A novel double-base heterojunction bipolar transistor for low-temperature bipolar logic

The authors report a novel double-base heterojunction bipolar transistor (DB-HBT) in the Si/SiGe material system which can operate with high gain in a single-transistor NAND gate at 77 K. The device structure is similar to that of a Si/SiGe/Si HBT, except for an intrinsic Si layer inserted into the p-SiGe base. The intrinsic Si layer is used to electrically isolate the two SiGe base layers of an n-Si/p-SiGe/i-Si/p-SiGe/n-Si DB-HBT, because the valence-band discontinuity at the SiGe/Si heterojunction acts as a barrier for holes. A key point is that the Si layer introduces only a very small barrier in the conduction band because most of the bandgap difference between unstrained Si and strained SiGe occurs at a valence band offset Delta E/sub V/.