This thesis deals with gallium nitride (GaN) high electron mobility transistors (HEMTs). The GaN HEMT is a transistor technology that is well suited for amplification of electrical signals at high frequencies and high power levels. Therefore, the GaN HEMTs are interesting for applications within i.e. mobile communication and security systems. In mobile communication solutions, a high operating frequency means that high data rates are achievable, and with a high power signal, a larger distance between sender and receiver is possible. Using GaN HEMTs in these systems will increase data rates and decrease power consumption compared to current technology. In radar applications, a large power signal means that the radar to detect objects that are further away. Furthermore, the high robustness of GaN HEMTs is highly valued in radar applications since it can survive jamming signals with high input powers meant to interfere or even damage the radar system.
GaN HEMTs are already commercially available and are to some extent used in the applications described above. However, some issues related to the technology limits a more widespread use. This thesis deals with some of these problems. For example, a long-standing issue for GaN HEMTs are so called trapping effects. These effects give the transistor a form of memory, meaning that its performance in the present is a function of what signals it has experienced in the past. These effects are due to different defects in the GaN material and can severely limit the performance of the transistors. Furthermore, GaN HEMTs are currently expensive to fabricate. A reliable fabrication process is of utmost importance to reduce the associated costs of the transistors.
In this thesis, trapping effects associated with defects in the material are characterized in order to understand their origin. With this information it may be possible to decrease the trapping effects in future transistors. Moreover, the results imply that a manufacturing process with high repeatability is achievable, which simultaneously can yield transistors with low loss. Overall, this leads to future communication and radar systems which offers higher performance while consuming less power at a reduced cost.