Gallium nitride (GaN) based power devices are merging as promising alternatives to Sibased counterparts in power switching applications, as they could deliver higher breakdown voltage, lower conduction resistance and higher frequency operation. Intensive efforts have been devoted to commercializing GaN technology. In spite of the significant achievements in epitaxial growth, processing and packaging techniques, challenges still exist that resists the penetration of GaN power devices into the Si dominated market, such as current collapse, threshold instabilities, reliabilities, et al.

This thesis is devoted to: (1) comparatively investigating the two most popular nitride (i.e. SiN_x and AlN) passivation techniques through TCAD simulation and experimental characterization, (2) identifying...[ Read more ]

Gallium nitride (GaN) based power devices are merging as promising alternatives to Sibased counterparts in power switching applications, as they could deliver higher breakdown voltage, lower conduction resistance and higher frequency operation. Intensive efforts have been devoted to commercializing GaN technology. In spite of the significant achievements in epitaxial growth, processing and packaging techniques, challenges still exist that resists the penetration of GaN power devices into the Si dominated market, such as current collapse, threshold instabilities, reliabilities, et al.

This thesis is devoted to: (1) comparatively investigating the two most popular nitride (i.e. SiN_x and AlN) passivation techniques through TCAD simulation and experimental characterization, (2) identifying the trap states in the p-GaN layer of Schottky-type p-GaN gate HEMTs and quantitatively revealing the dominating factor that causing threshold voltage instabilities, and (3) demonstrating surface reinforcement method that aiming to suppress the hotelectron- induced device degradation for GaN-based power HEMTs.

The mechanisms and switching properties of AlGaN/GaN high-electron-mobilitytransistors (HEMTs) passivated by SiN_x and monocrystal-like AlN are studied in the first part of this thesis work. The effects of interface traps and polarization charges on current collapse are investigated by TCAD simulations and experimental characterizations. Surface/interface deep levels can be compensated by both shallow donor-like traps (SiN_x passivation) and polarization charges (AlN passivation) at passivation/heterostructure interface, but with different levels of effectiveness under fast switching conditions. SiN_x-passivation introduces shallow donor-like trap states with short time constant that favors a fast emission of trapped electrons in the access region and suppressed current collapse, but nevertheless exhibits more severe time-dependent recovery of dynamic ON-resistance. For AlN passivation, interface traps are compensated by the fixed positive polarization charges and the OFF-state depletion region (in the 2DEG channel) is formed predominantly by electric-field effect, leading to an immediate accumulation of high channel electron concentration after switching the HEMT devices back to ON-state and instant response of drain current to gate and drain bias. The field plate structure is necessary in SiN_x-passivated devices for both current collapse suppression and electric field alleviation. With AlN passivation, the field plate can be solely designed for achieving more uniform electric field distribution for gate reliability concern without the concern of current collapse.

In the second part of this thesis work, the deep-level transient spectroscopy (DLTS) is conducted to investigate the gate stack of the p-GaN gate HEMT with Schottky gate contact. A metal/p-GaN/AlGaN/GaN heterojunction capacitor is prepared for the study. The DLTS characterization captures the transient capacitance change in the stack, from which the capacitance of the metal/p-GaN Schottky junction can be extracted. By proper selection of the rate window, the impacts of the hole insufficiency effect are avoided during trap states evaluation. Thus, the information of deep energy levels in the p-GaN layer is revealed, which consists of an electron trap state with activation energy of 0.85 eV and a hole trap state with activation energy of 0.49 eV. The identification of these trap states in the p-GaN layer provides a physical foundation for understanding the threshold voltage instability in Schottky-type p-GaN gate power HEMTs. The ∆VTH can be approximated based on the transient capacitance changes and the extracted trap concentrations, revealing the hole insufficiency phenomenon being the dominating factor.

In the third part of this thesis work, we report a processing technique to form a surface reinforcement layer (SRL) in GaN HEMTs with the aim to suppress device degradation caused by long-term hot-electron-induced degradation in dynamic ON-resistance (R_ON). This SRL is a crystalline GaON layer that is formed by reconstruction of the heterojunction surface through plasma oxidation and high temperature annealing. Using dynamic R_ON as the key indicator of device degradation, HEMTs with SRL exhibit substantially suppressed dynamic R_ON degradation than the conventional devices without SRL, after long-term “semi-on” hot-electron stress. The GaON SRL exhibits substantially enhanced thermal and chemical stability, and consequently strong immunity to energetic carriers. The SRL-enabled suppression of hot-electron-induced degradation is further verified by the electroluminescence characterizations.