a P-well
11
or a P-type substrate. A reset gate
14
is formed over the N-type BCCD region
12
with a gate insulating film
13
therebetween. An output gate
15
is also formed on the N-type BCCD region, spaced apart from the reset gate
14
. A floating diffusion region
16
is formed between the reset gate
14
and the output gate
15
, and a reset drain region
17
is formed on one side of the reset gate
14
opposite to the floating diffusion region
16
. A first transistor M
1
is controlled by a voltage detected at the floating diffusion region
16
, and a second transistor M
2
is connected to the first transistor M
1
in series. The first transistor M
1
and the second transistor M
2
have source followers. In an ideal case, the second transistor M
2
becomes a constant current source. Accordingly, an output current is constant when an output voltage Vout is constant. The source of the first transistor M
1
always follows a variation of the voltage provided to the gate.
FIG. 2
illustrates a layout of a signal detecting part for a CCD according to the related art. As show in
FIG. 2
, the layout of the related art signal detecting part shows the gate
21
of the first transistor M
1
disposed perpendicular to the floating diffusion region
16
. A source region
22
and a drain region
23
are disposed on both sides of the gate
21
of the first transistor M
1
. A metal layer
24
is disposed on the floating diffusion region
16
for electrical connection between the floating diffusion region
16
and the gate
21
of the first transistor M
1
. An output gate
15
and a reset gate
14
are disposed on both sides of the floating diffusion region
16
. A reset drain
17
is disposed on one side of the reset gate
14
opposite to the floating diffusion region
16
.
In the foregoing related art CCD, signal charges generated by incident lights pass through the HCCD and are collected at the floating diffusion region
16
. The amount of signal charges collected at the floating diffusion region
16
is Q, providing a voltage V according to Q=C×V. When a high voltage is provided to the reset gate
14
, the charges at the floating diffusion region
16
are transferred to the reset drain
17
, emptying the floating diffusion region
16
of signal charges and permitting reception of new signal charges. However, for a layout in which the reset gate
14
is disposed as close to the floating diffusion region
16
as possible, defining a source region
22
of the first transistor M
1
is difficult because of the limited space available.
Therefore, as shown in
FIG. 3
, a method for designing a layout is suggested, in which the reset gate
14
is disposed farther away from the floating diffusion region
16
for securing an adequate space for defining the source region
22
. However, the layout of
FIG. 3
has a larger floating diffusion region
16
, resulting in a reduction in the conversion ratio.
Therefore, the signal detecting part for a CCD according to the related art has the following problem. Even though different methods arc suggested to reduce a parasitic capacitance of the signal detecting part by changing a layout, there has been a limitation in reducing the parasitic capacitance of the signal detecting part because the related art layout is determined according to a design rule of a process.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a structure of a signal detecting part for a CCD that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a structure of a signal detecting part for a CCD which results in a reduced parasitic capacitance of a signal detecting part.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve and these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a signal detecting apparatus in a charge coupled device (CCD), which has a photodiode for converting an image signal into an electrical signal charge, a vertical CCD (VCCD) for transferring the signal charge in a vertical direction, a horizontal CCD (HCCD) for transferring the vertically transferred signal charge in a horizontal direction, and a sense amplifier for sensing the signal charge transferred from the HCCD, comprises: a floating diffusion region for collecting the signal charges transferred from the HCCD and detecting a voltage formed by the signal charges; a reset gate on one side of the floating diffusion region; a reset drain on one side of the reset gate opposite to the floating diffusion region; an output gate formed on the other side of the floating diffusion region; and a transistor having a gate connected to and formed at an angle with respect to a primary axis of the floating diffusion region the angle being greater than 0° and less than 90°.
In another aspect of the present invention, a signal detecting apparatus in a charge coupled device (CCD), which has a photodiode for converting an image signal into an electrical signal charge, a vertical CCD (
Hyundai Electronics Industries Co,. Ltd.
Le Que T.
Morgan & Lewis & Bockius, LLP
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