Hughes Aircraft Company

Raytheon Company

Chan-Shin Wu

Cheng P Wen

Robert N Sato

William E Stanchina

David B Rensch

Cheng-Keng Pao

A radiation detector, particularly suited for the detection of long wavelength infrared radiation, employs a plurality of multiple quantum well (MQW) superlattices in a unitary stack. The superlattices are electrically connected in parallel, and current outputs are obtained from the parallel connection as an indication of the incident radiation. Electrical contact layers are provided on the opposite sides of each superlattice, with adjacent superlattices sharing a common contact layer. The number of quantum well/barrier layer periods per superlattice is preferably reduced to about 20-30 divided by the number of superlattices in the stack, as compared to a single-superlattice detector with about 20-30 periods. This allows a common bias voltage applied to the superlattices to be similarly reduced by a factor approximately equal to the number of superlattices in the stack. The reduction in superlattice thickness and bias voltage yields a higher external quantum efficiency, improved signal-to-noise ratio and lower power consumption. The detector can be implemented either as a single color system with similar superlattices, or as a multi-color system in which each superlattice is tuned to a different radiation waveband.

Stacked multiple quantum well superlattice infrared detector

A multi-layer collector heterojunction transistor (10) provides for high power, high efficiency transistor amplifier operation, especially in the RF (radio frequency) range of operation. A larger band gap first collector layer (12), approximately 15% of the active collector region (11) thickness, is provided at the base-collector junction (13). A smaller band gap second collector layer (14) forms the remainder of the active collector region (11). The multi-layer collector structure provides higher reverse bias breakdown voltage and higher carrier mobility during relevant portions of the output signal swing. A lower saturation voltage limit, or 'knee' voltage, is provided at the operating points where linear operating regions transition to saturation operating regions as depicted in the output voltage-current (I-V) characteristic curves. The magnitude of the output signal swing of an amplifier may be increased, providing higher power amplification with greater power efficiency. The power supply voltage for the amplifier may be increased, providing for the use of a smaller, lighter power supply.

Multi-layer collector heterojunction transistor

An improved multiquantum well superlattice photodetector (10) for detecting long wavelength infrared radiation. Electron transport in a first excited energy state is enhanced in barrier layers (20) of the superlattice (16) by lowering the potential energy barriers of the barrier layers (20) to a predetermined level below the first excited energy state. The tunneling component of the dark current in a multiquantum well photodetector (10) may be substantially eliminated by placing a blocking layer (22) at one end of the superlattice (16). The thickness of the blocking layer (22) is also substantially greater than that of the barrier layers (20) of the superlattice (16) to prevent charge carriers which tunnel through the superlattice (16) from reaching the collector contact. The blocking layer (22) also has a potential energy barrier having a height at a level higher than that of the barrier layers (20) of the superlattice (16).

Dark current-free multiquantum well superlattice infrared detector

A multiple quantum well (MQW) radiation sensor eliminates tunneling current from the photoactivated current that provides an indication of incident radiation, and yet preserves a substantial bias voltage across the superlattice, by fabricating an intermediate contact layer between the superlattice and a tunneling blocking layer. Using the intermediate contact layer to apply a bias voltage across the superlattice but not the blocking layer, the photoexcited current flow through the intermediate contact and blocking layers is taken as an indication of the incident radiation. The width of the intermediate contact layer and the barrier energy height of the blocking layer relative to that of the superlattice barrier layers are selected to enable a substantial photoexcited current flow across the blocking layer. The thickness of the intermediate contact layer is preferably not more than about 1 photoexcited charge carrier mean-free path length, while the blocking layer's barrier energy height is preferably at least about 1 phonon energy level below that of the superlattice barrier layers.

Multiple quantum well superlattice infrared detector with low dark

A multiple quantum well (MQW) radiation sensor distinguishes radiation within a desired bandwidth, particularly long wavelength infrared radiation (LWIR), from background high intensity noise radiation that excites majority-minority charge carrier pairs. A minority charge carrier collector is provided at one end of the MQW, and the flow of minority charge carriers to the collector is sensed. The minority charge carrier flow provides an indication of the majority charge carrier flow that is attributable to the noise radiation rather than to radiation within the desired bandwidth, and can thus be used to provide a corrected indication of the desired bandwidth radiation.

Apparatus and method for discriminating against undesired radiation in a

A multiple quantum well (MQW) superlattice photodetector, is surmounted by a slab of transparent material having an angled surface that extends upward and away from the photodetector, such that incident radiation which is initially normal to the superlattice undergoes total internal reflection at the angled surface and is reflected onto the detector at a substantially non-normal angle. This off-normal angle allows the radiation to be partially absorbed by the detector. A reflection grating is preferably formed on the opposite side of the detector to redirect received radiation back through the MQW superlattice at an altered angle, such that remaining radiation can be absorbed by the detector during the second pass. The detector is formed upon a substrate, with the slab, substrate and quantum wells all preferably formed from the same type of material. Multiple detectors may be formed in an array upon a common substrate, with a slab providing a common reflective surface for the overall array.

Multiple quantum well photodetector for normal incident radiation

A multiple quantum well superlattice radiation detector is compositionally graded to establish an internal electric field within the superlattice that allows the device to operate with a reduced or zero externally applied bias voltage. The compositional grading can be implemented by grading the doping levels of successive quantum wells or the relative proportions of elements in successive barrier layers of the superlattice, or by a combination of the two. If a tunneling current blocking layer is employed, it can also be compositionally graded to inhibit a substantial increase in the blocking layer's barrier energy level near a charge carrier collector on the other side of the blocking layer from the superlattice. The charge carrier collector can itself be provided with a graded dopant concentration near the blocking layer to inhibit reverse bias voltage breakdown in the blocking layer.

Multiple quantum well superlattice infrared detector with graded

A multiquantum well superlattice photodetector for detecting long wavelength infrared radiation in which dark current is reduced by a blocking layer. The tunneling component of the dark current in a multiquantum well photodetector is substantially eliminated by placing a blocking layer at one end of the superlattice. The blocking layer has a potential energy barrier having a height at the same level of the barrier layers of the superlattice. The thickness of the blocking layer is substantially greater than the barrier layers of the superlattice to prevent charge carriers which tunnel through the superlattice from reaching the ohmic contact.

A multiple quantum well photodetector structure has superlattice which absorbs radiation polarized non-parallel to the superlattice during a first pass. Non-absorbed radiation polarized parallel to the superlattice is reflected back into the superlattice at a cross-angle to its incident angle, with its polarization shifted to a substantially non-parallel angle to the superlattice. At least part of this radiation is absorbed during its second pass through the superlattice, thereby increasing the efficiency of the device. An optical back grating is used to perform the cross-angle reflection, and a front grating may also be used to shift an incoming beam which is initially normal to the superlattice to an angle at which part of the beam is absorbed. The front grating is at a cross-angle to the back grating to enable a cross-angular shift by the back grating.

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Torrance, CA, United States of America

Chan-Shin Wu