The effect of high overdrive voltage on the positive bias temperature instability (PBTI) trapping behavior is investigated for GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) with LPCVD-SiN x gate dielectric. A higher overdrive voltage is more effective to accelerate the electrons trapping process, resulting in a unique trapping behavior, i.e., a larger threshold voltage shift with a weaker time dependence and a weaker temperature dependence. Combining the degradation of electrical parameters with the frequency–conductance measurements, the unique trapping behavior is ascribed to the defect energy profile inside the gate dielectric changing with stress time, new interface/border traps with a broad distribution above the channel Fermi level are introduced by high overdrive voltage.

Stress voltages on time-dependent breakdown characteristics of GaN MIS-HEMTs during negative gate bias stress (with V GS < 0, V D = V S = 0) and off-state stress (V G < V Th, V DS > 0, V S = 0) are investigated. For negative bias stress, the breakdown time distribution (β) decreases with the increasing negative gate voltage, while β is larger for higher drain voltage at off-state stress. Two humps in the time-dependent gate leakage occurred under both breakdown conditions, which can be ascribed to the dielectric breakdown triggered earlier and followed by the GaN layer breakdown. Combining the electric distribution from simulation and long-term monitoring of electric parameter, the peak electric fields under the gate edges at source and drain sides are confirmed as the main formation locations for per-location paths during negative gate voltage stress and off-state stress, respectively.

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The impact of carbon doping on the background carrier conduction in GaN has been investigated. It is found that the incorporation of carbon can effectively suppress the n-type background carrier concentration as expected. Moreover, from the fitting of the temperature-dependent carrier concentration and mobility, it is observed that high nitrogen-vacancy (V N ) dominates the background carrier at room temperature which consequently results in n-type conduction. The doping agent (carbon atom) occupies the nitrogen site of GaN and forms C N deep acceptor as revealed from photoluminescence. Besides, a relatively low hole concentration is ionized at room temperature which was insufficient for the compensation of n-type background carriers. Therefore, we concluded that this background carrier concentration can be suppressed by carbon doping, which substitutes the N site of GaN and finally decreases the V N .

High conductivity on the N-face of GaN has long been observed. For many years, sapphire was the basic substrate used for growing GaN layers. Hence, such conductivity was associated with a high concentration of defects at the GaN/sapphire interface. Due to recent advances in GaN growth techniques, bulk GaN in polar and non-polar orientations has become available. In this study, we compare GaN grown on sapphire with bulk GaN. It is found by numerical fitting that the sheet resistance of GaN layers on sapphire substrates is determined by both bulk GaN conduction and polar-induced screening-charge conduction. The same high conductivity on the N-faces of GaN layers is observed even without the interface with sapphire substrates, i.e., when the latter are peeled off and the GaN layers are chemically mechanically polished. A further comparison of three samples with different orientations (c-, a-, and m-planes) suggests that the observed high conductivity is caused by a mobile screening charge attracted to the N-face by the polar charge present on it. This conclusion follows from the fact that no high conductivity is observed in the samples with non-polar orientations.