Semiconductor superlattices: a model system for nonlinear transport

The results of computer simulations of submicron-scale Si and GaAs MESFET's which include carrier energy transport effects ("velocity overshoot") are presented. A new technique for solving the energy transport equation, which allows the description of short-time-scale carrier dynamics to be greatly improved without significantly increasing program complexity or computer time relative to conventional numerical simulations is described. The results indicate that the switching times of GaAs MESFET's should be less than would be predicted by conventional numerical models, and are consistent with experimental results.

Two-dimensional numerical simulation of energy transport effects in Si and GaAs MESFET's

On a semiphenomenological level, the explanation of the Gunn effect is one in terms of a time-independent, negative differential, bulk conductivity. This mechanism is based on the conduction-band structure of GaAs, which provides for two kinds of electrons, light and heavy ones. Light electrons dominate at low fields, heavy ones at high fields. Since the mobility of the heavy electrons is much lower than that of the light ones, there is a range of current decrease with increasing field, i.e., a negative conductivity. This negative conductivity leads to an electrical breakup of the crystal into alternating traveling domains of high and low fields, accompanied by alternating current. In a "mathematically perfect" crystal this instability would take the form of traveling negatively charged electron accumulation layers, separating the domains of high and low fields. In real crystals the inevitable spatial fluctuations in the impurity distribution lead to the experimentally observed dipole mode, wherein both negatively charged electron accumulation layers and positively charged electron depletion layers occur.

Nonlinear space-charge domain dynamics in a semiconductor with negative differential mobility

Computer calculations have been performed in order to investigate the influence of one‐dimensional random doping fluctuations and localized inhomogeneities on the performance of bulk GaAs oscillators. For the doping fluctuations assumed it was found that accumulation layers dominate in subcritically doped samples (n0×L<1012 cm−2) while dipole layers always form in samples doped above the critical level. Five different modes of oscillation are described and defined with the help of a mode chart. Special attention is paid to the ``quenched multiple‐dipole'' mode, which is difficult to obtain if localized inhomogeneities such as dislocations or doping gradients are incorporated in addition to the random fluctuations. It is concluded that a series‐connection of accumulation‐mode devices, which are not sensitive to doping fluctuations, provides an attractive approach to microwave or millimeter‐wave oscillator design.

Computer Study of Bulk GaAs Devices with Random One‐Dimensional Doping Fluctuations

An analytical treatment of uniformly propagating stable domains is developed for the case when the electrons have a field-independent diffusion coefficient and follow an arbitrary static velocity-field characteristic exhibiting a negative differential resistivity. For any value of the diffusion coefficient the domain velocity is equal to the electron velocity outside the domain and the peak domain field is determined by a dynamic characteristic which is derived from the static characteristic by a simple geometrical construction. In the limit of zero diffusion the domain consists of a depletion layer triangle to which is added a flat-topped region in long, low-resistivity samples at high bias voltages. Numerical results are given for a schematic three-line static characteristic which is representative of gallium arsenide and the variation of the domain parameters with resistivity, sample length and bias voltage is discussed.

https://iopscience.iop.org/article/10.1088/0508-3443/17/7/301/pdf

A simple analysis of stable domain propagation in the Gunn effect

A theory of negative-conductance amplification and of Gunn-effect oscillation in good agreement with experimental observations is developed for "two-valley" semiconductors such as GaAs and InP. The theory is based upon a conduction-band model in which the relative populations of two valleys of vastly different mobility are determined by the average electron temperature, the latter being described by energy transport equations. Numerical computer solutions of the dynamic equations identify different modes of sample behavior and give a clear indication of the microscopic physical processes relevant to each.

Theory of negative-conductance amplification and of Gunn instabilities in "two-valley" semiconductors

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Theory of negative-conductance amplification and of Gunn instabilities in "two-valley" semiconductors