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An anomalous output conductance that resembled short-channel effects was observed in long-channel N-polar GaN-channel/AlGaN-back-barrier/GaN-buffer high electron mobility transistors. The phenomenon could not be reasonably explained by drain-induced barrier lowering, leakage currents, or impact ionization events. We propose that the output conductance was caused by the ionization of a donorlike hole trap state at the negatively polarized AlGaN-back-barrier/GaN-buffer interface that shifted the threshold voltage at the drain side of the gate, where a high-field depletion region developed beyond current saturation. No evidence of increased output conductance or related device performance degradation was apparent under small-signal high-frequency conditions. The output conductance was suppressed by introducing photogenerated holes that compensated the traps. The effect of several typical back-barrier designs on the dc output conductance was examined.