The effect of base dopant out-diffusion and undoped SiGe or Si spacer layers at both sides of the base on the characteristics of microwave Si/SiGe/Si HBT´s

The effect of base dopant out-diffusion and undoped (i-) SiGe or Si layers at both sides of the base on the characteristics have been studied by device simulator ISE-TCAD. It has been found that small amounts of base dopant out-diffusion from heavily doped bases will degrade DC and frequency characteristics. The insertion of an i-SiGe layer at both sides of the base can improve collector current, current gain β and cut-off frequency f_T, the insertion of an i-Si layer between emitter and base and an i-SiGe layer between the base and collector can also improve β and f_T. These simulation results promise more freedom in the design of HBTs.