Advanced gallium nitride technology for microwave power amplifiers

Gallium nitride (GaN) based technology has been heavily researched over the past two decades due to its ability to deliver higher powers and higher frequencies that are demanded by the market for various applications. One of GaN’s main advantages lies in its ability to form heterojunctions to wider bandgap materials such as Aluminium Gallium Nitride (AlGaN) and Aluminium Nitride (AlN). The heterostructure results in the formation of the so called 2 dimensional electron gas (2DEG), which exhibits high electron densities of up to 6E13 cm−2 and high electron mobilities of up to 2000 cm2/V·s that enable the devices to support high current densities. Furthermore, it supports very high breakdown fields of 3.3 MV/cm due to its wide bandgap of 3.4 eV. The main objective of this work was to further advance the transistor technology using simple, cost effective and reliable techniques. The AlN/GaN material system exhibits higher sheet carrier concentrations compared to the conventional ternary AlGaN barrier, but introduces additional challenges due to its reduced thickness of 2-6 nm compared to 18-30 nm of AlGaN. The additional challenges of the thin AlN binary barrier include strain relaxation, high gate leakage currents and high Ohmic contact resistances due to its high bandgap of 6.2 eV. In this work, a thin (5 nm) in-situ SiNx passivation layer was employed to reduce the strain relaxation, reduce gate leakage currents and improve Ohmic contacts resistances. The optimised Ohmic contact annealing condition resulted in an Ohmic contact resistance of 0.4 Ω·mm and a sheet resistance of 300 Ω