Wideband Transferred-Substrate AlGaN-GaN Heterojunction Bipolar Transistors for Microwave Power Applications

This report reviews efforts to develop growth and fabrication technology for the GaN HBT at UCSB. Conventional devices are grown by plasma assisted MBE on MOCVD GaN templates on sapphire and fully MOCVD devices. HBTs were also fabricated on LEO material identifying threading dislocations as the primary source of collector-emitter leakage which was reduced by 4 orders of magnitude for devices on non-dislocated material. Base doping studies show that the mechanism of this leakage is localized punch-through caused by compensation near the dislocation. The cause of the large offset voltage in common emitter characteristics is discussed. The Mg memory effect in MOCVD grown GaN HBTs is investigated and MBE grown device layers are shown to produce sharp doping profiles. The low current gain of these devices, (3-6) is discussed. An air bridge and etch back process is used to fabricate transistors compatible with RF testing. The devices had a common emitter dilferential current gain of 3.5 with a short circuit current gain cutoff frequency of 2 CHz and an emitter current density of over 6 kA/cmA2. Remaining issues are high base resistance, low current gain, and emitter mesa etch process development.