Two-dimensional electron gas (2DEG) in a quantum well or inversion layer, unlike an ordinary grounded metallic plane, does not completely screen an applied transverse electric field. Owing to its Fermi degeneracy energy, a 2DEG manifests itself as a capacitor in series, whose capacitance per unit area equals CQ=me2/πℏ2, where m is the effective electron mass in the direction transverse to the quantum well. Partial penetration of an external field through a highly conducting 2DEG allows the implementation of several novel high-speed devices, including a three-terminal resonant-tunneling transistor and a gate-controlled thermionic emission transistor.

Das Buch wendet sich an Theoretiker wie auch Experimentatoren, die auf dem Gebiet der Festkfirperphysik arbeiten. E s kann aber auch Physik-Studenten der hoheren Studienjahre mit der Spezialisierungsrichtung FestkGrperphysik empfohlen werden.nT. JOHN

We have reconsidered the problem of the critical layer thickness hc for growth of strained heterolayers on lattice-mismatched substrates, using a new approach which allows us to determine the spatial distribution of stresses in a bi-material assembly and include the effects of a finite size of the sample. The possibility of dislocation-free growth of lattice-mismatched materials on porous silicon substrates is discussed as an example of a more general problem of heteroepitaxial growth on small seed pads of lateral dimension l, having a uniform crystal orientation over the entire substrate wafer. It turns out that for a given mismatch f, the critical film thickness hlc strongly depends on l, rising sharply when the latter is sufficiently small, l≲lmin. The characteristic size lmin( f ) below which, effectively, hlc( f )→∞, is determined in terms of the experimentally known (or calculated for growth on a monolithic substrate) function h∞c( f )≡hc( f ). When l≲lmin, then the entire elastic stress in the epitaxial film will be accommodated without dislocations.

The speed of operation of negative differential resistance (NDR) devices based on resonant tunneling in a double-barrier quantum-well structure is considered. It is shown that the intrinsic RC delay of a single barrier limits the frequency of active oscillations to fmax =1/(2πτ), where τ=εα−1(λ/c)exp(4πd/λ) with λ being the de Broglie wavelength of the tunneling electron, d the barrier thickness, ε the dielectric permittivity, c the speed of light, and α≊1/137 the fine-structure constant. The relevance of this estimate to recent experimental results is discussed. An alternative mechanism for the NDR is proposed—not involving resonant tunneling. It should be observable in various single-barrier structures in which tunneling occurs into a two-dimensional system of states. In a double-barrier structure, specially designed experiments are required to distinguish this effect from resonant tunneling.

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Abstract-We propose a genuinely temperature-insensitive quantum dot (QD) laser. Our approach is based on direct injection of carriers into the QDs, resulting in a strong depletion of minority carriers in the regions outside the QDs. Recombination in these regions, which is the dominant source of the temperature dependence, is thereby suppressed, raising the characteristic temperature 0 above 1500 K. Still further enhancement of 0 results from the resonant nature of tunneling injection, which reduces the inhomogeneous line broadening by selectively cutting off the nonlasing QDs.

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Abstract-We discuss in detail a new mechanism of nonlinearity of the light-current characteristic (LCC) in heterostructure lasers with reduced-dimensionality active regions, such as quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs). It arises from: 1) noninstantaneous carrier capture into the quantum-confined active region and 2) nonlinear (in the carrier density) recombination rate outside the active region. Because of 1), the carrier density outside the active region rises with injection current, even above threshold, and because of 2), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency int that holds true for QD, QWR, and QW lasers. This expression directly relates the power and threshold characteristics. The key parameter, controlling int and limiting both the output power and the LCC linearity, is the ratio of the threshold values of the recombination current outside the active region to the carrier capture current into the active region. Analysis of the LCC shape is shown to provide a method for revealing the dominant recombination channel outside the active region. A critical dependence of the power characteristics on the laser structure parameters is revealed. While the new mechanism and our formal expressions describing it are universal, we illustrate it by detailed exemplary calculations specific to QD lasers. These calculations suggest a clear path for improvement of their power characteristics. In properly optimized QD lasers, the LCC is linear and the internal quantum efficiency is close to unity up to very high injection-current densities (15 kA/cm 2 ). Output powers in excess of 10 W at int higher than 95% are shown to be attainable in broad-area devices. Our results indicate that QD lasers may possess an advantage for high-power applications.Index Terms-Quantum dots (QDs), quantum wells (QWs), quantum wires (QWRs), semiconductor heterojunctions, semiconductor lasers.