Double heterojunction GaAs/AlxGa1−xAs bipolar transistors prepared by molecular beam epitaxy

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT’s) grown by molecular beam epitaxy (MBE) were fabricated and tested. The incorporation of a graded base-collector junction improved the dc performance of a DHBJT over that obtained with an abrupt base-collector junction. Maximum current gains of 500, 900, and 1650 were obtained using graded collector and emitter junctions and base widths of 0.05, 0.2, and 0.1 μm, respectively. The reduction in current gain in going from a 0.1-μm base width to a 0.05-μm base width is attributed to a lowering of the electron lifetime resulting from the high base doping level used. The value of 1650 is comparable to the current gains obtained from liquid phase epitaxy (LPE)-grown HBJT’s and is the best obtained from MBE-grown HBJT’s. Common-emitter transistor turn-on voltages were found to correspond to a difference in the collector and emitter junction turn-on voltages. High collector growth temperature improved dc performance with 700 °C as the optimum growth temperature. Preliminary results from DHBJT’s with superlattice interfaces have shown that the current gain is almost constant through a wide range of collector current levels.