Numerical study of a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base

We consider a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base. The step height is slightly greater than the longitudinal optical (LO) phonon energy homega(LO) in order to facilitate LO-phonon-enhanced forward diffusion. Impurity and LO phonon scattering are considered as the dominant scattering mechanisms. We solve the Boltzmann transport equation (BTE) numerically in one dimension across the base and at arbitrary frequencies, unifying and extending previous theoretical treatments. The base transport factor is analyzed in the frequency domain by varying device parameters. The solution of the BTE is incorporated into the small-signal parameters of the intrinsic part of the device. Using a simple circuit approach for the extrinsic elements, high-frequency figures of merit are presented, casting light on the potential of the device as a high-gain amplifier and more importantly as a transit-time oscillator for millimeter-wave applications. (C) 2002 American Institute of Physics.