Process Development and Characterization of AlGaN/GaN Heterostructure Field-Effect Transistors

Abstract

The fabrication of AlGaN/GaN HFETs entailed first developing fundamental processing techniques such as isolation etching, low-resistance drain/source ohmic contacts, and Schottky gate formation. These efforts along with related processing issues will be discussed. Devices were then fabricated on epitaxial layers grown on sapphire substrates and on n-type, p-type, and insulating SiC. While sapphire is the substrate of choice for the nitrides, SiC substrates are better suited for high-power devices due to their significantly higher thermal conductivity. The dc, RF, and high-temperature performance of transistors on sapphire and SiC were investigated to determine the performance similarities and differences between using the various substrates. Forming low-resistance ohmic contacts to the wide bandgap heterostructures is difficult and as a result limits the overall device performance. In an effort to improve drain-source contacts, gate recessing through the use of low-damage inductively coupled plasma reactive ion etching was pursued. A wet chemical etching process called photoelectrochemical etching was also investigated.