CMOS active pixel sensor

Details CMOS active pixel sensor CMOS active pixel sensor

1999-05-27

2002-04-02

Williams, Alexander O. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S233000, C257S461000, C257S292000, C257S294000, C257S435000, C257S382000, C257S369000, C257S234000, C257S235000, C257S236000, C257S237000, C257S238000, C348S155000, C348S152000, C348S294000, C348S300000, C348S308000, C348S309000, C250S208100, C250S2140RC, C365S185030, C365S185330

Reexamination Certificate

active

06365950

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an image sensor, and more particularly to a CMOS active pixel sensor capable of compensating loss of sensor sensitivity due to leakage current in a light sensing device.
BACKGROUND OF THE INVENTION
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting images in the form of light energy into electrical signals. Advantages of CCD use include high sensitivity and full-factor. However, CCDs suffer from a number of weaknesses including limited readout rates and dynamic ranges, and difficulty in integrating CCDs with CMOS-based microprocessors.
FIG. 1
is a circuit diagram illustrating a conventional CMOS active pixel sensor. As shown in
FIG. 1
, the active pixel sensor (APS)
10
(or active pixel sensor cell) includes a photodiode
12
acting as light sensing means. This photodiode
12
has a cathode and an anode. The anode of the photodiode
12
is grounded, and the cathode thereof is collectively coupled to a source of N-channel reset transistor
14
and a gate of N-channel sense transistor
16
. A drain of the N-channel reset transistor
14
is coupled to a power supply voltage Vdd, a source thereof is coupled to the cathode of the photodiode
12
, and a gate thereof is coupled to a reset line
20
.
The N-channel sense transistor
16
has a drain coupled to the power supply voltage Vdd, a gate coupled to the cathode of the photodiode
12
and the source of the reset transistor
14
, and a source thereof coupled to a drain of N-channel row select transistor
18
. A gate of the row select transistor
18
is coupled to a select line
22
, and a source thereof is coupled to a bit line
24
.
An operation of the active pixel sensor cell
10
is performed in three steps: a reset step, wherein the cell
10
is reset from a previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, wherein the electrical signal is read out.
During the reset step, the gate of the reset transistor
14
is briefly pulsed with a reset voltage (for example, about 5 volts), which resets the photodiode
12
to an initial integration voltage of (V
R
−V
T
), wherein the voltage V
R
is the reset voltage and the voltage V
T
is a threshold voltage of the reset transistor
14
.
During the image integration step, light energy, in the form of photons, strikes the photodiode
12
, thereby creating a number of electron-hole pairs. Thereafter, the photogenerated holes are attracted to the ground terminal of the photodiode
12
, while the photogenerated electrons are attracted to the positive terminal of the photodiode
12
. For the additional electrons, the voltage across the photodiode
12
is reduced. The photodiode
12
is designed to limit recombination between the newly formed electron-hole pairs.
At the end of the image integration period, the voltage across the photodiode
12
is equal to (V
R
−V
T
−V
S
), wherein the voltage V
S
is a voltage changed by the photons absorbed in the photodiode
12
. Thus, voltage V
S
corresponds to the absorbed photons, V
S
can be determined by subtracting the voltage at the end of the image integration period from the voltage at the beginning of the image integration period. That is, the voltage V
S
is {(V
R
−V
T
)−(V
R
−V
T
−V
S
)}.
Following the image integration period, the active pixel sensor cell
10
is read out by turning on the row select transistor
18
(which has been kept off until this point). When the voltage across the photodiode
12
decreases, the gate voltage of the sense transistor
16
is also reduced, causing a reduction in the amount of current flowing to the bit line
24
through the transistors
16
and
18
. Thereafter, a voltage V
PX
(or referred to as “a pixel voltage”) on the bit line
24
is detected by a conventional current detector.
FIG. 2
is a graphical illustration of the pixel voltage of the active pixel sensor
10
before and after a light incidence. During a non-exposure time for the light incidence, the pixel voltage V
PX
, that is, the voltage on the bit line
24
of
FIG. 1
, is lowered by a voltage V
D
corresponding to the dark current of the photodiode
12
. During exposure from the incident light, the pixel voltage V
PX
is rapidly lowered. The non-exposure (reset) time t
1
before the light incidence and the exposure time t
2
during the light incidence can be different for different sensors dependant upon factors such as the size of the active pixel sensor cells.
After the photodiode
12
is reset by the reset integration voltage (V
R
−V
T
), as set forth above, the voltage of the photodiode
12
is lowered by a voltage V
D
corresponding to dark current (or leakage current) regardless of light energy before the incidence of light energy. As a result, the pixel voltage V
PX
=(V
R
−V
T
−V
S
−V
D
).
One problem of the above described CMOS active pixel sensor cell is that the dark current (or the leakage current) in the active pixel sensor cell
10
increases the noise level present at the gate of the select transistor. As a result, this decreases the sensitivity of the sensor. Since the amount of dark current occurring in active pixel sensor cells is different for different cells, it is difficult to compensate the dark current by a fixed offset.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to solve the above mentioned problems and to provide a CMOS active pixel sensor capable of compensating for performance loss due to leakage current in sensor.
According to an aspect of the present invention, a CMOS active pixel sensor is provided which comprises light sensing means for generating an output voltage at an output when light is incident thereupon, said light sensing means having an amount of leakage current before said light incidence, reset means for resetting the output voltage of the light sensing means to an initial reset voltage in response to a reset signal, a sense transistor having a source, a drain coupled to a power source, and a gate coupled to the output of the light sensing means, a select transistor having a drain connected to the source of the sense transistor, for providing a voltage at a source of the sense transistor to a bit line, in response to a select signal, and compensation means for supplying a voltage to the select transistor substantially corresponding to the output voltage of the light sensing means lowered by the leakage current.
Preferably, the compensation means comprises shielded light sensing means shielded from incident light, a first transistor for resetting an output voltage of the shielded light sensing means to the initial reset voltage in response to the reset signal, and a second transistor having a source, a drain coupled to the power source and a gate coupled to the output of the shielded light sensing means, for increasing current flow to the drain of the select transistor in an amount proportional to the leakage current of the shielded light sensing means.
Also provided is a method of increasing voltage readout sensitivity of a CMOS active pixel sensor, said sensor including a first photodiode having leakage current flow, a first reset transistor for resetting photodiode voltage and a first sense transistor connected to said photodiode, the method comprising the steps of: commonly connecting a second sense transistor at drain and source of said first sense transistor, said second sense transistor is of a type complementary to said first sense transistor; commonly connecting a second reset transistor to said first reset transistor for activating both first and second reset transistors by a reset signal; shielding a second photodiode from light incident on said first photodiode and connecting said second photodiode to said second reset transistor; and second source transistor, and reading out voltage upon illumination of incident light on said first photodiode by activating a sele

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