Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1997-11-03
2001-06-12
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C307S402000
Reexamination Certificate
active
06246436
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to an active pixel sensor. In particular, it relates to an active pixel sensor in which the sensitivity of the active pixel sensor is adjusted depending on the intensity of light being received by the active pixel sensor.
BACKGROUND
An electronic camera generally converts an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the camera. The electronic camera typically includes an array of image sensors or light sensitive sensors which detect the intensity of light received by the camera. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic camera systems to be low cost, compact and require low power.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems.
An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1
shows a circuit schematic of a prior art active pixel sensor. The active pixel sensor is generally included within an array of active pixel sensors. The active pixel sensor includes a photo-diode D
1
, a reset transistor Q
1
, a bias transistor Q
2
and a select transistor Q
3
. The photo-diode D
1
collects charge when the photo-diode D
1
is exposed to light. The photo-diode D
1
includes an inherent capacitance Cd which capacitively loads a signal node N
2
. The charge collected by the photo-diode D
1
is accumulated on the capacitance Cd of the photo-diode D
1
creating a photo-diode voltage which is proportional to the intensity of light received by the photo-diode D
1
. The photo-diode voltage is created at the cathode of the photo-diode D
1
.
The reset transistor allows the photo-diode D
1
to be reset by discharging the photo-diode capacitance Cd. A RST (reset) line discharges the photo-diode capacitance Cd by pulsing the RST line high to set the cathode of the photo-diode to a predetermined reset voltage. The predetermined reset voltage for the active pixel sensor shown in
FIG. 1
is voltage potential of the RST line minus the threshold voltage of the reset transistor Q
1
.
The select transistor Q
3
allows a controller to selectively sample the photo-diode voltage at a PIXOUT output of a particular active pixel sensor by pulsing a SELECT line to cause the select transistor Q
3
to conduct.
FIG. 2
is a plot of a signal voltage of the photo-diode D
1
of the active pixel sensor shown in FIG.
1
. The signal voltage is defined as a reference voltage minus the voltage potential of the PIXOUT output. The reference voltage is defined as the voltage potential of the PIXOUT output when the signal node N
2
is reset to the predetermined reset voltage. The greater the intensity of light received by the photo-diode D
1
, the greater the signal voltage. The charge conducted by the photo-diode D
1
is proportional to the intensity of light received by the photo-diode D
1
. As depicted by the plot, the signal voltage begins to saturate as the charge conducted by the photo-diode increases. The saturation voltage V
saturation
is the signal voltage in which an increase in the intensity of the light received by the photo-diode D
1
does not affect the signal voltage. The saturation of the photo-diode D
1
limits the dynamic range of the photo-diode D
1
. The range of the intensity of light being received by active pixel sensor which is usefully detectable is limited by the fact that the active pixel sensor saturates. Once the signal of the photo-diode D
1
of the active pixel saturates, it is impossible to detect changes in the intensity of the light being received by the active pixel sensor. Further, when the intensity of light received by the photo-diode D
1
is just below the intensity of light required to saturate the active pixel sensor, the response of the photo-diode D
1
is very non-linear. The operation of the active pixel sensor is limited to a range of light intensities in which the response of the photo-diode D
1
is linear.
It is desirable to have an active pixel sensor which allows the intensity of detectable light receive by the active pixel sensor to vary over a greater range than presently possible. The active pixel sensor would generate an analog voltage which represents the intensity of light received by the active pixel sensor over a greater range of light intensities than presently possible. Further, the active pixel sensor would be manufacturable using presently existing CMOS fabrication processes.
SUMMARY OF THE INVENTION
The present invention provides an active pixel sensor which provides detection of received light over a greater dynamic range of light intensity than presently possible. A gate capacitor is switched in parallel with a photo-diode within the active pixel sensor when the intensity of light being received by the active pixel sensor is potentially great enough to saturate the response of the active pixel sensor. The active pixel sensor maintains a high level of sensitivity at low levels of received light intensity. The active pixel sensor is compatible with low-cost CMOS fabrication processes.
A first embodiment of this invention includes an active pixel sensor. The active pixel sensor includes a photo-diode. The photo-diode conducting charge as a function of the intensity of light received by the photo-diode. The photo-diode includes a diode capacitance which collects charge conducted by the photo-diode which generates a photo-diode voltage. A switched capacitor is connected in parallel with the photo-diode when the photo-diode voltage drops below a pre-determined voltage potential. A capacitance of the switched capacitor adds to the diode capacitance when the switched capacitor is connected. The active pixel sensor further includes electronic circuitry to allow a controller to sample the photo-diode voltage.
Another embodiment of the invention is similar to the first embodiment, but includes the switched capacitor being a gate capacitor.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 4285582 (1981-08-01), Yamazaki
patent: 4583002 (1986-04-01), Kondo et al.
patent: 4669063 (1987-05-01), Kirsch
patent: 4839735 (1989-06-01), Kyomasu et al.
patent: 4943839 (1990-07-01), Kumano et al.
patent: 5289023 (1994-02-01), Mead
patent: 5329112 (1994-07-01), Mihara
patent: 5969758 (1999-10-01), Sauer et al.
Cham Kit M.
Chou Eric Y.
Lin Jane M. J.
Agilent Technologies , Inc
Garber Wendy R.
White Mitchell
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