The effect of base-emitter spacers and strain dependent densities of states in Si/Si/sub 1-x/Ge/sub x//Si heterojunction bipolar transistors

The authors describe two effects which can significantly affect the transport of electrons across the base region of Si-Si/sub 1-x/Ge/sub x/-Si heterojunction bipolar transistors and limit the effective bandgap reduction. These effects are the formation of a parasitic electron barrier due to a nonabrupt base-emitter junction and the reduction of the density of states in the Si/sub 1-x/Ge/sub x/ base layer because of the anisotropic strain. Junction spacers have been found to eliminate the parasitic barriers for optimum devices. For optimum device performance it is necessary to keep the Si-Si/sub 1-x/Ge/sub x/ interfaces in the junction depletion region. The strain dependence of the densities of states in the base region lowers collector current.<>