Experimental I-V characteristics of AlGaAs/GaAs heterojunction bipolar transistors with very thin bases
Npn abrupt AlGaAs/GaAs heterojunction bipolar transistors with thin base widths (WB) down to 200 Å have been fabricated for the first time, and their collector and base current-voltage characteristics have been studied. The experimental results show that the surface recombination base current and the base bulk recombination current are both significantly lower in 200 Å base HBTs than in comparable devices with 500 Å base width. For the thin base HBTS, the base bulk recombination current density is proportional to ~WB and the surface recombination current density is proportional to ~WB2. The experiment also showed that the collector current across a thin p+ GaAs base is limited, as expected, by the thermal velocity of the electrons rather than by conventional diffusive transport.