An investigation of graded and uniform base GexSi1-x HBT´s using a Monte Carlo simulation

A fully self-consistent Monte Carlo simulation has been used to investigate electron transport through the base-collector region of various Ge_x Si_1-x based heterojunction bipolar transistors. By considering a range of Ge content in the base of such devices we have shown that the base transit time decreases significantly as the Ge content of the base is increased from 0% to 15% but remains essentially unchanged by a further increase to 30%. Furthermore, it is shown that high current densities can beneficially affect the field distribution in the collector, substantially reducing the collector transit time. A modified form of the simulation has been used to investigate a graded base heterojunction bipolar transistor, with a maximum Ge content of 30%. By including the variation of effective masses across the base we have been able to show how deviations from linear grading can be modelled and to prove that such configurations can produce improvements for base transit times