Numerical simulation of AlGaAs/GaAs HBT's with various n--collector structures are performed to study the cutoff frequency characteristics. A thinner n--layer with higher doping density is desirable to achieve higher cutoff frequency. Effects of the velocity overshoot are pronounced in a case with a thinner n--layer.

GaAs MESFETs with a p-buffer layer (or a buried p-layer) are important devices for high-speed GaAs ICs. To study what conditions are required as a good substrate for ICs, we have investigated, by two-dimensional simulation, small-signal parameters and drain-current transients of GaAs MESFETs with a p-buffer layer on the semi-insulating substrate. It is shown that the introduction of a p-buffer layer is effective to improve the transconductance and the cuttoff frequeycy. These parameters are not degrade even if the p-layer doping is increased and a neurtral p-region exists. It is also shown that drain-current drifts and hysteresis in I-V curves can occur in a case with a p-buffer layer, too. It is concluded that the introduction of a relatively highly-doped p-layer on a substrate with low acceptor and electron trap (EL2) densities is effective to realize the stable and high performance of GaAs MESFETs.

Two-dimensional simulations of small-signal parameters (such as transconductance, gate capacitance and cutoff frequency) of GaAs MESFETs are performed in which impurity compensation by deep levels in the semi-insulating substrate is considered. It is shown that these are strongly affected by impurity densities in the substrate. For higher acceptor densities in the substrate, both transconductance and cutoff frequency become higher, because the substrate current is reduced. For low acceptor densities in the substrate, the gate capacitance takes a relatively large value even if the gate voltage is deeply negative, because the channel extends into the substrate and electrons there contribute to the capacitance. It is concluded that to utilize high-frequency performance of GaAs MESFETs, the acceptor densities in the semi-insulating substrate should be made high.

A numerical model for AlGaAs/GaAs HBT's is proposed in which the transport of electron energy is included by using hydrodynamic equation derived from the Boltzmann equation. The results are compared with those by the conventional model, indicating the importance of the energy transport effects.

Numerical simulations of GaAs MESFET's with deep acceptors Cr in the substrate are made for the first time. Acceptors are found to play an important role in determining I-V curves, whether they are shallow or deep. To minimize short-channnel effects, the acceptor density in the substrate must be high.

A simplified two-dimensional model that includes only an electron current equation is shown to be successfully applied to the calculation of current-voltage characteristics of GaAs MESFET's with a p-beffer layer or with the semi-insulating substrate including hole traps.