Measurement of the bandgap narrowing in the base of Si homojunction and Si/Si_1-xGe_x heterojunction bipolar transistors from the temperature dependence of the collector current

Abstract
A method is described for measuring the bandgap narrowing in the base of a Si homojunction or Si/SiGe heterojunction bipolar transistor from the temperature dependence of the collector current. The model includes the temperature dependence of the intrinsic carrier concentration, the bandgap, the minority carrier mobility, and freeze-out of dopant in the base. The analysis method is applied to transistors with epitaxial Si and SiGe bases, and also to Si devices with ion implanted bases. For the epitaxial base transistors, where the base boron profiles are sharp, a linear characteristic is obtained from which the bandgap narrowing in the base can be calculated. For a SiGe HBT with 16% Ge in the base, a bandgap narrowing of 117meV is obtained, which compares very well with the theoretical value of 117meV for the valence band offset. For the implanted base transistors, a linear characteristic is not obtained. This is explained by the presence of doping tails on the base profile.