(Invited) Performance Evaluation of III-N Bipolar Switches Grown on GaN Substrates

Abstract

The successful development of III-V nitride materials systems has led to active research of advanced electronic devices in the recent 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. Today, commercial GaN HFETs are available for microwave power amplifiers and power electronic circuits. On the other hand, the development of bipolar transistors and the related switch technologies are essential to fully exploit the potential of III-N materials systems. For instance, insulated-gate-bipolar transistors would require integrated bipolar transistor and unipolar transistor structures. The development of III-N bipolar switches in either two-terminal or three-terminal forms have therefore been actively sought. To this end, power switches that conduct the electric current in the vertical form are beneficial to achieve the ultimate Baliga’s Figure of Merit (BFOM) for switching. For GaN bipolar power switches development, low-defect-density native substrates, high-quality epitaxial growth technology, and proper device design and fabrication have become major focus areas of research. Early development of heterojunction bipolar transistors (HBTs) using GaN/InGaN heterojunctions at Georgia Tech showed promising device performance. 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 reported. In this presentation, we will review the development status of III-N bipolar switches and use GaN rectifiers as a case study to discuss the device performance pertinent to the choices of GaN substrates prepared by different methods. The device structures were grown and fabricated at Georgia Tech and the device performance correlation with respect to the substrates were evaluated. High-power operation was demonstrated for homojunction GaN PIN rectifiers grown on a 2-inch bulk GaN substrate using a metalorganic chemical vapor deposition (MOCVD) reactor. The GaN substrate was prepared using a near-equilibrium ammonothermal (NEAT) method. The device fabrication employed an ion-implantation isolation, field plate, and electroplated copper electrodes. The fabricated devices have dimensions up to 1.1 mm in diameter and showed uniform V on of 3.6±0.1 V and an ideality factor of 2.08. A blocking voltage (BV) of >1.2 kV and the on-state resistance (R ON A) of 0.4 mΩ·cm2 were measured. For PIN rectifiers with a dimension of 1.1 mm in diameter, we reported a BV of 1,200 V and an on-state current drive of greater than 11 A (1 kA/cm2). These results demonstrated the feasibility of using bipolar carrier transport mechanisms and PN junctions in III-N devices for power amplifications and switching circuits.