Photoelectric conversion device with stabilizing electrode

A photoelectric conversion device has a semiconductor region for accumulating the carriers generated by photoexcitation which reads the signals according to the accumulated carriers in said semiconductor region. A stabilizing electrode is provided above the surface where the semiconductor region is formed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photoelectric conversion device having a 
semiconductor region for accumulating carriers generated by 
photoexcitation, which device reads the signals according to the 
accumulated carriers in said semiconductor region. 
2. Related Background Art 
FIG. 1(A) is a schematic sectional view of one example of the photoelectric 
conversion device of the prior art, and FIG. 1(B) is its equivalent 
circuit diagram. 
In each Figure, photoelectric conversion cells are arranged on n silicon 
substrate 101, and each cell is electrically insulated from adjacent cells 
with an element separating device (not shown) consisting of Si0.sub.2, 
Si.sub.3 N.sub.4 or polysilicon, etc. 
Each cell has the constitution as described below. 
In the n.sup.- region 102 with low impurity concentration formed by 
epitaxial technique, etc., a p base region 103 and a n.sup.+ emitter 
region 104 are formed to constitute a bipolar transistor. Further, on the 
p base region 103, a capacitor electrode 106 is formed with an oxide film 
105 sandwiched therebetween to constitute a capacitor Cox for controlling 
base potential as opposed to the p base region. 
Also, p.sup.+ regions 107 and 108, and a gate electrode with the oxide film 
105 sandwiched therebetween are formed to constitute a PMOS transistor for 
performing refresh actuators. 
Otherwise, there are formed an emitter electrode 110 connected to the 
n.sup.+ emitter region 104, an electrode 111 connected to the p.sup.+ 
region 108, and a collector electrode 112 on the back of the substrate 110 
with an ohmic contact layer sandwiched therebetween, respectively. 
Next, the actuation of the above photoelectric conversion device will now 
be described. 
Light is incident from the side of the p base region 103. Carriers (here 
holes) corresponding to the dose of incident light are accumulated in the 
p base region (accumulated actuation). 
The base potential is changed by the carriers accumulated, and by reading 
its potential change from the emitter electrode 110, electrical signals 
corresponding to the incident dose can be obtained. Specifically, the 
emitter electrode 110 is maintained under floating state with a positive 
voltage being applied on the capacitor electrode 106. By this, the base 
potential is elevated to apply 1 bias in the ordinary direction between 
the base the emitter, and the accumulated voltage in the base being read 
from the emitter side (reading actuation). Even when the reading actuation 
may be completed, since the accumulated carriers in the p base region are 
not substantially reduced, the same signal can be read repeatedly 
(non-destructive reading). 
To perform a refresh actuation which eliminates the holes accumulated in 
the p base region 103, the emitter electrode 110 is grounded and 
simultaneously the electrode 111 is maintained at a constant potential. 
At first, a negative voltage is applied on the gate electrode 109 to turn 
on the pMOS transistor Qc. As a result the potential in the p base region 
103 becomes a constant value regardless of whether the accumulated 
potential is high or low. 
Subsequently, by applying a positive pulse for refresh on the capacitor 
electrode 106, a bias is applied in the ordinary direction between the 
base and the emitter as a result the accumulated holes are eliminated 
through the grounded electrode 110. When the refresh pulse has risen, the 
p base region 103 is returned to the initial state of a negative potential 
(refresh actuation). 
Thus, after the potential in the p base region 103 is made at a constant 
potential by the MOS transistor Qc, a refresh pulse is applied to erase 
the residual charges, and therefore fresh accumulation can be effected 
without dependence on the accumulated potential of the previous time. 
Also, the residual charges can be extinguished rapidly, whereby high speed 
actuation is rendered possible. 
Thereafter, the respective actuations of accumulation, reading, refresh are 
similarly repeated. 
The capacitor Cox may be sometimes not required to be provided, and 
photoelectric conversion reading can be performed also in the case of a 
photoelectric conversion cell in which only the transistor Qc is 
electrically connected to the base region 103. 
However, the photoelectric conversion device of the prior art as described 
above has the following problems. 
The surface of the base region 103 has a relatively lower impurity 
concentration for realizing a high Hfe, and therefore was susceptible to 
the influence from the potential on the cell surface through the oxide 
film 105 to be unstable. 
Further, the surface recombination current is generated from the silicon 
surface, and this current is greatly dependent on the surface potential. 
For this reason, in the photoelectric conversion device having a base 
region 103 with unstable surface, dark current caused by the above surface 
recombination current is greatly different between the cells to become a 
cause for noise. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a 
photoelectric conversion device, which can stabilize electrically the 
surface layer which generates surface recombination current and can obtain 
photoelectric conversion signals without variance. 
Another object of the present invention is to provide an improved 
photoelectric conversion device which has solved the problems possessed by 
the photoelectric conversion device of the prior art as described above. 
Still another object of the present invention is to provide a photoelectric 
conversion device which can effect photoelectric conversion constantly 
stably without receiving the influence from the potential of the cell 
surface. 
Still another object of the present invention is to provide a photoelectric 
conversion device with the dark current substantially unchanged between 
the cells and substantially without noise. 
Still another object of the present invention is to provide a photoelectric 
conversion device having a semiconductor region for accumulating the 
carriers generated by photoexcitation which reads the signals according to 
the accumulated carriers in said semiconductor region, characterized in 
that a stabilizing electrode is provided above the reference where said 
semiconductor region is formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the photoelectric conversion device of the present invention, by 
providing a stabilizing electrode above the surface of the semiconductor 
region for accumulating the carriers generated by photoexcitation and 
maintaining the stabilizing electrode at a constant potential, the surface 
potential in the above semiconductor is stabilized. 
By making the above stabilizing electrode at a constant potential, the 
surface potential of the above semiconductor region can be stabilized to 
inhibit the influence from an external disturbance. Accordingly, 
generation of the dark current can be inhibited, and the noise of reading 
signals due to variance in the dark current can be inhibited. 
Also, even if the above stabilized electrode may be maintained in the 
floating state, provided that it has sufficiently greater area as compared 
with the photoelectric conversion device, the surface potential of the 
above semiconductor region can be sufficiently stabilized. 
As the above stabilizing electrode, for example, a transparent electrode 
such as ITO, SNO.sub.2, etc. or polysilicon, etc. can be used. 
By use of a transparent electrode, the above effect can be obtained without 
changing the photoelectric conversion characteristics in the visible light 
region. 
By use of a polysilicon, a photoelectric conversion device for detection of 
IR-ray can be obtained as described below. In this case, since there is 
conformity with the conventional semiconductor process, there is the 
advantage that preparation can be simpler as compared with the case of 
using a transparent electrode. 
Now, preferred embodiments of the present invention are described in detail 
by referring to the accompanying drawings. 
FIG. 2(A) is a schematic plan view of a first preferred embodiment of the 
photoelectric conversion device according to the present invention, FIG. 
2(B) a cross-sectional view cut along its I--I line and FIG. 2(C) a 
partial cross-sectional view cut along its II--II line. The equivalent 
circuit of this embodiment is the same as shown in FIG. 1(B). 
The bipolar transistor for effecting photoelectric conversion is 
constituted of a n.sup.- collector region 2, a p base region 6 and n.sup.+ 
emitter region 12, and the capacitor electrode 10 for performing the 
respective actuations by controlling the base potential is opposed to the 
p base region 6 with an oxide film 8 sandwiched therebetween. The 
respective actuations of accumulation, reading and refresh in this 
embodiment are as already described. 
FIG. 3 is a graph showing the impurity profile in the p base region 6. 
The p base region is formed by the ion implantation method as described 
below, and it is formed at a lower concentration than the base 
concentration in conventional bipolar transistor for ensuring 
non-destructive readability. 
The impurity profile shown in FIG. 3 is that of the base region 6 which is 
formed at a dose amount of 1.times.10.sup.12 to 10.sup.13 /cm.sup.2, 
followed by completion of all the subsequent steps. 
As shown in the same graph, the concentration peak occurs at the position 
deeper than the surface, and the surface becomes a state approximate to a 
genuine semiconductor. 
However, in the state of the silicon surface approximate to a genuine 
semiconductor, surface recombination current shown by Schockley-Read-Hall 
theory becomes is liable to be generated. Such surface recombination 
current becomes the dark current in a photoelectric conversion device to 
cause deterioration of characteristics to occur. Besides, since the 
recombination current depends greatly on the potential of the surface, it 
is strongly subject to influence from external disturbance, etc. to appear 
as a variance in the dark current, and therefore variance in reading 
signals. 
Accordingly, by utilizing the fact that recombination current depends on 
surface potential, in this embodiment, a stabilizing electrode 11 for 
stabilizing the dark current by controlling the potential on the silicon 
surface is provided. 
The stabilizing electrode 11, as shown in FIGS. 2(A) and (C), is provided 
on a part of the surface of the p base region 6 and the surface of the 
n-collector region 2 therearound. In this embodiment, the material for the 
stabilizing electrode 11 is a transparent electrode material such as ITO, 
SnO.sub.2, etc. 
FIG. 4 is a graph showing the change in the dark current versus potential 
of the stabilizing electrode 11. 
As show in this graph, under the no control state (0 V) where no voltage is 
applied on the stabilizing electrode 11, dark current is greatly changed 
by an external disturbance, but when the surface potential is stabilized 
by maintaining the control voltage at -1 V or less, the dark current is 
inhibited to a small valve, and without change due to an external 
disturbance. By actuation under this state, reading signals with little 
variance can be outputted stably. 
Also, even if the stabilizing electrode 11 may be maintained at the 
electrically floating state without being maintained at a constant 
voltage, provided that the surface is made sufficiently greater as 
compared with the cell region, it is possible to stabilize sufficiently 
the silicon surface relative to an external disturbance. 
In the following, the preparation method of the present invention is 
described. 
FIGS. 5(A)-(D) illustrate schematically the preparation steps showing a 
part of the preparation steps of the embodiment shown in FIGS. 2(A) to 
(C). 
As shown in FIG. 5(A), n.sup.- region 2 with a thickness of 5 to 10 .mu.m 
is formed by epitaxial growth on the n-type silicon substrate 1. 
Subsequently, after formation of the oxide film 8a by oxidation of the 
surface, the oxide film 8a where p well region is desired to be formed is 
removed by patterning. 
Subsequently, with the oxide film 8a as the mask, boron ions are implanted 
and the p well region 3 is formed by diffusion by heat treatment. The p 
well region 3 is a region necessary for formation of NMOS of the 
circumferential circuit of the photoelectric conversion portion. 
Next, after the oxide film 8a was removed from the whole surface, oxidation 
is again effected, and further by selective oxidation, etc. by use of a 
silicon nitride film of 500 to 2000 .ANG., a n.sup.+ channel stop region 4 
and a thick oxide film 5 are selectively formed as shown in FIG. 5(B). 
Subsequently, after coating of a resistor, the portion where the base 
region is desired to be formed is removed by patterning. Boron ions are 
then implanted with the resist as the mask, and the p base region 6 is 
formed by heat treatment. 
The dose during the ion implantation is about 1.times.10.sup.12 to 
10.sup.14 /cm.sup.2, and the surface concentration about 10.sup.15 to 
10.sup.19 /cm.sup.3. 
The impurity profile is as shown in FIG. 3. Thus, by use of boron as the 
impurity, although the flight of ions is set toward the oxide film side 
from the interface between the oxide film which is the buffer layer and 
the n.sup.- region 2, the peak with concentration N is formed through 
local precipitation of boron ions, whereby the surface becomes the state 
approximate to genuine semiconductor. 
At the stage when the p base region 6 is formed, the thickness of the oxide 
film 8 formed thereon is about 1500 to 4000 .ANG.. 
Next, as shown in FIG. 5(C), n.sup.+ region 7 is formed by ion implantation 
or phosphorus deposition with POCl.sub.3, etc. Through the n.sup.+ region 
7, electrical connection to n.sup.- region 2 can be obtained, and it also 
becomes the portion for forming the collector necessary for reading 
circuit. 
Subsequently, after the portion constituting the capacitor Cox and the 
portion of MOS transistor Qc, a thin gate oxide film with a thickness of 
about 150 to 600 .ANG. is formed. And, a material such as a polysilicon, a 
high melting metal or metal silicide, etc. is deposited and subjected to 
patterning to form a gate electrode 9 and a capacitor electrode 10. 
The capacitor Cox is not necessarily required to be provided, but the 
capacitor Cox may be provided, if desired. 
Subsequently, a material such as ITO, SnO.sub.2, etc. is deposited 
according to the CVD method, the sputtering method, etc., followed by 
patterning to form a stabilizing electrode. The stabilizing electrode, as 
shown in FIGS. 2(A) and (C), may cover a part of the p-base region and a 
part of the surface of the n.sup.- region 2, or the whole region or only 
one of the regions. 
As described above, the gate electrode 9 and the capacitor electrode 10 
should desirably be formed prior to formation of the stabilizing electrode 
from the standpoint of ensuring the characteristics at the interface 
between the gate and the gate film, but it is also possible to form them 
in the reverse order. 
Next, as shown in FIG. 5(D), according to the method such as ion injection, 
etc., after formation of the n.sup.+ emitter region 12 and the p.sup.+ 
region 13 which is the source-drain region of the PMOS transistor Qc, an 
insulating film 20 such as SiO.sub.2, PSG, BPSG, etc. is formed according 
to the CVD method. The insulating film 20 is then subjected to patterning 
to open a contact hole, and the emitter electrode 12a, the electrode 14 
for the transistor Qc and the electrode 15 connected to the stabilizing 
electrode 11, and other electrodes are respectively formed. 
Subsequently, an interlayer insulating film 16 such as silicon nitride 
film, PSG, etc., a light-intercepting film 17 and a passivation film 18 
are successively formed to obtain a structure as shown in FIG. 3(B). 
FIG. 6 is a schematic illustration of one step in the preparation steps of 
a second preferred embodiment of the present invention. 
This embodiment is intended primarily to detection of IR-ray, and therefore 
the material of the stabilizing electrode 11 is not required to be 
transparent in the visible light region. In this embodiment, a polysilicon 
was employed. 
A polysilicon with a thickness of about 0.4 .mu.m can permit only about 6% 
of blue light (.lambda.=4500 .ANG.) to transmit therethrough, but can 
permit about 100% of IR light (.lambda.10000 .ANG.) to transmit 
therethrough. 
The preparation steps of this embodiment are the same as those in the first 
embodiment except for the stabilizing electrode of the polysilicon. 
Accordingly, only the step shown in FIG. 6 is replaced with the step shown 
in FIG. 5(C). 
In FIG. 6, after formation of the p base region 6 and the n.sup.+ region 7, 
the oxide film 8 at the capacitor portion, the emitter portion and the 
transistor Qc portion is removed by patterning, and a thin gate oxide film 
is formed by oxidation. 
Subsequently, the polysilicon is deposited by the LPCVD method, and after 
introduction of an impurity according to the ion implantation, thermal 
diffusion, etc., patterning is effected to form a gate electrode, a 
capacitor electrode 10 and a stabilizing electrode 11. 
Following subsequently the steps as described in FIG. 5(D), this embodiment 
can be constituted. 
Thus, in this embodiment, since the stabilizing electrode 11 is a 
polysilicon, it can be formed simultaneously according to the formation 
steps of the gate electrode 9 and the capacitor electrode 10, whereby the 
preparation steps can be simplified as compared with the case of the first 
embodiment. 
The effect of this embodiment is the same as that of the first embodiment 
except for aiming at detection of IR-ray. That is, dark current and its 
fluctuation can be inhibited by the stabilizing electrode 11. 
Also, in this embodiment, the capacitor may be provided, if desired. 
In the above respective embodiments, the case of npn bipolar transistor was 
shown, which, however, is not limittive of the present invention, as a 
matter of course. The present invention is also applicable for the pnp 
type, and further applicable for photoelectric conversion devices of other 
systems which accumulate photoexcited carriers. 
As described in detail above, since the photoelectric conversion device has 
a stabilizing electrode, by controlling this at a constant potential, the 
surface potential in the semiconductor region can be stabilized, and also 
the influence from the external disturbance can be inhibited. For this 
reason, generation of dark current can be controlled, and the noise of the 
reading signals due to variance of dark current can be inhibited. 
Also, even when the above stabilizing electrode may be maintained at a 
floating state, provided that it has sufficiently larger area as compared 
with the photoelectric conversion device, the surface potential in the 
above semiconductor region can be sufficiently stabilized.