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Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN x surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaNbuffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by currentvoltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.

Current–voltage characteristics of a nominally undoped AlGaN/GaN two-dimensional electron gas channel is measured at a room temperature, and electron drift velocity is deduced under assumption of uniform electric field and field-independent electron density. No velocity saturation is reached at fields up to 130 kV/cm, when the effect of Joule heating is minimized through application of nanosecond pulses of voltage. The estimated drift velocity is near 2×107 cm/s at 130 kV/cm. Monte Carlo simulation of the drift velocity is carried out with and without effects of channel self-heating for a many-subband model, with hot phonons and electron gas degeneracy taken into account.