MONOLAYER DELTA-DOPED HETEROJUNCTION BIPOLAR-TRANSISTOR CHARACTERISTICS FROM 10 TO 350 K

Heterojunction bipolar transistors (HBTs) with bases composed of a single monolayer of dopant are found to have exponentially increasing current gain as the temperature is lowered, in contrast to the gains of homogeneously doped HBTs, which have little dependence on temperature. We present GaAs-AlGaAs HBTs with base doping as high as 3 X 10(14) cm-2 with a room-temperature current gain of 14. The sheet resistance of such a layer is 270-OMEGA/square at room temperature, and the carriers are restricted to a 15 angstrom layer, implying an extremely short base transit time. AT 77 K the gain increases to 220, while the base resistance decreases to 190-OMEGA/square. Therefore by operating this HBT at low temperature we have achieved for the first time high gain and low base resistance in a HBT with an infinitesimally thin base.