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.

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.

The stationary characteristics of gain-guiding semiconductor lasers with a symmetrical structure are theoretically examined, solving the wave equation by the Galerkins method and taking the effect of transverse carrier diffusion into consideration. For wide stripe lasers, the carrier density distribution becomes flat with the increase of the current density. And with the more increase of the current density, a hole-burning, a mode deformation and a kink" in the current density-light output characteristics appear. For the narrower stripe lasers, the current density where the hole-burning occurs increases and surpasses the normal driving current level. Hence the transverse mode profile can be approximated by the gaussian one with a small numerical error, which verifies the assumption in our previous papers(8),(9). The influence of the current profile on the characteristics is larger for wide stripe lasers than for narrow ones. However it less affects them compared with the transverse carrier diffusion even for wide stripe lasers. That is, the effect of current density profiles on the characteristics is weakened by the transverse carrier diffusion.

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.

The performance of the tapered velocity coupler for single mode fibers is investigated theoretically and experimentally. An improved analytical solution convenient for analyzing and designing the tapered velocity couplers is derived. The mode cross-talk in the tapered region and the mode conversion loss due to the discontinuity at the ends of the fibers are measured and are compared with the theoretical results. Simultaneous shaping of the multiple single mode fibers into tapered structure is successfully realized and a low loss splicing (less than 0.2 dB) is achieved either by using adhesives or by the thermal fusion technique.

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.