Investigation of Transport Phenomena in Semiconductors and Semiconductor Devices: Drain Noise, Two-Phonon Scattering, and Phonon Drag

Citation

Esho, Iretomiwa (2024) Investigation of Transport Phenomena in Semiconductors and Semiconductor Devices: Drain Noise, Two-Phonon Scattering, and Phonon Drag. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/b30w-cr73. https://resolver.caltech.edu/CaltechTHESIS:01242024-044612209

Abstract

The dynamics of charge carriers in semiconductors set the foundation for semiconductor device performance. Devices crucial for fields like radio astronomy rely on transistor amplifiers where hot electron dynamics impact noise significantly. The overarching goal of this work is to contribute towards the development of better transistor amplifiers by investigating electron transport in existing devices and emerging materials.

The physical mechanisms governing noise in a class of semiconductor devices called high electron mobility transistors (HEMTs) are not completely understood. HEMTs are transistors that use a junction between two materials of different band gaps as the channel. HEMTs are used as amplifiers by translating a small signal applied at the gate terminal to a large current at the drain terminal or output. The noise added at the input is well-characterized by the device physical temperature, while the origin of the noise added at the output is still up for debate. We attempt to fill this knowledge gap by proposing a theory of noise occurring at the drain terminal of these devices as a type of partition noise arising from two possible electron paths. This theory emphasizes the critical role of the conduction band offset between epitaxial layers of the device: a larger offset maximizes the channel sheet density and minimizes electron transfer between layers, potentially improving noise performance. The theory accounts for the magnitude and dependencies of the drain temperature and suggests strategies to realize devices with lower noise.

We then investigate phonon-limited charge transport in the semiconductor boron arsenide. Boron arsenide has drawn significant interest due to reports of simultaneous high thermal conductivity and ambipolar charge mobility, desirable properties for integration in electronic devices. The theoretical prediction of high electron and hole mobility assumed the dominance of charge carrier scattering by one phonon. We consider the effects of two-phonon electron and hole scattering processes in boron arsenide, and find that inclusion of these higher-order processes reduces the computed room-temperature electron and hole mobility significantly from the one-phonon value. Despite its potential, our predictions of electron and hole mobility contradict recent experimental reports based on photoexcited charge carrier diffusion. Several factors may explain this discrepancy, including another type of two-phonon scattering not considered in this work, superdiffusion of hot carriers, induced carrier concentration, or a combination of all or some of the above elements.

At high carrier concentrations, the phonon system may interact with the electron system on the timescale of the phonon-phonon interaction. When this happens, the nonequilibrium state of phonons becomes important for electron transport, and vice versa as these systems interact in a coupled manner. This coupled interaction could lead to an inflated value of the experimentally reported mobility. We quantify this effect, known as phonon drag, with a coupled electron-phonon Boltzmann transport equation framework and demonstrate that the electron mobility is indeed enhanced significantly at the relevant carrier densities.

Thesis Files