| GaN/SiC heterojunction bipolar transistors (HBTs) appear to be a promising candidate for applications in broadband, high power amplifiers. Both the emitter injection efficiency and the base transport factor of the (npn) transistors are expected to be high mainly due to the wider bandgap of the GaN emitter and the long electron lifetime of the SiC base, which should lead to a potentially high common-emitter current gain. As a result, there should be more freedom to optimize the device design to improve the high frequency performance of the HBT without significant constraints by the current gain consideration. The large bandgap energies and the high breakdown fields of the material system, along with the high thermal conductivity of SiC, should allow the HBT to operate with a high power density at a high temperature.; This dissertation reports the development process and the measurement results of GaN/SiC HBTs. Two types of HBTs, using an n-GaN/p-SiC and an n-AlGaN/pGaN/p-SiC emitter-base junction respectively, were designed with an emphasis on improving the high frequency performance of the transistor. The common-emitter current gains of the GaN/SiC and the AlGaN/GaN/SiC HBTs were theoretically estimated to be approximately 80 and in the range of 9–80, respectively. The group-III nitride device layers were grown by molecular beam epitaxy (MBE), while the SiC device layers were deposited by the SiC wafer manufacturer on semi-insulating 4-HSiC substrates. In the fabrication process of the HBTs, the pattern transfer was made by either photolithography or e-beam lithography. The mesa formation was made by the chlorine-based dry etch. Common-emitter mode transistor action was observed only in the AlGaN/GaN/SiC HBTs with a current gain not higher than 0.02. The extracted current gain of the GaN/SiC HBTs from the Gummel plot was not higher than 0.001. The unexpectedly low current gains of both types of HBTs probably result from the electron-hole recombination in the emitter-base junction. |
