Abstract

During the past two decades AlGan/GaN Heterostructure Field Effect Transistors (HFETs) have been the target of much attention in high power microwave applications. Crystal imperfections in AlGan/GaN HFETs have been pointed out as the cause of many reliability concerns such as drain-current collapse, gate lag, and excessive gate leakage-current. Current collapse and reliability degradation due to electron trapping at the surface layer of AlGan/GaN HFETs are major impediments for commercialization of these devices. Even though there have been remarkable improvements in crystal growth and device fabrication technology, trapping effects in AlGan/GaN HFETs, specially under high drain-voltage conditions, have not been completely removed. Therefore, an assured simulation of HFET with incorporation of trapping effects is needed. In this thesis, in order to substantiate the hypothesis of electron trapping at deep surface states as the cause of semi-permanent current collapse this phenomenon is studied with the use of CADtool Medici. Monte Carlo simulation of electronic transport at AlGan/GaN channels reveals that in addition to the steady-state velocity overshoot there exists a pronounced kink in the low electric-field region of the drift-velocity versus electric-field characteristics of these channels. Existence of the inflexion points attributed to this kink and the large width of the overshoot pattern in conjunction with the large electric-fields conventionally applied to these wide band-gap semiconductors, make the modeling of electronic devices fabricated in this technology different than those of other 111-V semiconductors. An analytical model for drain current/voltage characteristic of AlGan/GaN HFETs with incorporation of steady-state drift-velocity overshoot and the inflexion points in the electronic drift transport characteristics is also presented in this thesis. The wide peak and pronounced inflexion points in the transport characteristics of AlGan/GaN heterojunctions are modeled through considering a drift-diffusion channel rather than a drift-only transport channel. Simulation results have been compared to a non-diffusion type channel implemented with the assumption of Ridley's saturating transport model. The model is based on applying an iterative approach between Poisson's equation and current-continuity equation, which relieves the results from the burden of the choice of fitting parameters. With the advancement of this technology, development of a versatile analytical model with incorporation of these considerations is vital for understanding the full range of capabilities of III-Nitride material system.