Numerical simulation and analysis of silicon(1-x)germanium(x) pseudomorphic heterojunction bipolar transistors

Numerical simulation was used to analyze performance of Si$\\sb{1-x}$Ge$\\sb{x}$ pseudomorphic heterojunction bipolar transistors (PHBTs). Models for different material properties were developed and used in 2-D drift-diffusion-equations and 1-D hydrodynamic-equations simulation programs. N-p-N and P-n-P PHBTs were analyzed. For N-p-N PHBTs both metal- and poly-Si-emitter contacts were investigated. The figures of merit used are unity current gain frequency f$\\sb{T}$ and maximum frequency of oscillation f$\\sb{max}$. In all cases analyzed, Si$\\sb{1-x}$Ge$\\sb{x}$ offers significant advantages over equivalent Si devices: metal emitter N-p-N Si$\\sb{1-x}$Ge$\\sb{x}$ devices can have f$\\sb{T}$ up to two to three times higher than Si; poly-Si emitter N-p-N Si$\\sb{1-x}$Ge$\\sb{x}$ up to 50% or more higher f$\\sb{T}$; P-n-P metal-emitter Si$\\sb{1-x}$Ge$\\sb{x}$ devices shows similar improvements. f$\\sb{max}$ is also improved in all devices, but not as much as f$\\sb{T}$. Some devices are actually limited by their f$\\sb{max}$ and not f$\\sb{T}$ and to improve their performance, f$\\sb{max}$ must be increased first by reducing parasitic resistances and capacitances.