Effects of self-heating on the microwave performance of SiGe HBTs

In this study, the effects of self-heating on the fT and fmax frequencies have been investigated in NpN SiGe heterojunction bipolar transistors (HBTs) by two-dimensional numerical device modeling using a commercial simulator. The device studied was a single emitter stripe, double mesa configuration with self-aligned base contacts where a Gaussian distribution was assumed for the base's boron profile. Increases in the base transit time and the collector delay time due to degradation in the electron mobility and saturation velocity and an increase in the base-collector depletion layer width were found to degrade the fT while increased base resistance also contributed to the reduction in fmax. The onset of significant device self-heating and degradation in fT and fmax were observed for collector current densities of 5 × 104 A/cm2 for VCE = 2 V.