The majority of transistors currently in use are made from silicon, a semiconductor which has received a large amount of attention within the electronics industry. These silicon transistors with impurity doping perform well at room temperature, but their properties begin to degrade as carriers freeze out when the temperature is decreased. Nitride-based transistors are a promising development; unlike their silicon counterparts, nitride devices do not rely on impurity doping as the dominant mechanism of conductivity. Heterostructures based on nitrides are also capable of exhibiting very high electron mobility, a desirable quality for such components. The performance of these GaN high electron mobility transistors – HEMTs – has been observed to improve as the temperature is lowered.
This improved functionality at low temperature forms the basis of my PhD project. I aim to explore the behaviour of various GaN HEMT structures, grown in-house, and investigate the effects of decreasing the temperature on their properties – both of the material itself and the whole device. If low temperatures are not found to be destructive to these devices, a possible emerging application is within the engines of high-power electric vehicles. For maximum efficiency, it is desirable to use superconducting components within these engines, which require cryogenic temperatures to operate. It is therefore advantageous for other electrical components to operate reliably at these temperatures. The electrical properties of the devices will be explored using magneto-transport measurements, and the materials will be characterised using a range of techniques including scanning probe microscopy and Nomarski optical microscopy.
