Ph.D Thesis

Ph.D StudentMehari Shlomo
SubjectA Study of Gate Leakage Current and Electron Trapping
Effects in AlGaN/GaN-Based Heterostructure
Field Effect Transistors
DepartmentDepartment of Electrical and Computer Engineering
Supervisors PROF. Dan Ritter
Full Thesis textFull thesis text - English Version


Gallium nitride (GaN) and its compounds, AlXGa1-XN, InXGa1-XN and InXAl1-XN, are considered second to silicon in importance. The GaN-based technology has already revolutionized the lighting industry, enabling very efficient semiconductor based LEDs and UV detectors. The GaN-based heterostructure field effect transistors (HFETs), have a unique combination of material properties, including high 2D electron gas (2DEG) concentration, high 2DEG mobility, and high breakdown fields. These features make GaN-based HFETs not only very suitable for high-power and high-frequency applications, such as radars, and power converters, but also prime candidates to replace current Si based power technologies, which are rapidly approaching their limit. But, to date, the implementation of GaN-based HFETs is still impeded due to a lack of a full understanding of the anomalies this technology has, such as electron trapping effects that cause the current collapse phenomenon, and excessive off-state leakage current.

The main goal of this work is to provide new physical understanding regarding the phenomena that impede GaN-based devices realizing their full intrinsic potential.

Within the main part of the research, a novel methodology was developed, based on gated van der Pauw transient measurements, which provides an opportunity to study the location and the nature of electron traps in GaN-based HFETs. Interface traps can be probed using an insulated gate, and bulk traps can be probed using a Schottky gate. As a demonstration, the interface trap density (Dit) between a PECVD SiNX gate dielectric and GaN-based HFET layers were obtained across a large energy range. In contrast to conventional methods, no specific models are required as the effect of the interface charge is directly measured.

A further detailed analysis of the 2DEG concentration transient response at different temperatures and gate voltages using both Schottky and insulted gated van der Pauw test structures has provided interesting new findings. First, it was shown that electron transport mechanisms across the AlGaN barrier may determine the transients observed in insulated gate structures. It was also demonstrated that barrier transport has a dominant role in the trade-off phenomenon between dynamic stability and low Schottky gate leakage current. These findings differ markedly from the conventional interpretation of the activation energies measured in GaN-based HFETs. Second, the electron trap energies at different spatial locations in GaN-based heterostructures were identified which are otherwise almost impossible to detect.  

The study has also sought to identify suitable gate dielectric and passivation layers for GaN-based MIS-HFET devices. Simulations and measurements of GaN-based MIS capacitors have revealed that gate leakage current across atomic layer deposited Al2O3 gate insulator is effectively suppressed and reaches the theoretical tunneling limit at high electrical fields. Such an outcome is very important for high-power switching applications. However, it was found that the Al2O3/GaN interface is of poor quality, and caused severe threshold voltage instabilities. Alternatively, a nitride based dielectric layer was implemented in GaN-based MIS-HFET successfully. It was shown that a single-step PECVD SiNX dielectric is useful for: gate insulation, surface passivation, and surface protection during Ohmic formation.