HBT on LEO GaN

Dramatic progress in GaN electronics has led to increased interest in bipolar transistors. Although there have been reports of GaN bipolars from several groups, the development of the GaN bipolar transistor is still in its fundamental stages. In the case of GaN, the usual correlation between common base, Gummel, and common emitter characteristics does not exist due to significant collector-emitter leakage, leaving only the common emitter characteristic as a reliable measure of DC device performance. We identify the source of this leakage as threading dislocations and clarify the effect of this leakage on the transistor DC characteristics. Furthermore, we conclude from various growth structures and methods of device fabrication that the electron lifetime in the neutral base is currently the limiting factor in GaN NPN transistor performance. Typical GaN material has high threading dislocation densities, 10/sup 7/-10/sup 9/ cm/sup -2/, due to lattice mismatch with the substrate, typically sapphire or SiC. To study the effects of threading dislocations on GaN bipolar transistors, we have fabricated devices on material grown using the lateral epitaxial overgrowth technique, LEO. To the authors' knowledge, this is the first demonstration of GaN bipolar transistors grown on nondislocated material. The LEO substrate allows us to compare devices grown on material with a negligible dislocation density with those grown on a standard template.