Failure modes and mechanisms of AlGaN/GaN high-electron-mobility transistors are reviewed. Data from three de-accelerated tests are presented, which demonstrate a close correlation between failure modes and bias point. Maximum degradation was found in "semi-on" conditions, close to the maximum of hot-electron generation which was detected with the aid of electroluminescence (EL) measurements. This suggests a contribution of hot-electron effects to device degradation, at least at moderate drain bias (VDS 30-50 V), new failure mechanisms are triggered, which induce an increase of gate leakage current. The latter is possibly related with the inverse piezoelectric effect leading to defect generation due to strain relaxation, and/or to localized permanent breakdown of the AlGaN barrier layer. Results are compared with literature data throughout the text.

Reliability of GaN High-Electron-Mobility Transistors: State of the Art and Perspectives

This timely second edition has been substantially expanded to keep students and practicing power conversion engineers ahead of the learning curve in GaN technology advancements. Acknowledging that GaN transistors are not one-to-one replacements for the current MOSFET technology, this book serves as a practical guide for understanding basic GaN transistor construction, characteristics, and applications. Included are discussions on the fundamental physics of these power semiconductors, layout and other circuit design considerations, as well as specific application examples demonstrating design techniques when employing GaN devices.

/pdf/gan-transistors-for-efficient-power-conversion-1vxm76211t.pdf

GaN Transistors for Efficient Power Conversion

Recent success with the fabrication of high-performance GaN-on-Si high-voltage HFETs has made this technology a contender for power electronic applications. This paper discusses the properties of GaN that make it an attractive alternative to established silicon and emerging SiC power devices. Progress in development of vertical power devices from bulk GaN is reviewed followed by analysis of the prospects for GaN-on-Si HFET structures. Challenges and innovative solutions to creating enhancement-mode power switches are reviewed.

https://iopscience.iop.org/article/10.1088/0268-1242/28/7/074011/pdf

Gallium nitride devices for power electronic applications

GaN HEMT reliability

Trapping is one of the most deleterious effects that limit performance and reliability in GaN HEMTs. In this paper, we present a methodology to study trapping characteristics in GaN HEMTs that is based on current-transient measurements. Its uniqueness is that it is amenable to integration with electrical stress experiments in long-term reliability studies. We present the details of the measurement and analysis procedures. With this method, we have investigated the trapping and detrapping dynamics of GaN HEMTs. In particular, we examined layer location, energy level, and trapping/detrapping time constants of dominant traps. We have identified several traps inside the AlGaN barrier layer or at the surface close to the gate edge and in the GaN buffer.

http://www.mtl.mit.edu/~alamo/pdf/2011/RJ-128.pdf

A Current-Transient Methodology for Trap Analysis for GaN High Electron Mobility Transistors

We have found that there is a critical drain-to-gate voltage beyond which GaN high-electron mobility transistors start to degrade in electrical-stress experiments. The critical voltage depends on the detailed voltage biasing of the device during electrical stress. It is higher in the OFF state and high-power state than at VDS = 0. In addition, as |VGS| increases, the critical voltage decreases. We have also found that the stress current does not affect the critical voltage although soft degradation at low voltages takes place at high stress currents. All of our findings are consistent with a degradation mechanism based on crystallographic-defect formation due to the inverse piezoelectric effect. Hot-electron-based mechanisms seem to be in contradiction with our experimental results.

Critical Voltage for Electrical Degradation of GaN High-Electron Mobility Transistors

We have carried out systematic experiments of the electrical reliability of state-of-the-art GaN HEMTs. We have found that degradation is mostly driven by electric field and that there is a critical electric field below which negligible degradation is observed. Device degradation is associated with the appearance of prominent trapping behavior. Degradation is consistent with a model of defect formation in the AlGaN barrier as a result of the high electric field. We postulate that lattice defects are introduced by excessive stress associated with the inverse piezoelectric effect. Electron trapping at these defects reduces the extrinsic sheet carrier concentration and the maximum drain current.

Mechanisms for Electrical Degradation of GaN High-Electron Mobility Transistors

We have investigated the surface morphology of electrically stressed AlGaN/GaN high electron mobility transistors using atomic force microscopy and scanning electron microscopy after removing the gate metallization by chemical etching. Changes in surface morphology were correlated with degradation in electrical characteristics. Linear grooves formed along the gate edges in the GaN cap layer for all electrically stressed devices. Beyond a critical voltage that corresponds to a sharp increase in the gate leakage current, pits formed on the surface at the gate edges. The density and size of the pits increase with stress voltage and time and correlate with degradation in the drain current and current collapse. We believe that high mechanical stress in the AlGaN layer due to high-voltage stressing is relieved by the formation of these defects which act as paths for gate leakage current and result in electron trapping and degradation in the transport properties of the channel underneath.

/pdf/evolution-of-structural-defects-associated-with-electrical-1tgey378iz.pdf

Jungwoo Joh

Papers

GaN HEMT reliability

A Current-Transient Methodology for Trap Analysis for GaN High Electron Mobility Transistors

Critical Voltage for Electrical Degradation of GaN High-Electron Mobility Transistors

Mechanisms for Electrical Degradation of GaN High-Electron Mobility Transistors

Evolution of structural defects associated with electrical degradation in AlGaN/GaN high electron mobility transistors