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

Abstract

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 3×10 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 epilayer. 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 , the LDE must be thin (≤ 50 nm) and highly doped (≥ 3×10 17 cm -3 ).