Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-09-07
2004-10-12
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140AG, C330S308000
Reexamination Certificate
active
06803555
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electro-optical detectors and in particular to amplifiers for interfacing with electro-optical detectors, such as photovoltaic detectors.
BACKGROUND
Many modem systems rely on electro-optical detectors, or sensors, to sense a portion of the electromagnetic spectrum. These systems might include telecommunications systems, fiber-optic systems, imaging systems, cameras, and other commercial and military systems. The electro-optical sensors of these systems can be critical components in determining performance, sensitivity, cost, and dynamic range of the overall system.
To achieve a very high level of performance, many modem electro-optical sensors include two primary functional components. The first component is a detector element or detector array. One detector element commonly used is a photovoltaic detector element. The second functional component is the readout multiplexer.
For electro-optical sensors operating in the visible spectrum and up to approximately 1.0 &mgr;m wavelength radiation, silicon is commonly used to fabricate both the detector (e.g., a single detector element or an array of detector elements) and the readout multiplexer. For optical sensors operating at significantly shorter or longer wavelengths, alternative semiconductor materials may be selected for the detector to provide more efficient sensitivity for the desired region of the electromagnetic spectrum. In this case, it may be desirable to use different materials to fabricate the detector and the readout multiplexer, since the readout multiplexer can still be fabricated in silicon.
Electrical signals from individual detector elements are processed by electronics signal chains, which have become increasingly sophisticated in modern electro-optical sensors. The signal chains are now designed to optimize the impedance interface to the detector elements; the integration of the electrical signals; the noise performance of the sensor; and the signal storage, multiplexing, and processing to an optimized systems interface.
FIG. 1A
is a circuit diagram and
FIG. 1B
is a cross-sectional side view of a typical pn junction photovoltaic detector element
10
. As shown in
FIG. 1A
, detector element
101
is a diode structure including an anode
11
and a cathode
12
. A terminal
13
is electrically coupled to anode
11
, and a terminal
14
is electrically coupled to cathode
12
. Detector element
10
may be fabricated by diffusing a p-type region
15
into an n-type semiconductor
16
, thereby forming a pn junction as shown in FIG.
1
B. Since detector element
10
is a diode structure that is responsive to illumination, detector element
10
is also called a photodiode.
In electro-optical systems, an electromagnetic image is spatially sampled in units called pixels. Detector element
10
can be used to sample a single pixel at a time. Thus, detector element
10
is also sometimes referred to as a pixel. Depending on the application and the format of detector array, the image may or may not be scanned. If the image is scanned, it may be scanned in one dimension or in two dimensions. For example, to sample a line of an electro-magnetic image, a line array of detector elements
10
is provided, or the image is scanned across the single detector element
10
.
FIG. 2A
is a circuit diagram and
FIG. 2B
is a perspective view of a typical pn junction photovoltaic detector array
20
. In
FIG. 2A
, detector array
20
includes four detector elements
10
, each with terminals
13
and
14
. Typical line arrays of this type in current systems may include as many as
512
, or more, detector elements
10
. Individual detector elements
10
are fabricated in close proximity to each other in the necessary quantity to support the system application. In
FIG. 2B
, four p-type regions
15
(one for each detector element
10
) are shown arranged in a line and diffused into n-type semiconductor
16
. Sampling of a two-dimensional image can be accomplished by fabrication of a plurality of detector elements
10
arranged in a two-dimensional array, also called a staring array. Typical two-dimensional arrays in current systems may include 1024×1024, or more, detector elements
10
.
FIG. 3
is a graphical illustration of a current-voltage (IV) characteristic of pn junction photovoltaic detector element
10
of
FIG. 1A
under illumination. The right and left halves of the diagram are referred to as the forward bias (FB) and reverse bias (RB) regions, respectively. Under forward bias, the zero current intercept, also called the forward voltage (V
F
), of detector element
10
is a function of the illumination level. Similarly, under reverse bias, the reverse bias current is also a function of the illumination level. The reverse bias current, however, may also include a junction leakage current component and, under high reverse bias, a reverse bias breakdown current component.
Depending on material quality and properties, the magnitudes of the leakage current and/or the reverse bias breakdown current may be as large as, or larger than, the detector element photocurrent, which is the signal of interest. The extraneous leakage and reverse bias breakdown currents may degrade performance and dynamic range of the electro-optical sensor.
For a detector with a single detector element
10
, it is reasonable to interface between the readout multiplexer and detector element
10
using wires or printed circuit board traces. In one-dimensional line arrays or two-dimensional staring arrays, however, the detector element count may be as large as
512
detector elements
10
, or even over one million detector elements
10
, respectively. In these cases, wire and circuit board trace interfaces are unrealistic, and it is desirable to have the readout multiplexer of the electro-optical sensor in close physical proximity to detector elements
10
to facilitate electrical coupling of detector elements
10
to the readout multiplexer.
Direct electrical coupling of detector elements
10
to the readout multiplexer allows the sizes of detector elements
10
to be small, reducing the overall size of the detector array. Integrated circuit wire bonding and bump bonding techniques have been employed to achieve such electrical interfaces.
FIG. 4
is a perspective illustration of an electro-optical sensor
40
including an electro-optical detector
41
in close proximity to an integrated circuit readout multiplexer
42
. Detector
41
includes a plurality (i.e., an array) of detector elements
10
, each of which is coupled to an electronics signal chain for processing the signal from each detector element
10
.
FIG. 5
is a circuit diagram of an array
50
of four detector elements
10
(i.e., photodiodes) each coupled to an integrating amplifier
51
. Each integrating amplifier
51
includes, due to materials and manufacturing variations, a unique input offset voltage (labeled Vos
1
, Vos
2
, Vos
3
, and Vos
4
, respectively) shown explicitly coupled between terminal
13
of each detector element
10
and the input of each integrating amplifier
51
.
FIG. 6A
is a graphical illustration of a current-voltage (IV) characteristic for array
50
of
FIG. 5
, for a large input offset voltage distribution. The relatively large variation in the values of the input offset voltages Vos
1
, Vos
2
, Vos
3
, and Vos
4
of each integrating amplifier
51
is shown for illustration purposes. The effect of variations in the input offset voltages of integrating amplifiers
51
is to cause each detector element
10
to operate at a different bias point on its IV curve. The current from each detector element
10
will thus show an offset variation that is dependent on the IV characteristic of the detector element
10
and the magnitude of the input offset voltage distribution from integrating amplifiers
51
. These offset currents introduce variations in the output signals for each detector element
10
. In some cases, these variations can represent a significant portion of the dynamic range of the signal levels of detector
Aziz Naseem Y.
Parrish William J.
Indigo Systems Corporation
MacPherson Kwok Chen & Held LLP
Michelson Greg J.
Porta David
Yam Stephen
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