| During the past decade, AlGaN/GaN heterostructure field effect transistors (HFETs) have been the focus of the device research community as an alluring candidate for high power, high voltage microwave applications. Intensive theoretical and experimental investigations on III-Nitride material system have opened new frontiers for the electronic devices. Extraordinary high sheet carrier concentration and strong confinement of the carriers at specific heterointerfaces of unintentionally doped AlGaN/GaN heterostructures are known to be consequences of strong spontaneous and piezoelectric polarization fields. Despite the inferiority of the low field electron mobility in GaN compared to the main contender in the III--V notch of electronic device market (i.e. GaAs), this material enjoying a much larger peak electron velocity, larger saturation velocity, larger thermal stability, higher two dimensional electron gas (2DEG) concentration, and better confinement as a consequence of its larger bandgap, has raised many hopes as being the perfect material for the channel of microwave power devices.; Despite all the interesting aspects of AlGaN/GaN Modulation Doped Field Effect Transistors (MODFET), these devices have usually been reported to demonstrate much lower microwave gain compared to their DC-predicted values. This dissertation is focused on the reliability issues of AlGaN/GaN MODFETs using the highly sensitive tools of low frequency noise measurement. Overall, in this study the impact of growth technique, surface passivation, and Al composition on the reliability of AlGaN/GaN MODFETs under RF and DC stresses are evaluated. Studies on the impact of RF and DC stress demonstrate the existence of considerable hot carrier-caused degradations. Operation under RF large signal is observed to further these destructive effects. Drain low frequency noise, is found to be capable of predicting the long term reliability of the gate characteristics of these devices. For these wide-bandgap materials, low frequency noise is found to be extremely dependent on the interface roughness, while essentially independent of barrier's Al composition and surface treatment. Due to this matter, in this material system low frequency noise would be a valuable tool in interface roughness assessment. |
