Development of gallium nitride materials for heterojunction bipolar transistors

The progress of GaN-based heterojunction bipolar transistors (HBTs) has been limited by the difficulties associated with the growth and processing of the p-type GaN base layer in Npn HBTs. Some of these difficulties include obtaining a high hole concentration in the p-type GaN base, obtaining low-resistance contacts to the etched base, and the simultaneous requirement of a thin base and low-sheet resistance base. The goal of this work is to identify and understand these difficulties and find solutions to overcome them.; A chemically-assisted ion-beam etching system is built and characterized to obtain low damage etching while still maintaining a decent etch rate. An optimized recipe offering good repeatability is developed to etch GaN materials.; Anomalously high ideality factors (n >> 2.0) found in base-emitter and base-collector junctions of GaN-based HBTs are detrimental to the device performance. We have developed a new theoretical model supported by experimental and simulation results to account for these high ideality factors. Prior to this work, no detailed theory had been published to explain the origin of such high ideality factors in GaN-based diodes.; Theoretical calculations and Silvaco ATLAS device simulation are used to help with the transistor structure design, specifically the base thickness. A complete HBT fabrication process flow is established and the devices are grown, fabricated, and characterized in our laboratory. It is observed that the base-emitter and the base-collector junctions of the fabricated HBTs are rectifying. Transistor action has been observed in the common-base and common-emitter configuration.; A new HBT design is proposed to reduce the high base access resistance in GaN HBTs. This design incorporates polarization-enhanced ohmic contacts and selective epitaxial growth technique. Experimental results using transmission line measurement technique yield specific contact resistances of 5.6x10 -4 Ocm2 for polarization-enhanced p-type contacts and 7.8x10-2 Ocm2 for conventional p-type contacts indicating the significant benefit of polarization-enhanced contacts. Detailed theoretical calculations indicate an increase of 70% in the extrinsic base epitaxial resistance and that of 87% in the maximum oscillation frequency of the proposed HBT structure compared to the conventional structure.