On the electron minority carrier mobility and the effective bandgap in heterojunction bipolar transistors with strained Si1-xGex-base

This paper describes the experimental extraction of the ratio of the minority carrier diffusion constant in strained p-type Si_1-xGe_x to the value in Si at the same doping level. The extraction is based on the application of new insights in the effective density of states in strained Si_1-xGe_x. It will be shown how different experimental observations suggest a strong reduction of the valence band density of states, a fact already predicted by theoretical calculations [1]. Together with the minority carrier diffusion-ratio constant we will also extract the effective bandgap difference between Si and strained Si_1-xGe_x. It will be shown how the reduction of the effective valence band density of states leads to a significantly lower value for the heavy-doping induced apparent bandgap narrowing in strained Si_1-xGe_x than in Si. Due to this effect the collector current enhancement by the use of strained Si_1-xGe_x-layers in the base will become smaller at base doping levels exceeding 10¹⁹ cm^-3 and Ge-fractions larger than 10%.