Analysis of the Emitter Charge Storage in SiGe Heterojunction Bipolar Transistors with a Lightly Doped Emitter

摘要

An analysis is presented of the emitter charge storage in SiGe HBTs with a lightly doped emitter (LDE) and the mechanisms that lead to the experimentally observed reduction of the f_T are identified. The charge storage in the LDE is important for doping concentrations below 3xl0~(17) cm~(-3) and scales with the thickness. It also depends on the Gummel number of the base and the doping concentration of the collector epi-layer. For LDE concentrations below 10~(17) cm~(-3) and collector currents above 10~4 A/cm~2 the gradient of the hole distribution can even become negative and cause excessive charge storage. However, under very high injection ― beyond the f_(T,max) - the LDE storage decreases which gives a second peak in the f_T characteristics. In order to achieve the maximum f_T, F the LDE must be thin and highly doped (≥ 3xI0~(17) cm~(-3)).