The temperature dependence of the dc characteristics of silicon germanium bipolar transistors

Abstract

Silicon germanium bipolar transistors provide excellent high frequency performance within the context of silicon, encroaching dramatically on III–V semiconductor’s territory. Applications of the technology, however, will require the performance specifications with regard to the temperature range of operation. Using devices accessible from a BiCMOS technology, adapted to incorporate SiGe, we have measured the temperature dependence of properties such as the dc gain, ideality factors, and VBE at various base current levels, from 100 to 450 K. The ideality factors are close to unity above 250 K but greater than 2.5 at 100 K. We suggest that tunneling at the emitter perimeter can account for the high base ideality factors at low current and low temperature. Our data show a decrease in the gain with increasing current and temperature in normal operating ranges—devices with “box” Ge profiles in the base layer do not have this feature. This is important in preventing thermal runaway in power transistors, and may remove the necessity of emitter ballast resistors and provide higher power-added-efficiency in an amplifier block. We also show that the gain-Early voltage product is roughly linear with 1/T above ambient temperature.