The successful development of III-V nitride materials systems have led to active research of electronic devices in the past decades. III-nitride (III-N)-based devices have a bandgap engineering flexibility, large energy gap, direct band structure, and high carrier mobility when compared to Si- and SiC -based devices, providing new performance improvement opportunities for high-speed and energy-efficient electronics. In particular, tremendous efforts have been devoted to the quest for heterojunction field effect transistors (HFETs). Today, commercial GaN HFETs are available for microwave power amplifiers and power electronic circuits. On the other hand, bipolar transistors are also essential devices in order to fully exploit the potential of III-N materials systems and the associated semiconductor technologies. For instance, insulated-gate-bipolar transistors would require integrated bipolar transistor and unipolar transistor structures to optimally operate such devices in a vertical current conduction form. The development of III-N bipolar switches in either two-terminal or three-terminal forms have therefore been actively sought. Unfortunately, myriad technological challenges in the growth and the fabrication processing have limited the progress of III-N bipolar transistors for years. These particular device technologies are still in the early stage of the technology development.

In this presentation, we will review the development status of III-N bipolar transistor switches and vertical GaN rectifiers. Early development of heterojunction bipolar transistors (HBTs) using AlGaN/GaN heterojunctions was unsuccessful because of the limited current drive and current gain. The GaN/InGaN HBTs, on the other hand, showed promising device performance. We studied optimized epitaxial growth in a metalorganic chemical vapor deposition (MOCVD) reactor, developed a direct-growth (or non-regrowth) mesa-type III-N HBT fabrication processing technology, and achieved state-of-the-art of GaN/InGaN NPN HBT performance.[1]^,[2],[3],[4] These devices exhibited high-current gain of >100, high current drive density of >100 kA/cm², and high d.c. power handling capability of > 3 MW/cm². Small-signal microwave amplification with a cut-off frequency (f_T) of 8GHz was also demonstrated experimentally. Similarly, GaN-based rectifiers have been studied in recent years and these devices have reportedly demonstrated switching performance approaching theoretical values. At Georgia Tech, our preliminary study demonstrated that 800-V homojunction GaN PIN rectifiers can achieve a specific on-state resistance of R_ON = 2.8 mOhm-cm². [5]The large turn-on voltage of GaN PIN diodes, however, could be problematic in circuit applications. To address this problem, GaN junction-barrier Schottky (JBS) rectifiers are also under study. Using a nickel as the Schottky contact material and an additional MOCVD regrowth step, our preliminary results demonstrated a GaN JBS diode with a turn-on voltage of ~0.8V, a value similar to typical silicon PN diodes. These results demonstrated the feasibility of using bipolar carrier transport mechanisms and PN junctions in III-N devices for power amplifications and switching circuits. Further device design and processing techniques that help achieve these performance characteristics will be discussed in the conference.