Mathematical modeling of a SiGe HBT: Effect of Ge profiles

Abstract

The performance of a silicon-germanium heterojunction bipolar transistor (SiGe HBT) has been studied with three different Ge doping profiles in the base region. These profiles are triangular (graded), trapezoidal (graded and box combination), and box (constant). The parameters studied are current gain (β), early voltage (Va), βVa, cutoff frequency (fT), and maximum frequency of oscillation (fmax). Additional effects such as temperature, base pushout, low collector current injection effect, bandgap narrowing, and collector voltage variations have been included in the model. The modeled results show that in box profile, β, and emitter delay (τe) are highly improved whereas in the triangular profile, the parameters Va, βVa, and base transit time are highly improved. The trapezoidal approach has intermediate results between the other two approaches. The f_T and fmax are highly improved as compared to Si BJT in all the three cases. The results obtained from the developed model show good agreement with the existing reported results.