Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1998-10-16
2001-02-06
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S184000, C257S440000, C257S443000, C257S448000, C250S338400, C250S339020, C438S054000, C438S066000, C438S074000, C438S094000
Reexamination Certificate
active
06184538
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.
FIELD OF THE INVENTION
The present invention relates to radiation detection and devices thereof, and more specifically, to a quantum-well radiation sensing array.
BACKGROUND
Quantum-well semiconductor devices can be configured to detect radiation with improved performance compared to many other types of radiation detectors. Unique properties of the quantum-well structures allow for a high quantum efficiency, a low dark current, compact size and other advantages.
In particular, various quantum-well structures can be formed by artificially varying the compositions of lattice matched semiconductor materials to cover a wide range of wavelengths in infrared (“IR”) detection and sensing. An intraband transition, that is, photoexcitation of a carrier (i.e., an electron or a hole) between a ground state and an excited state in the same band (i.e., a conduction band or a valance band), can be advantageously used to detect radiation with a high responsivity in the IR range at a selected wavelength by using a proper quantum-well structure biased at a proper voltage. For example, the absorption wavelength of a quantum-well structure formed of Al
x
Ga
1−x
As/GaAs can be changed by altering the molar ratio x (0≦x≦1) of aluminum or the thickness of GaAs layer. Other materials for infrared detection include Hg
1−x
Cd
x
Te and Pb
1−x
Sn
x
Te. See, for example, Gunapala and Bandara, “Recent Developments in Quantum-Well Infrared Photodetectors,” Physics of Thin Films, Vol. 21, pp. 113-237, Academic Press (1995) and a commonly assigned pending Application Ser. No. 08/785,350 filed on Jan. 17, 1997, which are incorporated herein by reference.
Infrared sensing arrays formed of quantum-well structures are desirable due to their applications in night vision, navigation, flight control, environmental monitoring (e.g., pollutants in atmosphere) and other fields. Many conventional infrared arrays respond to radiation only in a specified wavelength range, such as a short-wavelength infrared range (“SMIR”) from about 1 to about 3 &mgr;m, a mid-wavelength infrared range (“MWIR”) from about 3 to about 5 &mgr;m, a long-wavelength infrared range (“LWIR”) from about 8 to about 12 &mgr;m, or a very-long-wavelength infrared range (“VLWIR”) that is greater than about 12 &mgr;m. All sensing pixels in a quantum-well sensing array operating at a specified radiation wavelength are biased at a predetermined voltage. A readout multiplexer having an array of readout pixels corresponding to sensing pixels is usually used to supply this common bias voltage and to read out the signals from the sensing pixels.
A sensing array may have sensing pixels that each include a MWIR detector and a LWIR detector to form a dual-band array. Hence, simultaneous detection of radiation signals can be achieved at two different IR ranges in the same array.
Several dual-color single-element quantum-well detectors have been proposed. Two quantum-well detectors for two different wavelengths can be stacked together to form a single detector for detecting two radiation at two different wavelengths. See, for example, Tidow et al., “A High Strain Two-Stack Two-Color Quantum Well Infrared Photodetector”, Applied Physics Letters, Vol. 70, pp. 859-861 (1997) and a commonly assigned pending Application Ser. No. 08/928,292 filed on Sep. 12, 1997, which are incorporated herein by reference. Two different voltages are supplied to the detector to provide proper bias to different quantum-well detectors for substantially optimized responsivities. U.S. Pat. No. 5,552,603 to Stokes discloses a three-color quantum-well sensing array that requires two bias voltages for each sensing pixel.
This requirement of two different bias voltages presents a difficulty in forming a dual-band sensing array. A sensing array requires a multiplexer for readout but most commercial readout multiplexers can only provide a single bias voltage to the sensing pixels. It may be possible to design a special readout multiplexer capable of supplying two voltages. However, this increases the cost of the device. In addition, the need of two voltages complicates the circuitry.
SUMMARY
The present invention provides a quantum-well sensing array capable of simultaneously detecting radiation of two or more different wavelengths. Two or more quantum-well sensing stacks are implemented in each pixel and are biased at a common voltage difference. A readout multiplexer array of a single reference voltage can be coupled to the sensing array and to provide power to and read signals from the sensing array.
One embodiment of such a dual-band quantum will sensing array includes a plurality of sensing pixels. Each sensing pixel has a first semiconductor contact layer doped to have a predetermined type of conductivity, a first quantum-well sensing stack formed over the first semiconductor contact layer and configured to have a plurality of alternating semiconductor layers which form a first number of quantum wells of a first well width, at least one second semiconductor contact layer doped to have the predetermined type of conductivity and formed on the first quantum-well sensing stack, a second quantum-well sensing stack formed over the second semiconductor contact layer and configured to have a plurality of alternating semiconductor layers which form a second number of quantum wells of a second well width, and a third semiconductor contact layer doped to have the predetermined type of conductivity and formed on the second quantum-well sensing stack.
The first and second quantum-well sensing stacks are configured to respond to radiation at first and second operating wavelengths, respectively, to produce charged carriers. The first and third semiconductor contact layers are maintained at a common bias electrical potential with respect to the second semiconductor contact layer so that the first and second quantum-well sensing stacks are biased by a common voltage difference.
The doping level, well width and the number of quantum wells of each quantum-well sensing stack can be configured to substantially maximize the amount of produced charged carriers in responding to each received radiation photon at the respective operating wavelength under the common bias voltage difference. In particular, for a given common bias voltage difference, the doping levels, well widths and the numbers of quantum wells of the two quantum-well stacks can be selected relative to each other so that the amounts of radiation-induced charged carriers in the two stacks are of the same order of magnitude when the intensities of the received radiation energies at the two different wavelengths are different from each other by more than one order of magnitude.
Reflective grating layers or reflecting layers with features on the order of the operating wavelength may be formed on the second and third contact layers to direct normal incident radiation received from the first contact layer back to the first and second sensing stacks at angles to induce absorption of the radiation.
These and other aspects and associated advantages of the present invention will become more apparent in light of the following detailed description, the accompanying drawings, and the claims.
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M.Z. Tidrow et al., A high strain two-stack two-color quantum well infrared photodetectorAppl. Phys. Lett.70 (7), Feb. 17, (1997).
Gunapala et al., 9-um Cutoff 256 X 256 GaA
Bandara Sumith V.
Gunapala Sarath D.
Liu John K.
Baumeister Bradley W.
California Institute of Technology
Fish & Richardson P.C.
Jackson, Jr. Jerome
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