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The gate and drain bias dependence of hot electron-induced degradation in GaN-based metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) was investigated in this work. Devices exhibit an abnormal increase in peak transconductance ( ${G}_{m}$ ,max) during hot carrier stress (HCS) and a partially quick recovery of that after removing the electrical stress. A physical model is proposed to explain the abnormal electrical characteristics caused by HCS. By using density functional theory (DFT), we calculated the energy for electrons to dehydrogenate preexisting [NGaH3]−1 complexes in GaN layer during stress. The dehydrogenation of defects affects the Gm,max of devices. Meanwhile, the neutralization of donor traps in AlGaN barrier layer also plays a significant role in the increase of Gm,max and the detrapping effect of electrons from these traps after removing the electrical stress accounts for the partially quick recovery of ${G}_{m, \text{max}}$ .

Deubiquitinases (DUBs) are a group of enzymes that regulate myriad biological processes by removing ubiquitin posttranslational modifications. They can either cleave ubiquitin directly from a substrate protein or disassembling a ubiquitin chain. Currently, about 100 human DUBs were identified and they were classified into seven families, and majority of them are cysteine proteases. DUBs exhibit distinct activities and preferences towards a wide variety of complex Ub chain with different linkages and architectures. Understanding how DUB achieves the selectivity towards specific substrates is important to elucidate how Ub system is regulated. This dissertation discusses our efforts to investigate the molecular mechanism of DUB, specifically the proteasome associated DUB UCH37/UCHL5. We have combined biochemical assay, biophysical characterization, mass spectrometry, and cell biology to address how DUB interacts with different ubiquitin chains to achieve substrate specificity. These lead us to uncover a novel mechanism that DUBs may use distinct surfaces for distinct activities. Finally, the discovery of the new mechanism of DUB would also provide new avenues for the DUB inhibitors development for therapeutics.

An atomic-level controlled etching (ACE) technology is invstigated for the fabrication of recessed gate AlGaN/GaN high-electron-mobility transistors (HEMTs) with high power added efficiency. We compare the recessed gate HEMTs with conventional etching (CE) based chlorine, Cl2-only ACE and BCl3/Cl2 ACE, respectively. The mixed radicals of BCl3/Cl2 were used as the active reactants in the step of chemical modification. For ensuring precise and controllable etching depth and low etching damage, the kinetic energy of argon ions was accurately controlled. These argon ions were used precisely to remove the chemical modified surface atomic layer. Compared to the HEMTs with CE, the characteristics of devices fabricated by ACE are significantly improved, which benefits from significant reduction of etching damage. For BCl3/Cl2 ACE recessed HEMTs, the load pull test at 17 GHz shows a high power added efficiency (PAE) of 59.8% with an output power density of 1.6 W/mm at V d = 10 V, and a peak PAE of 44.8% with an output power density of 3.2 W/mm at V d = 20 V in a continuous-wave mode.