Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Charge transfer device
Reexamination Certificate
2002-02-12
2003-07-08
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Charge transfer device
C257S215000
Reexamination Certificate
active
06590238
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of image sensors and, more particularly, to such image sensors having a capacitive control gate for eliminating an undesired capacitive effect caused by a reset gate on a floating diffusion and controlling the capacitance of a floating diffusion.
BACKGROUND OF THE INVENTION
As shown in
FIGS. 1 and 2
, prior art charge-coupled devices
10
typically include a substrate
20
of a p-type having a buried channel
30
of the n-type for transferring charge packets of electrons. A plurality of gates
40
(two gates for the two-phase device shown in
FIG. 1
) are connected to the buried channel
30
for controlling charge packet transfer in the buried channel
30
. A non-clocked output gate
50
is positioned between one of the gates
40
a
and a floating diffusion
60
for preventing capacative coupling of the last clocked gate
40
a
to the floating diffusion
60
.
The floating diffusion
60
provides a mechanism for sensing the size of the charge packet for subsequent measurement and the like. A reset transistor
70
provides a mechanism for resetting the voltage level of the floating diffusion after appropriate sampling. A reset drain
80
is adjacent the reset transistor
70
for receiving drained charge from the floating diffusion
60
, as is well known in the art.
Referring to
FIG. 3
, a typical timing sequence of the charge-coupled device
10
is shown. At time T
0
, the reset gate
70
is clocked high. This turns on the reset transistor
70
and resets the floating diffusion
60
to the reference voltage of the reset drain
80
. Then at time T
1
, the reset gate
70
is clocked low. Capacitive coupling (represented by C
reset
) between the reset gate
70
and the floating diffusion
60
causes the voltage of floating diffusion
60
to be pushed to a more negative voltage when the reset gate
70
is turned off. The floating diffusion
60
voltage then remains stable and is sampled at time T
2
. Then at time T
3
, the gates
40
are clocked which changes their levels and transfers a new charge packet over the output gate
50
and onto the floating diffusion
60
. The magnitude of the voltage change at time T
3
on the floating diffusion
60
is proportional to the size of the charge packet and the capacitance of the floating diffusion
60
. The relationship is given by V=Q/C where V is the voltage change, Q is the size of the charge packet (coulombs), and C is the capacitance of the floating diffusion (farads). The new voltage on the floating diffusion
60
is sampled at time T
4
and then the timing cycle is repeated.
Although the currently known and utilized sensor and associated method for transferring charge is satisfactory, it includes drawbacks. The shortcoming of the output structure is the capacitive coupling of the reset gate to the floating diffusion, which causes the voltage glitch between times T
0
and T
1
on the amplifier input node. This short glitch also restricts the type of electronics that are used to process the output signal of the CCD. Another disadvantage of the prior art is the voltage change of the floating diffusion is fixed to a constant value by the floating diffusion capacitance. Consequently, it is desirable to have a structure, which allows the voltage gain to be changed for some means of altering the floating diffusion capacitance, and which eliminates undesired capacative coupling of the reset gate
70
.
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 is a charge-coupled device having a plurality of cells for forming the charge-coupled device, each of the cells capable of retaining charge; a transfer mechanism within the charge-coupled device for moving charge through the plurality of cells; an output region for receiving charge moved through the plurality of cells under control of the transfer mechanism; a floating diffusion to receive charge moved across the output region; a reset gate to remove charge from the floating diffusion and reset the floating diffusion to a reference voltage level; and a capacitance control gate adjacent to the floating diffusion for canceling capacitance coupling of the reset gate. A capacitance control gate covers a portion of the floating diffusion and the voltage of the capacitance control gate is adjusted to alter the capacitance of the floating diffusion. The capacitance control gate is clocked opposite that of the reset gate to cancel the capacitive effects of the reset 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 producing a smoother waveform going to the CCD amplifier input, which is easier for the electronics of the camera to process. The present invention also provides a means for changing the effective gain of the CCD output.
REFERENCES:
patent: 5495441 (1996-02-01), Hong
patent: 5612554 (1997-03-01), Funakoshi
Dang Phuc T.
Eastman Kodak Company
Nelms David
Watkins Peyton C.
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