Abstract
This thesis shows a novel and unique method of how Gallium Nitride Field Effect Transistors
(GaN FETs) are characterised in an electromagnetically defined environment. This
measurement technique enables the device characteristics to be determined without the circuit
influencing them, so that different devices can be compared on the same basis. This is done
because measuring the switching transients of GaN FETs often results in distorted waveforms
caused by unknown or undefined parameters. Designers are forced to guess the transient
characteristics rather than have a waveform representing the exact operation. Transient
parameters of the devices need to be known so that GaN FETs can be used in converters that
switch faster, are more efficient and occupy a smaller footprint compared to those that use
Silicon based semiconductor devices.
Two variables must be measured in order to accurately describe the GaN FET device
operation and these are the current through the device and the voltage across the device. In
this thesis, “conventional” voltage and current measurement techniques are addressed with
special attention given to resistive type current shunts. Current measurement technology has
trailed behind voltage measurement since current must first be converted to a voltage before
any measurement instrument can interpret it. This current to voltage conversion process, the
voltage measurement across the device as well as the test circuit are all prone to coupling
stray electromagnetic fields which will affect the measurement. To solve this, a well-defined
electromagnetic environment is required.
Coaxial transmission lines are well defined electromagnetic structures. In addition, correctly
terminated transmission lines are a way of theoretically creating a purely resistive load for all
frequencies. In this thesis, GaN FET devices are switched into a coaxial transmission line
terminated into its characteristic impedance. The well-defined electromagnetics of the
transmission line offers a reliable noise free measurement platform, and a pure resistive load
allows for accurate representation of the current to be measured as well as creates conditions
where the device-and not the circuit- determines the switching transient. Attention is given to
current shunts and their problematic behaviour during fast switching transients. As an
extension to the concept of a coaxial current shunt, it is shown that a well terminated resistive
coaxial transmission line offers a defined electromagnetic environment for switching
transient measurement. The contribution of this thesis is the novel idea of using a terminated
coaxial transmission line as a defined electromagnetic environment for accurate measurement
and characterisation of fast switching devices under resistive conditions, while
simultaneously using the line structure as the current measurement device.
D.Ing. (Electrical and Electronic Engineering Science)