Date of Award

8-30-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Engineering Physics

First Advisor

John W. McClory, PhD

Abstract

Microscale beams of germanium ions were used to target different locations of aluminum galliumnitride/gallium nitride (AlGaN/GaN) high electron mobility transistors (HEMTs) to determine location dependent radiation effects. 1.7 MeV Ge ions were targeted at the gap between the gate and the drain to observe displacement damage effects while 47 MeV Ge ions were targeted at the gate to observe ionization damage effects. Electrical data was taken pre, during, and post irradiation. To separate transient from permanent degradation, the devices were characterized after a room temperature anneal for at least 30 days. Optical images were also analyzed pre and post irradiation. This is the first use of localized dynamic irradiation testing has been used to compare AlGaN/GaN HEMT performance to the results of stress testing via in situ measurements of the gate and drain currents. The 6 MV Tandem Accelerator at Sandia National Laboratories using the Micro-ONE system was used to induce displacement and ionization damage. Displacement damage was caused by 1.7 MeV Ge ions targeting the gate-drain gap of ten HEMTs in the off, on, and semi-on bias states where a fluence dependent delayed response between ion deposition and gate current degradation in the semi-on and on bias state was observed. The delayed response was also observed in the drain current degradation when biased in the semi-on state, while occurring immediately in the on state. Ionization damage was induced by 47 MeV Ge ions targeting the gate region in the semi-on bias state where gate current degradation occurred during the initial irradiation of the gate active region. Drain current degradation occurred in both the initial and subsequent irradiations. Pre and post irradiation output and transfer performance characteristics indicate drain current and transconductance degradation for both experiments in all bias states. No threshold voltage shift was observed during the displacement damage experiment with 1.7 MeV Ge ions. During the ionization experiment, the threshold voltage increased after the initial irradiation with 2 x 1010 cm-2 47 MeV Ge ions across the length of the gate. Subsequent irradiation over the same location and after a 60 day room temperature anneal did not change this threshold voltage shift and the decrease in the drain current and transconductance persisted, indicating permanent damage. The same performance characteristics changes have been associated with reliability stress testing causing similar effective damage in both the gate-drain gap and the gate regions. The observed degradation in device characteristics are consistent with the inverse piezoelectric effect in the displacement damage experiment and charge trapping in the gate region in the ionization experiment. These results show that radiation induced degradation can be captured by using a targeted ion beam in order to determine location dependent fluence limits, thereby informing both reliability and radiation hardness models.

AFIT Designator

AFIT-ENP-DS-19-S-025

DTIC Accession Number

AD1084466

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