Investigation of transit times in SiGe structures: a Monte-Carlo approach

Self-consistent Monte Carlo simulations of the base-collector regions of Ge_xSi_1-x/Si heterostructure bipolar transistors are discussed. High-current effects, which cause inversion of the collector field, are shown to decrease the base-collector transit time from 9.3 ps to 5.7 ps. Base and base-collector transit times are investigated for a range of Ge content in the base. The base transit time is found to decrease as the Ge content of the base is increased from 0% to 30%. The effects of the electron injection boundary conditions on transit times are studied, and it is shown that a complete knowledge of the valley occupancy on injection is unimportant for many aspects of the simulation of carriers passing through the device but that it is necessary to have a good knowledge of the momentum distribution within individual valleys