A theoretical toolbox for the simulation and design of HBTs constructed in the Al Ga₁₋ As and Si₁₋ Ge material systems

A theoretical toolbox for the simulation of Heterojunction Bipolar Transistors (HBTs), including the effects of tunneling, recombination, and the optimum non-linear base profile (for the minimisation of the base transit time), is developed. The models developed are applicable to a general material system, and are analytic. Extensions specifically required by the complex S₁₋xGex material system are also developed. The optimum (to minimise base transit time) base doping is found to be non-exponential, and the optimum base bandgap grading is not linear. A general transport model for HBTs, including recombination processes, is developed that accounts for the complex nature of charge transport throughout the entire device. Unique methods for optimising HBT metrics, which cannot be employed for Bipolar Junction Transistors (BJTs), are also presented. A description of charge transport within the emitter-base Space-Charge Region (SCR), which accounts for tunneling and is not beholden to the usual drift-diffusion analysis, is developed. The implications of having different electron effective masses in the two sides of the hetero-junction, leading to what is termed a mass boundary, is fully explored. It is found that the tunneling of electrons within the emitter-base SCR leads to a non-Maxwellian minority-particle ensemble distribution entering the neutral base. Finally, transport within SiGe HBTs is considered, with all of the relevant material models presented and multi-band transport models developed. This treatment leads to a variety of interesting conclusions regarding the operation of present-day SiGe HBTs and possible future designs.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat