Novel GaN-based Vertical Field Effect Transistors for Power Switching

Novel GaN-based vertical structures are investigated to exploit the high voltage and high power capability of GaN. Two distinctive structures – vertical high electron mobility transistor (VHEMT) and vertical junction field effect transistor (VJFET) are studied. The trade-off between on-state resistance (Ron), threshold voltage (Vth) and breakdown voltage are modelled using technology computer aided design (TACD) simulation and real devices are fabricated and characterized.
In the VHEMT structure. A novel crystallographic wet etching technique is developed to obtain a c-plane sidewall in a V-shape groove (V-groove). Based on this technique, a V-groove metal-oxide-semiconductor (VMOS) structure is established. The threshold voltage shifts from 2 to 6 V after multiple sweeps which indicates presence of interfacial traps. In addition, an AlGaN/GaN heterostructure is successfully regrown by molecular beam epitaxy (MBE) on the V-groove surface which forms the platform for the two dimensional electron gas.
Subsequently, the design of VJFET structures are discussed. Contrary to the published result in the literature, the simulation suggests that a higher Vth can be achieved without compromising Ron. As a preparation for the VJFET structure, p-type GaN and GaN1-xAsx based diodes grown by MBE are characterized. A hole concentration of as high as 8.5×1019 cm−3 is achieved in the GaN1-xAsx structure which improves the conductivity and contact resistivity. Trench regrowth VJFET structures using p-GaN and p-GaN1-xAsx are characterized. A high leakage current is observed which is thought to be caused by defects at the regrowth interface. The regrowth structures are further studied in detail by transmission electron microscopy (TEM).