Is self-heating responsible for the current collapse in GaN HEMTs?

AlGaN/GaN high-electron mobility transistors (HEMTs) are a very promising technology for switching and radio frequency power applications due to the high saturation velocity and large breakdown field of the GaN material. However, the electrical reliability of this material system in both the on and the off-state operation regimes is still a fundamental problem to be solved before the widespread use of this technology can be made. In the present work, an electro-thermal particle based device simulator has been developed for a GaN HEMT structure. It consists of a Monte Carlo-Poisson solver which is self consistently coupled with an energy balance solver for both the acoustic and optical phonons. We observe that for a given drain bias, the lattice temperature increases to its steady state value after several iterations (Gummel cycles), whereas the electron temperature starts reducing. This implies that the velocity of the carriers is degrading with the temperature increase which, in turn, degrades the on-current. It is also observed that the lattice temperature and electron temperature are highest near the gate to drain transition region as the carrier velocity is highest in this region. The incorporation of self-heating effects in the model leads to higher electric fields at the gate-drain extension which, in turn, can contribute towards larger charge trapping at the surface and a possibility of formation of pits and cracks, thus leading to reliability concerns.