Silicon-Germanium-Base Heterojunction Bipolar Transistors By Molecular Beam Epitaxy

We report the first SiGe-base heterojunction bipolar transistors (HBT's). The devices were fabricated using molecular beam epitaxy (MBE), low-temperature processing, and germanium concentrations of 0, 6, and 12 percent. The transistors demonstrate current gain, and show the expected increase in collector current as a result of reduced bandgap due to Ge incorporation in the base. For a 1000-Å base device containing 12-percent Ge, a 6 times increase in collector current was measured at room temperature, while a 1000 times increase was observed at 90 K. The temperature dependence of the collector current of the Si0.88Ge0.12-base transistor is consistent with a bandgap shrinkage in the base of 59 meV. For the homojunction transistors, base widths as thin as 800 A were grown, corresponding to a neutral base width of no more than 400 Å. © 1988 IEEE.