Degree Name

Doctor of Philosophy


School of Engineering Physics


Solid-state nanostructure thermionic devices offer the potential of reliable and scalable refrigeration and power generation at high efficiencies. Theory is developed allowing the analysis of thermionic devices with arbitrary ballistic electron transmission. This is used to show that the nature of the energy spectrum of electrons transmitted in the device has a significant effect on the device performance. Electronic transport in multilayer and nanowire solid-state thermionic devices is considered in detail. Devices that select electrons for emission according to their total momentum, in contrast to conventional devices in which selection is based on longitudinal momentum only, are shown to be superior in a number of ways. While the efficiency of conventional devices is shown to be limited to values less than the Carnot value, total momentum filtered devices may achieve the Carnot efficiency. Total momentum filtered devices also have a more easily optimised electron energy spectrum and produce significantly higher currents than conventional devices. In any thermionic device it is shown that electronic efficiency is increased by transmitting a narrower range of electron energies and using a transmission probability that sharply rises to full transmission. Conditions for maximum power and efficiency with losses are also presented. It is shown that the formalisms used to describe thermionic and thermoelectric devices reduce to the same form when considered over distances around the electron mean free path. This implies that both have a similar dependence on the nature of the electron energy spectrum. Wide single-barriers are shown to give electron energy spectra that rise sharply. Analysis of a number of barrier potential profiles reveals that structures yielding electron reflection above the barrier are undesirable. It is suggested that short-period superlattice barriers, which may have reduced thermal conductivity for enhanced performance, might be the best barrier structure. These are analysed and it is discussed how they should be structured for good electronic transport. Equations for thermionic emission in heterostructure nanowires are derived and used to propose a correction to measurements of nanowire barrier heights. Calculations predict that, in principle, appropriately doped thin nanowires are capable of outperforming conventional thermionic devices. Quantum dot embedded nanowires are then considered and their energy levels discussed in detail. It is proposed that such a device may be used for a low-temperature proof-of-principle experiment to measure efficiency approaching the Carnot limit. Finally, it is shown that the electron energy spectrum principles developed apply to more than just solid-state thermionic devices by considering nanometre-gap vacuum thermionic refrigerators and hot carrier solar cells. The general principles explored in this thesis are therefore likely to be relevant to other energy conversion devices.