Determination of transport properties in gallium arsenide and related compound semiconductors

Accurate measurements of the electrical properties of semiconductor materials are important for device modeling and optimization. The femtosecond laser characterization laboratory at Purdue University has been designed to make a wide range of time-resolved measurements available for virtually any semiconductor system. The laser system has been successfully employed to characterize the mobility versus doping in both p$\sp+$-GaAs and p$\sp+$-$\rm In\sb{0.53}Ga\sb{0.47}As.$ In these material systems, the minority electron mobilities are comparable to or higher than the majority electron mobilities. For doping densities greater than $1\times 10\sp{19}$ cm$\sp{-3},$ the mobility is found to increase with increasing doping density. The observed high mobilities in degenerately doped p-GaAs and p-InGaAs indicate the diffusion velocity of electrons through the base of a Np$\sp+$n heterojunction bipolar transistor may approach the electron thermal velocity. Indeed, direct experimental evidence demonstrates a thermal velocity limit to diffusive transport in np$\sp+$n GaAs bipolar transistors. This thermal velocity limit must be taken into account when examining the steady state and transient characteristics of GaAs bipolar transistors. When corrected for thermal velocity limits, the equilibrium np product may be extracted from previous measurements of minority carrier injection currents in GaAs bipolar transistors. The femtosecond laser characterization laboratory has also been employed for ultrafast measurements of the lifetime of photo-excited carriers in low temperature growth GaAs and AlGaAs. These materials exhibit extremely short carrier lifetimes, which make them useful for ultrafast photoconductive switch applications. The femtosecond laser characterization laboratory has provided considerable insight into the carrier transport processes affecting high speed semiconductor devices