Investigation of deep traps in silicon–germanium epitaxial base bipolar transistors with a single polysilicon quasi self-aligned architecture

Abstract

The electrical properties of Si/Si 1− x Ge x bipolar transistors have been analysed at temperatures ranging from 77 to 500 K. The investigated SiGe base transistors were fabricated using a BiCMOS single-polysilicon quasi self-aligned process, where base implant had been replaced by selective epitaxy on the base active area. At low temperature, static current–voltage measurements show a degradation of base current ideality, whereas collector current remains ideal over the whole temperature range. By studying forward and reverse currents at emitter–base and base–collector junctions, we have established that deep levels were involved in conduction phenomena at these junctions. Detailed measurements using capacitance transient spectroscopy (with different reverse and filling pulse voltages and different filling pulse durations) have revealed the presence of two deep levels along the periphery of the emitter. These deep levels have been found with identical characteristics at both junctions. It is demonstrated that these traps are most probably induced by the extrinsic base implantation.