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Title: Neutron- and photon-induced charge dynamics in group III-nitride semiconductor devices
Author: Butler, Peter A.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2019
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Semiconductor devices formed from the group III-nitride materials, aluminium gallium nitride (AlGaN) and gallium nitride (GaN), have the potential to increase the efficiency and performance of electrical systems for safety-critical applications and harsh radiation environments. In this thesis, the electrical characteristics and irradiation response of AlGaN and GaN heterostructures, and High Electron Mobility Transistors (HEMTs), were studied using electron conduction, trapping, and generation. An admittance-based method was developed to measure mobility in a two-dimensional electron gas (2DEG) without using Ohmic contacts. Using the technique, the 2DEG density dependence of mobility in AlGaN/AlGaN heterostructures was measured from 80 K to 400 K, providing insights into mobility-limiting scattering mechanisms and the sources of carriers in the 2DEG. A current transient spectroscopy technique was developed to study charge trapping effects in HEMTs exposed to neutron irradiation. Two mechanisms contributing to irradiation-induced performance degradation were identified: increases in trap density, and changes to inter-trap coupling behaviour. In situ measurements were made of Schottky-gate and Metal-insulator-semiconductor-gate (MIS) HEMT response to the highly challenging Omega laser Inertial Confinement Fusion (ICF) pulsed neutron environment. Schottky-gate HEMTs proved more irradiation-sensitive than MIS HEMTs, but recovered in as little as 10 μs after irradiation, with no catastrophic failures observed for drain biases of up to 150 V. These differences will inform the design of circuits for use in neutron radiation environments, and merit further investigation. A pulsed femto-second laser system was developed to simulate irradiation-induced Single Event Effects (SEE), and to probe HEMT internal electric field profiles, by using localised two photon absorption (TPA). The system induced ion irradiation-like enhanced charge collection effects in HEMTs, with distinct prompt and delayed conduction processes. Peak currents and charge collected at HEMT contacts displayed power law dependences on pulse energy. Locations of electric field peaks within HEMTs, associated with reliability-limiting degradation processes and susceptibility to SEE, were located, found to depend on buffer doping, and shown to move when electrical biasing was altered. These findings are important for testing the safe operating area of HEMTs. The magnitude and duration of the SEE-like electrical transients varied with irradiation pulse repetition frequency (PRF): peak current increased as a sub-unity power of PRF, suggesting a dependence upon excess carrier lifetime and defect density. SEE susceptibility testing using repetitive irradiation sources should account for the PRF effect. Finally, a fast-acting mechanism was found to limit the combined effect of two discrete irradiation pulses that arrive in quick succession. For inter-pulse delays of the order ≈100 ps, peak drain current and total charge collected decrease by up to a third, as compared to ≈ns delays, where the effects of two pulses combine constructively. This limiting mechanism fundamentally connects the prompt and delayed current components of irradiation response, and may help to explain the lack of catastrophic breakdown events observed in HEMTs irradiated at pulsed neutron sources.
Supervisor: Kuball, Martin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available