Germanium Gradient Optimization for High-Speed Silicon Germanium Hetero-Junction Bipolar Transistors

The effect of germanium trapezoidal profile shape on the direct current (DC) current gain (β_F), cut-off frequency (f_T) and maximum oscillation frequency (f_MAX) of silicon-germanium (SiGe) hetero-junction bipolar transistors (HBTs) has been investigated. The energy balance (EB), hydrodynamic (HD) and drift-diffusion (DD) physical transport models in SILVACO technology computer aided design (T-CAD) simulator were used. It was found that the current gain values using energy balance model are higher than hydrodynamic and much higher than those corresponding to drift-diffusion. Moreover, decreasing the germanium gradient slope towards the collector side of the base enhances the maximum oscillation frequencies using HD and EB models whilst, they remain stable for DD model.