Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-09-17
2001-12-04
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S022000, C257S025000, C257S184000, C438S093000, C438S094000
Reexamination Certificate
active
06326639
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor hetereostructure radiation detector for wavelengths in the infrared spectral range. The semiconductor heterostructure radiation detector is provided with an active layer composed of a multiplicity of periodically recurring single-layer systems each provided with a potential well structure having at least one quantum well with subbands (quantum well), the so-called excitation zone, which is connected on one side to a tunnel barrier zone, whose potential adjacent to the excitation zone is higher than the band-edge energy of a drift zone adjoining on the other side of the potential-well structure.
STATE OF THE ART
Photodetectors of the aforementioned type are a special sort of quantum-well intersubband photodetectors, abbreviated QWIP. The QWIP structures usually employed are photoconductors, whereas the aforedescribed detectors have photovoltaic properties, i.e. upon illumination, the detector structure generates a photocurrent inside the detector without application of external electrical voltage to the detector. Detectors having such photoresistive photovoltaic properties have the special advantage that they are not subject to any generation recombination noise in the unilluminated state, because no dark current flows in them. This permits, in particular, the production of detectors with very large dynamic properties.
A typical representative of such a quantum-well intersubband photodetector optimized for detection of infrared waves is described in DE 42 20 620 C1. The presented quantum-well intersubband infrared photodetector is provided with a multiplicity of quantum wells which are spatially separated by wide barrier layers. With respective doping, each single quantum well has an asymmetric potential course resulting in the creation of tunnel barriers at both ends of the barrier layer and form electronic conditions inside it, permitting in this manner the selective rerelaxation of the charge carriers excited in the barrier region. These carriers form a photocurrent without application of an external voltage to the layer sequence.
H. Schneider et al.'s article “Transport Asymmetry and Photovoltaic Response in (AlGa)As/AlAs/GaAs/(AlGa)As Single-Barrier Quantum-Well Infrared Detectors” Appl. Phys. Lett. 60(12), Mar. 23, 1992, pp. 1471 to 1473, describes QWIP structures whose active detector layer is composed of a periodic sequence of a so-called absorption zone, a tunnel barrier zone and a drift zone. The essential feature of these state-of-the-art structures is that the potential edge of the tunnel barrier zone is higher than the drift zone so that the charge carriers, raised from the absorption zone by optical excitation, cannot propagate in the direction of the immediately adjacent tunnel barrier zone but rather go in the opposite direction via the drift zone. According to the authors, this asymmetric transport mechanism is composed of the asymmetric potential courses and the scatter processes at the transition regions. The charge carriers migrating in the direction of the drift zone reach the tunnel barrier zone of the next layer sequence through which the charge carriers tunnel into the absorption zone.
C. Sirtori et al's article “Photocurrent Reversal Induced by Localized Continuum Resonances in Asymmetric Quantum Semiconductor Structures', Appl. Phys. Lett. 63, S. 2670 (1993) describes the use of a multiplicity of quantum-well structures each provided with different potential well widths.
Although all the aforedescribed photovoltaic detectors have the advantage that they are subject to substantially less noise than conventional photoconducting detectors, because they generate no dark current due to the fact that no external voltage is applied, as already mentioned in the preceding. Johnson noise is also extremely low due to the very great impedance of the detectors. As a photocurrent flows in the present detector structures although no voltage is applied, thus there is a resistive—not only—a capacitive coupling. Detector arrays of this detector type are very compatible with readout electronic devices, such as those used for InSb respectively HgCdTe detectors.
However, the state-of-the-art photovoltaic detectors have the disadvantage that they are much less sensitive than the conventional detectors operating with an external supply voltage.
DESCRIPTION OF THE INVENTION
For this reason, the present invention provides an improved semiconductor heterostructure radiation detectors, generically based on the photovoltaic principle, in such a manner that detection sensitivity is improved.
The present invention is based on the further development of a state-of-the-art semiconductor heterostructure radiation detector which is provided with a layer that is active for the detection of wavelengths in the infrared spectral range. This layer, which is active for the detection of wavelengths in the infrared range, is composed of a multiplicity of periodically recurring single-layer systems. The single-layer systems each have a potential structure having at least one quantum well with subbands, the so-called quantum well, hereinafter referred to as the excitation zone. The quantum well is connected on one side to a tunnel barrier zone whose potential adjacent to the excitation zone is higher than the band-edge energy of a drift zone adjacent on the other side of the potential- well structure. Such a layer system is, as already described in the preceding, also known as a “single-barrier well” structure.
An element of the present invention is that the state-of-the-art structure is further developed in that the drift zone is adjacent to a trap zone provided with at least one subband-containing quantum-well structure and is connected to a tunnel-barrier zone of another periodically recurring single-layer system. Another element of the present invention is that the energy levels of the subbands of the quantum-well structures inside the excitation and trap zone and the thickness of the tunnel barrier zone are dimensioned in such a manner that there is sufficient tunnel probability for the charge carriers to tunnel from the trap zone through the tunnel barrier zone into the excitation zone.
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Schneider Harald
Walter Martin
Antonelli Terry Stout & Kraus LLP
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Mintel William
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