Unipolar hot electron transistors (HETs) represent a tantalizing alternative to established bipolar transistor technologies. During device operation electrons are injected over a large emitter barrier into the base where they travel along the device axis with very high velocity. Upon arrival at the collector barrier, high-energy electrons pass over the barrier and contribute to collector current while low-energy electrons are quantum mechanically reflected back into the base. Designing the base with thickness equal to or less than the hot electron mean free path serves to minimize scattering events and thus enable quasi-ballistic operation. Large current gain is achieved by increasing the ratio of transmitted to reflected electrons. Although III-N HETs have undergone substantial development in recent years, there remain ample opportunities to improve key device metrics.

In order to engineer improved device performance, a deeper understanding of the

operative transport physics is needed. Fortunately, the HET provides fertile ground

for studying several prominent electron transport phenomena. In this thesis we present

results from several studies that use the III-N HET as both emitter and analyzer of

hot electron momentum states. The first provides a measurement of the hot electron

mean free path and the momentum relaxation rate in GaN; the second relies on a new

technique called electron injection spectroscopy to investigate the effects of barrier height

inhomogeneity in the emitter. To supplement our analysis we develop a comprehensive

theory of coherent electron transport that allows us to model the transfer characteristics

of complex heterojunctions. Such a model provides a theoretical touchstone with which

to compare our experimental results. While these studies are of potential interest in their

own right, we interpret the results with an eye toward improving next-generation device

performance.