Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1997-10-16
2002-11-19
Moe, Aung S. (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S308000, C348S241000
Reexamination Certificate
active
06483541
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a solid state imaging device, a signal processing method and a driving method therefor and a camera, and more particularly to an X-Y address type solid state imaging device as represented by an amplification type solid state imaging device and a signal processing method for the solid state imaging device as well as a camera which employs an X-Y address type solid state imaging device as an imaging device. The present invention further relates to an amplification type solid state imaging device wherein pixels themselves have an amplification function and signals of the pixels are outputted as voltages and a driving method for the amplification type solid state imaging device.
An X-Y address type solid state imaging device includes, as shown in
FIG. 14
, a pixel section
111
wherein a large number of pixels are arranged in rows and columns, a vertical scanning circuit
112
for successively selecting the rows of the pixel section
111
, a horizontal scanning circuit
113
for successively selecting the columns of the pixel section
111
, and an output circuit (charge detection circuit)
114
for outputting a signal. The vertical scanning circuit
112
and the horizontal scanning circuit
113
are each formed from, for example, a shift register and successively generate a vertical scanning (vertical selection) pulse signal &phgr;V and a horizontal scanning pulse signal &phgr;H one by one for each row and each column, respectively.
In this X-Y address type solid state imaging device, when the horizontal scanning circuit
113
holds signal charge of the pixels in capacitors and outputs the pixel signals of the capacitors to the output circuit
114
from a horizontal signal line via horizontal switches each formed from a MOS transistor, dispersions in threshold voltage Vth of the horizontal switches (MOS transistors) are superposed on the pixel signals, and they appear as vertical string-like fixed pattern noises on a screen, deteriorating the picture quality.
Here, a generation mechanism of vertical string-like fixed pattern noises generated from the horizontal scanning circuit
113
is described with reference to an equivalent circuit of a signal path for the nth column of the horizontal scanning circuit
113
shown in FIG.
15
. As a presupposition of description, notice is taken here of a dispersion in threshold voltage Vth of a horizontal switch.
Referring to
FIG. 15
, a pixel signal held by a capacitor
121
flows as charge to a horizontal signal line
123
when a horizontal scanning pulse &phgr;Hn is supplied from a horizontal shift register (not shown) to the gate electrode of a horizontal switch
122
so that the horizontal switch
122
is put into a connecting state, and is then demodulated into a voltage by and outputted from an output circuit
124
. In this instance, if the threshold voltage Vth which defines the boundary between a disconnecting state and a connecting state of the horizontal switch
122
exhibits a dispersion for each of the horizontal switches of the individual columns, then charge represented by the product of the dispersion in threshold voltage Vth and a variation amount in capacitance generated between the horizontal switch
122
and the horizontal signal line
123
appears on the horizontal signal line
123
. Therefore, a vertical string-like fixed pattern noise which corresponds to the charge amount is superposed with the pixel signal.
The manner of such superposition is described with reference to
FIGS. 16A and 16B
which show equivalent circuits where the horizontal switch
122
is converted into a capacitance model. More particularly,
FIG. 16A
shows an equivalent circuit where the horizontal switch (MOS transistor)
122
is in a disconnecting (off) state, and FIG.
16
B shows an equivalent circuit where the horizontal switch (MOS transistor)
122
is in a connecting (on) state.
In
FIG. 16A
, the horizontal switch
122
is in a disconnecting state and, as a capacitance model, a gate-drain capacitance
122
a
is produced between the gate electrode of a horizontal switch (MOS transistor) to which a horizontal scanning pulse &phgr;Hn is applied and the capacitor
121
by which a pixel signal is held while a gate-source capacitance
122
b
is produced between the gate electrode of the horizontal switch and the horizontal signal line
123
, and the capacitor
121
and the horizontal signal line
123
are disconnected from each other.
On the other hand, in
FIG. 16B
, the horizontal switch
122
is in a connecting state and the capacitor
121
is connected to the horizontal signal line
123
, and a gate-channel capacitance
122
c is produced between the gate electrode of the horizontal switch (MOS transistor) to which a horizontal scanning pulse &phgr;Hn is applied and the horizontal signal line
123
. Here, the capacitance of the gate-channel capacitance
122
c
is considerably higher than the total capacitance of the capacitance
122
a
and the capacitance
122
b.
Since the two states of
FIGS. 16A and 16B
are changed over with reference to the threshold voltage Vth of the horizontal switch
122
by the voltage of the horizontal scanning pulse &phgr;Hn applied to the gate electrode of the horizontal switch
122
, if the horizontal switch of each column has a dispersion in threshold voltage Vth, then the product of the dispersion in threshold voltage Vth and the difference between the capacitances of the horizontal switch
122
in the two states of
FIGS. 16A and 16B
appears as dispersion charge on the horizontal signal line
123
and makes a vertical string-like fixed pattern noise.
Now, where the capacitances of the capacitance
122
a
, capacitance
122
b
and capacitance
122
c
are represented by Cdg, Cgs and Cg, respectively, the dispersion of the threshold voltage Vth of the horizontal switch
122
is represented by &Dgr;Vth, the dispersion charge appearing on the horizontal signal line
123
is represented by &Dgr;q, the capacitance of a detection capacitor
125
of the output circuit
124
is represented by Cd, and a vertical string-like fixed pattern noise appearing on the output is represented by &Dgr;Vout, the dispersion charge &Dgr;q and the fixed pattern noise &Dgr;Vout are given by
&Dgr;q=(Cg−Cgd−Cgs)·&Dgr;Vth
&Dgr;Vout=&Dgr;q/Cd
Particularly, giving an example of numerical values, if Cgd and Cgs are 1 fF, Cg is 20 fF, the dispersion &Dgr;Vth of the threshold voltage Vth is 50 mV and the capacitance Cd of the detection capacitor
125
is 0.5 pF, then the fixed pattern noise &Dgr;Vout is 1.8 mv.
Driving timings of the ordinary X-Y address type solid state imaging device and a manner in which vertical string-like fixed pattern noises appear are illustrated in a timing chart of
FIG. 17. A
vertical scanning pulse signal &phgr;V (&phgr;V
1
, . . . , &phgr;Vm, &phgr;Vm+1, . . . ) for selecting pixel elements of the same row successively rises for each horizontal blanking period, and an operation pulse signal &phgr;OP rises in synchronism with the vertical scanning pulse signal &phgr;V. The operation pulse signal &phgr;OP is applied to the gate electrode of an operation switch (not shown) formed from a MOS transistor for reading out a pixel signal to the capacitor
121
.
As the operation pulse signal &phgr;OP rises, pixel signals of a selected row are read out into the capacitors
121
. The pixel signals of the certain row held in the capacitors
121
are, when a horizontal image period is entered, read out from the output circuits
124
as the horizontal switches
122
are successive put into a connecting state when the horizontal scanning pulse signal &phgr;H (&phgr;H
1
, . . . , &phgr;Hn, &phgr;Hn+1, . . . ) outputted from the horizontal shift register successively rises.
In this instance, if it is assumed, for example, that an equal signal amount is outputted from all pixels and only the threshold voltages Vth of the horizontal switches
122
have individual dispersions, then as seen in the timing chart of
FIG. 17
, an output signal OUT d
Ueno Takahisa
Yonemoto Kazuya
Moe Aung S.
Sonnenschein Nath & Rosenthal
Sony Corporation
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