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 fT are identified. The charge storage in the LDE is important for doping concentrations below 3×10 cm 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 cm and collector currents above 10 A/cm the gradient of the hole distribution can even become negative and cause excessive charge storage. However, under very high injection – beyond the fT,max the LDE storage decreases which gives a second peak in the fT characteristics. In order to achieve the maximum fT, the LDE must be thin (≤ 50 nm) and highly doped (≥ 3×10 cm).