Temperature Dependence of the Current Gain of Si/Si1-xGex Heterojunction and Si Homojunction Bipolar Transistors

This paper describes a method for extracting the bandgap narrowing in the base of Si homojunction or Si/Si1-xGex. heterojunction bipolar transistors from the temperature dependence of the collector current. The method is applied to a Si homojunction transistor with a 6 × 10¹⁸ cm^¿3 epitaxial base, and a doping-induced bandgap narrowing of 39meV is obtained. The method is also applied to a Si/Si0.84Ge0.16HBT with nominally the same base doping concentration, and a bandgap narrowing of 119meV is obtained due to the Ge. This compares with a theoretical valence band offset of 117meV. The effect of boron out-diffusion on the extracted value of bandgap narrowing is also described.