Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2001-03-09
2002-09-24
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S184000, C257S190000
Reexamination Certificate
active
06455908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to detection of radiation, and more particularly to multispectral radiation detectors for detection of radiation in multiple spectral bands.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
It is desirable to employ radiation detectors to convert electromagnetic radiation, such as infrared (IR) radiation, into electrical signals. The term photodetector is sometimes used, and is used herein, to refer to any type of radiation detector, i.e. a detector that detects electromagnetic radiation. Such detectors may be used in a variety of applications, including thermal imaging and transmission of information using signals having infrared wavelengths. One type of photodetector is the junction photodetector, or photodiode, which has a semiconductor p-n junction that produces electrical current under illumination with electromagnetic radiation. Other approaches, such as the use of bolometers which detect temperature changes caused by incident radiation, or quantum-well intra-subband detectors in which incident radiation causes excitation of electrons between confined energy states of a quantum well, generally provide lower sensitivity and/or slower frequency response.
The operation of a basic semiconductor p-n junction photodetector is illustrated by the energy band diagrams of
FIGS. 1A-B
. An energy band diagram, showing electron energy vs. distance, of a p-n junction under equilibrium conditions is shown in FIG.
1
A. This diagram includes conduction band edge
10
, valence band edge
12
, and Fermi energy level
14
. The energy difference
16
between the conduction and valence band energies is known as the energy gap, or bandgap, of the semiconductor. Because the same semiconductor material is used throughout the junction of
FIG. 1A
, this energy gap is shown as constant with distance throughout the junction.
Conduction in a semiconductor can generally be described in terms of the movement of electrons in the conduction band (having energy at and above that of conduction band edge
10
) and holes in the valence band (having energy at and below that of valence band edge
12
). The proximity of Fermi level
14
to conduction band edge
10
on the left side of the junction indicates that this portion of the semiconductor is doped n-type, while the right side of the junction is doped p-type. On the n-type side of the junction the majority carriers are electrons and the minority carriers are holes, while the reverse is true on the p-type side. The p-n junction includes a built-in electric field &egr;
0
in the junction region where the conduction and valence band edges are bent. The field exerts a force moving any holes appearing in this junction region to the right (in the direction of the field, as shown by the arrow in FIG.
1
A), and moving any electrons appearing in the junction region to the left (opposite the direction of the field).
A photodiode is typically operated with the p-n junction reverse-biased, as shown in FIG.
1
B. As in the case of built-in electric field of
FIG. 1A
, the larger applied electric field of
FIG. 1B
forces electrons toward the n-type side of the junction and holes toward the p-type side. Electrons and holes may be generated in the junction region by absorption of an incident photon such as photon
18
. If photon
18
has energy higher than energy gap
16
, absorption of the photon may provide energy to excite an electron from the valence band to the conduction band, creating conduction electron
20
and hole
22
. The junction region
24
over which electric field &egr; appears may be considered the collection region of the photodiode (collecting the photogenerated carriers), while the outer neutral n-type and p-type regions may be considered absorber regions, as Well as contact regions for connecting the photodiode to a surrounding circuit. Because electrons are collected on the n-type side and holes on the p-type side, each contact collects photogenerated majority carriers. The designation of a “majority” or “minority” carrier is dependent upon the location of the carrier within the device. A hole formed by absorption of a photon on the n-type side of the photodiode is a minority carrier when formed, and becomes a majority carrier upon being transported by the electric field to the p-type side of the photodiode.
When properly biased, the photodiode thus produces a current related in a known manner to the electromagnetic radiation incident thereon. Photodiodes are used, for example, to detect short-, mid-, and long-wavelength IR radiation having wavelengths from about 1 &mgr;m to about 30 &mgr;m.
Semiconductor-based IR photon detectors (photodiodes), as well as other types of radiation detectors, are generally characterized as having an energy gap (bandgap) that is suitable for absorbing radiation within a specified spectral region. Infrared detectors that gather data in more than one IR spectral band can determine increased information from the scene to further improve sensitivity above that of single-band detection. Because a new dimension of contrast is obtained, the detection of radiation within two or more spectral regimes using a single detector has been established as a desirable goal. A single detector capable of detecting radiation of two or more distinct spectral regions, or “colors,” may be referred to as a multispectral, or multicolor, photodetector.
There are two primary ways to achieve multicolor capability: “multiple detector” systems having separate detectors for each spectral band; and “multispectral detectors” that provide separate but spatially and temporally collocated signals from multiple IR spectral bands using a single detector element. To obtain multiple spectral band sensitivity, multiple detector systems currently rely on cumbersome imaging techniques that either disperse the optical signal across multiple IR detectors or use a filter wheel to spectrally discriminate the image focused on single detecting element. In comparison, integrated multispectral detectors offer separate and simultaneous sensitivity to different spectral bands within the same detector unit cell (spatially collocated). The use of an integrated multispectral detector eliminates the need for aligning two or more detectors and also reduces the number of on-board optical components, thereby providing significant reduction of weight and power in a simpler, more reliable, and less costly package. Furthermore, the temporal and spatial co-registration between each spectral field occurs on the pixel level, which enables high-performance signal processing.
Referring now to
FIG. 2A
, there is shown a perspective view of the wafer layer structure of an exemplary single-color photodetector
200
. Such a photodiode detector is formed from a plurality of absorption layers stacked on a common substrate. To fabricate such a photodetector, an n-type absorption layer
212
is grown on a transparent substrate
211
, followed by a p-type barrier layer
213
to form the IR photodiode. Light
201
within the spectral absorption band
212
of the n-type absorber layer which impinging upon the photodiode of detector
200
is transmitted through transparent substrate
211
, and absorbed by n-type absorber region
212
. When the photodiode of the junction of layers
213
,
212
is properly biased, the absorption of the light causes a corresponding current to flow across the p-n junction of the photodiode. Photodetector
200
thus produces a current having a magnitude related in a known manner to the intensity of radiation within the spectral band of region
212
impinging on the photodiode of detector
200
.
Two-color detection is achieved by extending the structure of photodetector
200
by one additional absorbing layer tuned to respond to the IR radiation that transmits through the lower layers. Referring now to
FIG. 2B
, there is shown a perspective view of the wafer layer structure of a
Johnson Jeffery L.
Lin Chih-Hsiang
Applied Optoelectronics, Inc.
Chaudhuri Olik
Kinsella N. Stephen
Wille Douglas A.
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