Active pixel sensor with bootstrap amplification

Details Active pixel sensor with bootstrap amplification Active pixel sensor with bootstrap amplification

2000-01-26

2001-04-03

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C348S302000

Reexamination Certificate

active

06211510

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field Of The Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to variable biasing of the readout transistor in an active pixel sensor to improve sensitivity, reduce noise and to provide compressive non-linearity in the charge-to-voltage gain.
2. The Prior Art
In the art of CMOS active pixel sensors, the sensitivity, noise, and nature of the gain of an active pixel sensor present issues of concern. The sensitivity of an active pixel sensor in measuring the charge generated by the photons striking the active pixel sensor is typically characterized by determining the volts generated per photon of light striking the active pixel sensor and is termed charge-to-voltage gain. The readout amplifier in an active pixel sensor represents a substantial source of noise that in prior art pixel sensors has required design tradeoffs. The gain in prior art active pixel sensors is most often expansive, though it is preferred to be compressive.
The sensitivity of an active pixel sensor is determined by at least three factors. The first factor is related to the percentage of the area in the active pixel sensor available for converting photons to electrons. This is known as the fill factor. An increase in the area leads to an increase in the amount of charge generated. A second factor affecting the sensitivity of the active pixel sensor is related to the capacitance that is available for the integration of the charge sensed by the active pixel sensor. It will be appreciated that the voltage on the capacitor for given amount of charge is inversely proportional to the size of the capacitor. Accordingly, when the capacitance increases, the voltage decreases for the same amount of charge. A third factor is the gain of the readout amplifier for the active pixel sensor. Since the readout amplifier in the prior art is typically a transistor configured as a source follower, the gain is less than one.
One source of noise in an active pixel sensor is created by threshold fluctuations in the readout transistor. The amount of threshold fluctuation is related to the size of the readout transistor. As the size of the readout transistor is increased, the amount of threshold fluctuation, and hence the amount of noise decreases.
In compressive nonlinear gain, the gain at high light levels is less than the gain at low light levels. Those of ordinary skill in the art will appreciate that it is typically desirable to have greater sensitivity in converting photons-to-voltage at lower rather than higher light levels, because this increases the signal-to-noise ratio at lower light levels and, accordingly, the usable dynamic range of the active pixel sensor is increased.
The CMOS active pixel sensor art includes active pixel sensors that may or may not have embedded storage.
FIGS. 1A and 2A
illustrate typical CMOS active pixel sensors without and with embedded storage, respectively.
In an active pixel sensor
10
of
FIG. 1A
, a photodiode
12
employed to collect charge has an anode connected to a fixed voltage potential, shown as ground, and a cathode connected to the source of an N-channel MOS reset transistor
14
and the gate of an N-channel MOS readout transistor
16
. The gate of N-channel MOS reset transistor
14
is connected to a RESET line, and the drain of N-channel MOS reset transistor
14
is connected to a voltage reference, Vref. The drain of N-channel MOS readout transistor
16
is connected to a fixed potential Vcc, and the drain of N-channel MOS readout transistor
16
is connected to an N-channel MOS row select transistor
18
. Typically, the voltage Vref and the voltage Vcc are the same. In the active pixel sensor
10
, the capacitance available for the integration of the charge sensed by the active pixel sensor includes the photodiode
12
capacitance and the gate capacitance of the N-channel MOS readout transistor
16
.
The operation of the active pixel sensor
10
as it is typically performed is well understood by those of ordinary skill in the art. A timing diagram corresponding to the operation of active pixel sensor
10
is depicted FIG.
1
B. The active pixel sensor
10
is first reset by a RESET signal, during a reset step, that turns on N-channel MOS reset transistor
14
to place the voltage Vref on the cathode of the photodiode
12
. An integration step begins when the RESET signal makes a transition from HIGH to LOW wherein photo-generated electrons are collected on the photodiode
12
to reduce the voltage on the cathode of the photodiode
12
from the value Vref placed there during the reset step. During a readout step, a ROW SELECT signal will be asserted to turn on N-channel MOS select transistor
18
to place the voltage at the source of N-channel MOS readout transistor
16
on the column output line for detection. It should be appreciated that the voltage on the gate of N-channel MOS readout transistor
16
formed by the charge accumulated on the cathode of the photodiode
12
will be followed by the source of N-channel MOS readout transistor
16
.
In
FIG. 2A
, the CMOS active pixel sensor
30
has embedded storage. The active pixel sensor
30
includes a photodiode
32
having an anode that is connected to ground and a cathode that is connected to the source of N-channel MOS reset transistor
34
. The gate of N-channel MOS reset transistor is connected to a RESET line, and the drain of N-channel MOS reset transistor
34
is connected to a voltage Vref. The cathode of photodiode
32
is also connected to the source of N-channel MOS transfer transistor
36
. The gate of N-channel MOS transfer transistor
36
is connected to a XFR line, and the drain of N-channel MOS transfer transistor
36
is connected to a first plate of a capacitor
38
and to the gate of N-channel MOS readout transistor
40
. The drain of N-channel MOS readout transistor
40
is connected to Vcc, and the source of N-channel MOS readout transistor
40
is connected to N-channel MOS select transistor
42
. Typically, the voltage Vref and the voltage Vcc are equal to one another.
In the active pixel sensor
30
, the capacitance available for the integration of the charge sensed by the active pixel sensor
30
includes the capacitance of a photodiode
32
, the capacitance of the storage capacitor
38
, and the gate capacitance of the N-Channel MOS readout transistor
40
. It should be appreciated, however, that because the voltage at the drain of the N-channel MOS readout transistor
40
is high, the capacitance at the gate of the N-channel MOS readout transistor
40
is small and the gate capacitance of the N-channel MOS readout transistor
40
is not typically a preferred charge storage element.
A timing diagram corresponding to the operation of active pixel sensor
30
is depicted FIG.
2
B. In the operation of the active pixel sensor
30
, with the N-channel MOS transistor
34
turned on by a RESET signal to place the voltage Vref at the cathode of the photodiode
32
, the N-channel MOS transfer transistor is also turned on by a signal asserted on the XFR line. When the N-channel MOS reset transistor
34
is turned off, the integration of photons striking the photodiode
32
begins. Since the N-channel MOS transfer transistor
36
is turned on, the capacitor
38
adds to the capacitance of the photodiode
32
during integration to increase the charge capacity and therefore, the intensity range of the storage pixel sensor
30
. At the end of the integration period, the N-channel MOS transfer transistor
36
is turned off and the N-channel MOS row select transistor
42
is subsequently turned on so that the voltage at the gate of the N-channel MOS readout transistor
40
will be followed by the source of N-channel MOS readout transistor
40
to be placed on the column output.
In both active pixel sensors
10
and
30
, by minimizing the gate area of the N-channel MOS readout transistors
16
and
40
, respectively, the area provided to the photodiodes
12
and
32
, respectively, can be made larger to improve the sen

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