Development of A1GaN/GaN HBTs and HFETs for high power and high frequency operation.

GaN-based semiconductors show promise for the fabrication of electronic
components capable of high power, high frequency operation. This has lead to the
development of both AlGaN/GaN heterostructure field effect and bipolar junction
transistors. In order to increase the efficiency of these devices novel fabrication
technologies are examined. Contact resistances to buried p-type layers, exposed using
a novel wet etch, were found to be superior when compared to those formed on dry
etched structures. Utilising this technology in conjunction with a novel inverted n-p-n
AlGaN/GaN HBT is found to increase the emitter-base heterojunction quality and
also reduces growth complexity. Replacing the n-type collector with a Schottky diode
was found to significantly reduce leakage in the component and enable normal
operation under common emitter bias conditions.
Bulk and surface trapping effects in AIGaN/GaN heterostructures are independently
identified. Bulk traps are found to be located close to or at the me~-semiconductor
interface which results in a modification to the HFET band structure. Gate leakage
current along the AIGaN surface is found to be controlled by injection from the gate
and a surface hopping conduction mechanism which dominates at high and low
temperature respectively. Passivation of the structure using SiN reduces current flow
at the AIGaN surface. Employing a plasma pre-treatment, contamination at the
surface of the device can be removed allowing intimate contact between the
passivation and AlGaN films. Using a CF4 plasma treatment is found to remove these
contaminants and deposits a thin film of AlF3 on the AlGaN surface which reduces
current collapse in AlGaN/GaN HFETs to negligible levels. GaN capping layers on
HFETs can reduce both parasitic contact resistances and also reduce current collapse.
Methods of selectively etching through the GaN capping layer are presented in order
to develop a suitable self-aligned gate recess process.