K. M. S. V. Bandara

Exchange interactions in quantum well subbands

It is shown that exchange interactions in the two‐dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Exchange interaction effects in quantum well infrared detectors and absorbers

Infrared excitation energies between the ground‐state subband and the first excited‐state subband in quantum wells are analyzed including the effect of exchange interactions on the ground‐state subband. Analytic and numerical calculations relevant to infrared absorption and infrared detection are performed. Previous work on exchange interaction effects in narrow wells is extended to deal with well widths which can provide absorption peaks and photocurrent peaks throughout the long‐wavelength infrared and very‐long‐wavelength infrared regime. Exchange effects are shown to be substantial.

GaAs/AlGaAs superlattice miniband detector with 14.5 μm peak response

Extended long wavelength infrared detection with a miniband‐type AlGaAs/GaAs superlattice structure is reported. The experimental response band of the detector is peaked near 14.5 μm in good agreement with the theoretical response, provided that electron‐electron interactions are taken into account. The detector operates at a low bias voltage, which could lead to important advantages in application to IR focal plane arrays.

Long-wavelength infrared detection in a Kastalsky-type superlattice structure

The first successful demonstration of long‐wavelength infrared (LWIR) detection with a Kastalsky‐type AlGaAs/GaAs superlattice structure is reported. The experimental response band of the detector is centered near 10 μm in very good agreement with the theoretical response band provided that electron‐electron interactions are taken into account. The detector operates at significantly lower bias voltage than photoconductive multiple quantum well LWIR detectors. This could lead to important advantages in applications to photovoltaic detector arrays. The response at 83 K is about 50% of the response at 24 K. Optimization of the response, operating temperature, or bias voltage has not been carried out.

Electron‐electron interactions and resonant tunneling in heterostructures

A Hartree–Fock approach [Z. Physik 61, 126 (1930)] is used to analyze electron‐electron interactions in heterostructures. The analysis includes estimates of the effect of electron‐electron interactions on localized states in quantum wells and resonant tunneling. Resonant tunneling is treated in a self‐consistent narrow resonance approximation. The Hartree–Fock approach yields an explicit exchange interaction subband energy shift which is completely omitted in Poisson equation treatments. Exchange effects are found to be of the same order of magnitude but opposite in sign to direct interactions at typical electron densities.

Is intrinsic bistability really intrinsic tristability

The issue of intrinsic bistability in resonant tunneling devices is examined analytically and quantitatively. The results indicate that the putative argument for intrinsic bistability is actually an argument for intrinsic tristability. If a third stable configuration really exists, it might be hard to access experimentally although its existence could have consequences for high‐speed device applications. If an intrinsic bistable regime is ipso facto a tristable regime, then an experimental search for a third stable configuration could provide persuasive evidence for the physical mechanism commonly advocated as the basis for intrinsic bistability.

Spectral linewidths of quantum well intersubband transitions

Abstract The effect of electron-electron interactions on quantum well intersubband transition spectral linewidths is examined. It is shown that the effect of exchange interactions on absorption linewidths can be comparable in magnitude to observed absorption linewidths, so that previous assumptions about the origin of observed widths may need to be revised. The exchange interaction width mechanism involves differences in the in-plane momentum dependence of electron energies in the ground state subband and the excited state subband. These differences are primarily due to intrasubband exchange interactions in the ground state subband.

Interfacing multispectral sensors to real time processors based on neural network models

Experiments have been carried out to demonstrate and study the interfacing of silicon sensors to silicon devices which represent first stage elements of an artificial neural network. Sensor outputs (network inputs) are photocurrents associated with infrared, visible, or ultraviolet light. First stage linear coding of input current into the pulse rate of a stereotypical neuronlike spiketrain output has been achieved with a dynamic range of more than 106. For 1 pA inputs, the estimated noise referred to input is 10 fA. Network elements are shown to obey equations of the same form as equations which occur in nonlinear neural network models recently analyzed by Hopfield [Proc. Natl. Acad. Sci. 81, 3088 (1984)] and previously studied by Sejnowski [J. Math. Biology 4, 303 (1977)].

Quantum effects and bit errors in mesoscopic logic and memory circuits

Bistability in well known circuits employing devices with nonlinear current‐voltage characteristics is shown to be a classical concept which can be modified by quantum mechanical effects. The classically stable states ‘‘0’’ and ‘‘1’’ are shown to correspond to a single stable quantum mechanical state and a metastable state. Bit error rates associated with metastable state lifetimes are estimated for circuits employing quantum well devices and found to be appreciable in the mesoscopic regime (feature sizes ≲0.2 μm).

Breit–Wigner description of resonant tunneling

Analytic properties of scattering amplitudes are used to develop Breit–Wigner parametrizations of resonant tunneling currents in heterostructures. The analysis reveals a problem in using conventional Breit–Wigner resonance expressions to describe the important regions of peak current and negative differential resistance which occur when resonances are near the emitter threshold. Commonly used Breit–Wigner expressions (a) do not incorporate the correct threshold branch points, (b) do not possess the correct limit at threshold, and (c) do not satisfy unitarity both above and below threshold. Expressions which avoid such difficulties are deduced and used to parametrize tunneling current formulas.