(Invited) Toward an Understanding of Defects and Device Reliability in GaN Hemts

Abstract

GaN device reliability has become a major factor in technology acceptance for various electronics applications including high radiation and space applications. GaN’s properties make it ideal for a variety of harsh conditions including high energy particle radiation, high power RF biasing, high temperature, and simultaneous combinations of these stressors. Understanding how trapping behavior impacts the electrical characteristics and how defects form or are activated with these various stressors is important to predicting lifetime and end of life performance and approaches to reduce the degradation rate, improve the radiation hardness, etc. This work explores different modes of GaN high electron mobility transistors (HEMT) degradation when subjected to proton irradiation and electrical stressing. Specific trap states associated with different stress conditions are revealed using conventional and HEMT-based deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) methods. These trap characterization techniques provide quantitative information about degradation-limiting traps that can exist anywhere within the device bandgap profile and can be distributed within specific regions of the device structure. In addition, using the newly developed scanning-DLTS, we will show the ability to quantitatively map specific defects on the nanometer scale to identify how defects are related to specific microstructure. Multiple types of AlGaN/GaN HEMTs were characterized in a systematic fashion using conventional, HEMT-based, and scanning-DLTS and DLOS before and after the application of RF switching, high voltage switching and irradiation. Overall, radiation is found to form defects that primarily impact the threshold voltage where shallower traps cause dynamic threshold voltage VT instabilities and deeper traps cause static VT shifts. In particular, proton irradiation is found to increase the concentration of traps at EC-1.3 eV and EC-3.28 eV in the GaN buffer, which appear to be responsible for large and persistent VT shifts in GaN HEMTs, and a state at EC-0.72 eV in the GaN buffer that separately correlates with an increase in dynamic VT instability. The RF accelerated life testing (ALT) has led to different degradation pathways in different generations of HEMTs and different vendors but has consistently been in the drain access region where the electric fields are maximized, which is quite different than the radiation-induced degradation. Early generations consistently showed correlation of the RF output power with the concentration of the EC-0.57 eV level, which we have recently mapped with scanning-DLTS to directly correlate the EC-0.57 eV level with edge dislocations and Fe doping. Newer generations have degraded due to deeper, near mid-gap defects indicating that several degradation mechanisms are possible in GaN HEMTs and they evolve as the materials and device designs are improved. Semi-on DC ALT exhibits degradation somewhat different to RF ALT where the degradation is still due to defects in the drain access region but the traps responsible are different indicating that different testing mechanisms may not be predictive of actual degradation in some cases. The use of DLOS and DLTS to track the activation of these defects will be shown as essential to accurately determine exact degradation mechanisms to establish accurate predictive reliability models and guidelines for improved end of life material and device designs.