Solid-state image sensing device and method of driving the same

Details Solid-state image sensing device and method of driving the same Solid-state image sensing device and method of driving the same

1999-09-13

2002-11-26

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C348S299000

Reexamination Certificate

active

06486460

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a solid-state image sensing device and a method of driving a solid-state image sensing device.
BACKGROUND OF THE INVENTION
When a solid-state image sensing device used as an image pickup device is applied to an electronic camera, it is necessary to secure a sufficiently large dynamic range. Because the dynamic range of solid-state image sensing device is significantly narrower than that of silver film.
So, Japanese patent application laid-open No. 8-9260 (1996) discloses a technique that the dynamic range is enlarged by varying a voltage of substrate within an imaging period to switch the amount of accumulable charge of a photodiode. This conventional technique is explained below.
FIG. 1
is a plan view showing cell part in, e.g. a CCD type solid-state image sensing device. The cell part is composed of a photoelectric conversion section
101
, a vertical charge transfer electrode
102
, a first charge transfer electrode
105
and a second charge transfer electrode
106
.
FIG. 2
is a cross sectional view cut along the line I-I′ in FIG.
1
. As shown, the cell part is composed of an N type semiconductor substrate
107
, a P
−
type semiconductor substrate
108
, an N type semiconductor substrate
109
, a P
+
type semiconductor substrate
110
, a first charge transfer electrode
105
formed with first-layer polysilicon
111
, a second charge transfer electrode
106
formed with second-layer polysilicon
112
. aluminum film
113
serving as shade film, insulating film
114
, and cover insulating film
115
.
FIG. 3
is a characteristic diagram showing the electronic potential of photoelectric conversion section.
First, in order to reset unnecessary electric charge before accumulating charge into photodiode, a substrate voltage VH
sub
is applied to the N
−
type semiconductor substrate
107
, depleting completely the N type semiconductor substrate
109
composing the photoelectric conversion section
101
and the P
−
type semiconductor substrate
108
with a low concentration formed just hereunder, moving all the unnecessary charge to the N
−
type semiconductor substrate
107
.
Such a structure is generally called “vertical overflow drain structure (vertical OFD)” (reference: J. of Institute of Television Engineers of Japan, Vol. 37, No. 10 (1983), pp. 782-787.
Subsequently, a substrate voltage Vb
sub
(hereinafter referred to as ‘substrate voltage’) is applied to the N
−
type semiconductor substrate
107
, the photoelectric conversion section
101
starts accumulating a signal charge according to amount of incident light. Hereupon, by adjusting the substrate voltage arbitrarily, such excessive charge that cannot be accumulated in the photoelectric conversion section
101
is moved into the N
−
type semiconductor substrate
107
using the vertical OFD structure, thereby the controlling of amount of accumulable charge is conducted.
Using this technique, the solid-state image sensing device is controlled so that the amount of accumulable charge in solid-state image sensing device is switched sequentially from a first amount of accumulable charge (Q
sat
(
1
)≠0) to a second amount of accumulable charge (Q
sat
(
2
)≠0, Q
sat
(
1
)<Q
sat
(
2
)) within one imaging period.
By changing the substrate voltage applied to OFD (overflow drain) of solid-state image sensing device at time t(
1
) within imaging period to conduct this operation, the substrate voltage is controlled so that the amount of accumulable charge in solid-state image sensing device is kept Q
sat
(
1
) from the beginning to time t(
1
) within one imaging period and, after time t(
1
), switched into Q
sat
(
2
).
FIG. 4
shows the relationship (solid line) between an amount of accumulable charge within one imaging period and charge accumulation time, in a solid-state image sensing device having such a function.
FIG. 5
shows the relationship (solid line) between an amount of accumulable charge within one imaging period and an amount of light.
Dotted lines in
FIGS. 4 and 5
indicate characteristics in the case that the amount of accumulable charge does not vary within one imaging period.
As shown in
FIGS. 4 and 5
, comparing with the case that the amount of accumulable charge does not vary, the dynamic range can be enhanced.
Namely, by providing a means for switching the amount of accumulable charge in solid-state image sensing device sequentially from the first amount of accumulable charge (Q
sat
(
1
) ≠0) to the second amount of accumulable charge (Q
sat
(
2
)≠0, Q
sat
(
1
)<Q
sat
(
2
)) within one imaging period, the dynamic range can be enhanced.
However, in the conventional solid-state image sensing device, when t(
1
) is set within one imaging period and Q
sat
(
1
) and Q
sat
(
2
) are set only under the condition of Q
sat
(
1
)<Q
sat
(
2
), the dynamic range may not be improved sufficiently. Also, there may occur a case chat the dynamic range is improved little more than the case that the amount of accumulable charge does not vary. The reason is as explained below.
FIGS. 6A
to
6
C show the relationship between charge accumulation time where t(
1
) varies among t(
1
a
), t(
1
b
) and t(
1
c
) and the amount of accumulable charge.
FIGS. 7A
to
7
C show the relationship between an amount of incident light and the amount of accumulable charge. Meanwhile, t(
1
a
)<t(
1
b
)<t(
1
c
) is satisfied, t(
1
b
) is the middle point of one imaging period, and
2
Q
sat
(
1
)=Q
sat
(
2
) is satisfied. Dotted lines indicate characteristics in the case that the maximum amount of accumulable charge is constant.
As seen from
FIGS. 6A
to
6
C and
7
A to
7
C, in case of t(
1
c
). the dynamic range is enhanced comparing with the case that the maximum amount of accumulable charge is constant. However, in case of t(
1
a
) and t(
1
b
), the dynamic range is not enhanced comparing with the case that the maximum amount of accumulable charge is constant.
This is because t(
1
), Q
sat
(
1
) and Q
sat
(
2
) are determined only under the condition of Q
sat
(
1
) Q
sat
(
2
). At this condition, the dynamic range cannot be improved surely comparing with the case that the amount of accumulable charge is constant. Further, the circuit is complicated comparing with the case that the amount of accumulable charge is constant.
Japanese patent application laid-open No. 1-253960 (1989) discloses a solid-state image sensing device where the saturation amount of signal transfer is made larger than the amount of signal charge in saturation of light-receiving element. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
Also, Japanese patent application laid-open No. 5-22728 (1993) discloses a technique that the amount of accumulable charge is varied according to the gain of amplification circuit corresponding to solid-state image sensing device and the gain of white-balance adjustment circuit. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
Further, Japanese patent application laid-open No. 10-150183 (1998) discloses a solid-state image sensing device equipped with a drive system that reduces an OFD bias to solid-state image sensing element when reading an amount of charge. However, it does not describe about that the amount of accumulable charge is varied in the form of multiple stages within one imaging period.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a solid-state image sensing device and a method of driving a solid-state image sensing device that the dynamic range is improved effectively.
It is an object of the invention to provide a solid-state image sensing device and a method of driving a solid-state image sensing device that even when the photoelectric conversion efficiency is varied with amount of light, an image that does not give uncomfortable feeling to eyes can be produced.
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