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I. INTRODUCTIONEMTS based on the GaN/AlGaN materials system are rapidly becoming the semiconductor device of choice for RF and power switching applications. These devices require a semi-insulating buffer to suppress leakage and punch-through. RF devices frequently make use of iron (Fe) doping to render the GaN insulating, but for the higher voltages required for many power switching applications, it has been found that carbon (C) doping delivers higher breakdown voltage and lower off-state leakage [1,2]. Unfortunately it has also been found that using carbon can result in a transitory increase in R ON , also known as current-collapse (CC), when switched from the off to the on-state [2,3]. With field plates now universally used to control surface effects, it is clear that the remaining CC in these devices mostly results from charge storage in deep levels in the buffer. Our previous studies have shown that the difference in CC between Fe and C doping results from their acceptor trap levels pinning the bulk Fermi level in the upper and lower halves of the bandgap respectively [4]. GaN:C is p-type with its low hole density, and hence high resistivity, giving long time constants for charging processes (a hole density of only 10 4 cm -3 was inferred in [5]

We report on a floating buffer model to explain "kink," a hysteresis in the output characteristics of Fe-doped AlGaN/GaN HEMTs observed at low drain bias. Unintentionally doped background carbon can make the GaN buffer p-type allowing it to electrically float. We further note that reverse bias trap-assisted leakage across the junction between the 2DEG and the p-type buffer can provide a mechanism for hole injection and buffer discharging at just a few volts above the knee, explaining the "kink" bias dependence and hysteresis. We show that HEMTs with a different background carbon have dramatically different kink behaviors consistent with the model. Positive and negative magnitude drain current transient signals with 0.9-eV activation energy are seen, corresponding to changes in the occupation of carbon acceptors located in different regions of the GaN buffer. The observation of such signals from a single trap calls into question conventional interpretations of these transients based on the bulk 1-D deep-level transient spectroscopy (DLTS) models for GaN devices with floating regions.

The role of buffer traps (identified as C N acceptors through current DLTS) in the off-state leakage and dynamic Ron of 650V rated GaN-on-Si power devices is investigated. The dynamic Ron is strongly voltage-dependent, due to the interplay between the dynamic properties of the C N traps and the presence of space-charge limited current components. This results in a complete suppression of dyn Ron degradation under HTRB conditions between 420V and 850V.