Energy transport simulation for graded HBT's: Importance of setting adequate values for transport parameters

An energy transport simulation method for graded AlGaAs/GaAs heterojunction bipolar transistors (HBT's) is presented in which Al composition-, doping density-, and energy-dependences of transport parameters are considered. Fo r several representative Al composition anti doping densities, parameters s uch as electron mobility, energy relaxation time, and upper-valley fraction are evaluated as a function of electron energy by a Monte Carlo method. Fo r the other Al composition, these are determined by a linear interpolation method, Calculated cutoff frequency characteristics and electron velocity p rofiles are compared with those obtained by using more simplified approache s, demonstrating the importance of giving adequate transport parameters, pa rticularly in analyzing graded band-gap base HBT's.