The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.

Electronic properties of two-dimensional systems

This review presents in a comprehensive and tutorial form the basic principles of the Monte Carlo method, as applied to the solution of transport problems in semiconductors. Sufficient details of a typical Monte Carlo simulation have been given to allow the interested reader to create his own Monte Carlo program, and the method has been briefly compared with alternative theoretical techniques. Applications have been limited to the case of covalent semiconductors. Particular attention has been paid to the evaluation of the integrated scattering probabilities, for which final expressions are given in a form suitable for their direct use. A collection of results obtained with Monte Carlo simulations is presented, with the aim of showing the power of the method in obtaining physical insights into the processes under investigation. Special technical aspects of the method and updated microscopic models have been treated in some appendixes.

The Monte Carlo method for the solution of charge transport in semiconductors with applications to covalent materials

Electron transport in Si inversion layers at 300 K is studied using a self-consistent Monte Carlo solution of the Boltzmann transport equation coupled to the two-dimensional Poisson equation and the one-dimensional Schr\"odinger equation. Physical elements included in the model are (1) nonparabolicity effects to treat quantization in the inversion layer; (2) static screening of the Coulomb interactions accounting for the population of many subbands; (3) anisotropy of the deformation-potential interaction, shown to be quite important in the case of a two-dimensional electron gas (2DEG); (4) a careful analysis of the dynamic screening of the deformation-potential interaction, showing that the interaction between electrons and acoustic phonons can be approximated by the unscreened interaction in the nondegenerate limit of a 2DEG; and (5) the inclusion of interface ${\mathrm{SiO}}_{2}$ optical phonons. Up to ten subbands have been included to study the 2DEG together with a bulk-transport model employed to handle high-energy electrons. We have obtained mixed results: In the Ohmic regime, we have found a phonon-limited mobility that exhibits the correct dependence on carrier density, but which is about 20% larger than the experimental data. This still represents an improvement upon previous nonempirical theories, and even better quantitative agreement is obtained at the very low and very high carrier densities at which Coulomb scattering and scattering with surface roughness, respectively, control the mobility. At high longitudinal fields we find a bulklike saturated velocity, in agreement with some experimental results, but not with many others that we consider more reliable.

Monte Carlo study of electron transport in silicon inversion layers.

The physics of hot-electron degradation of Si MOSFET's: Can we understand it?

Molecular beam epitaxy (MBE)-grown L/sub g/=1.7- mu m pseudomorphic Al/sub 0.38/Ga/sub 0.62/As/n/sup +/-In/sub 0.15/Ga/sub 0.85/As metal-insulator-doped channel FETs (MIDFETs) are presented that display extremely broad plateaus in both f/sub T/ and f/sub max/ versus V/sub GS/, with f/sub T/ sustaining 90% of its peak over a gate swing of 2.6 V. Drain current is highly linear with V/sub GS/ over this swing, reaching 514 mA/mm. No frequency dispersion in g/sub m/ up to 3 GHz was found, indicating the absence of electrically active traps in the undoped AlGaAs pseudoinsulator layer. These properties combine to make the pseudomorphic MIDFET highly suited to linear, large-signal, broadband applications. >

A pseudomorphic AlGaAs/n/sup +/-InGaAs metal-insulator-doped channel FET for broad-band, large-signal applications

A study of DX centres in AlGaAs layers is presented, both experimental and theoretical. It is shown that owing to DX centres, at room temperature, the microwave transconductance of HEMTs can be considerably higher than that obtained from the DC regime, and approaches the values which could be expected if DX centres did not exist. As a result, for room temperature applications, special structures designed to eliminate DX centres are not really necessary.

Investigation of influence of DX centres on HEMT operation at room temperature

An attempt is presented in modelling at room temperature the carrier dynamics in the inversion layer using a Monte Carlo simulation. The interaction of inversion carriers with the surface is regarded successively as perfectly specular and perfectly diffuse. A comparison of the results with the experimental data of Coen and Muller gives reasonable agreement in a wide range of electric field.

Electron dynamics in P-Si m.o.s.f.e.t. inversion channels

A very simple analytical relation, without any fitting parameters, between the gate current in a HIGFET or SISFET and the 2D electron concentration giving rise to the drain current is proposed. The theoretical background which allows us to obtain this relation is first presented. Then good agreement is shown with experimental results obtained with an AlGaAs/GaAs HIGFET. Lastly, characteristics of structures which should yield high values of 2D electron concentration without too high a value of gate current are discussed.

Gate current and 2D electron concentration in HIGFET and SISFET

By using a model which considers velocity overshoot, it is shown that the performance of GaAs MESFETs in enhancement mode depends strongly on the geometrical and electrical characteristics of the access region between source and gate. The sheet resistance of the unrecessed epilayer, and the distance between the source-end of the recessed region and the gate, have to be as small as possible. 300 nm gate length MESFETs with very low values for these parameters were realised with an n-GaAs active layer (6*10/sup 17/ cm/sup -3/). These devices exhibit very high microwave transconductances (800 mS/mm) with good cutoff frequencies (up to 55 GHz). This result suggests that very high transconductance MESFETs can be fabricated from not-too-heavily doped active layers provided that the characteristics of the source-gate access region is properly optimised.

Realisation of very high transconductance GaAs MESFETs

Based on very simple design criteria, a reliable technological process has been achieved allowing us to realise high performance 0.2 μm gate GaAs MESFETs characterised by low source and gate access resistances (less than 0.2 Ωmm and 0.25 Ωmm, respectively) and high transconductances (more than 1 S/mm) and exhibiting high current and power gain cut-off frequencies (close to 100 GHz).

Design and realisation of very high performance 0•2 μm gate GaAs MESFETs

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