Studies on the influence of quiescent-gate (V-gs0) and quiescent-drain (V-ds0) bias stresses in rf-plasma MBE grown AlGaN/GaN high-electron-mobility transistors (HEMTs) were performed. The increase of drain current (I-D) collapse by quiescent-bias-stress in AlGaN/GaN HEMTs were observed using pulsed (pulse width = 200 ns; pulse period = 1 ms) I-DS-V-DS characteristics. The Si3N4 passivation suppressed about 80% I-D collapse in quiescent-bias-point stressed HEMTs. The remaining 20% I-D collapse were not suppressed which may be coming from buffer-related traps. However, more than 10% of I-D collapse suppression was observed on un-stressed or fresh-HEMTs. Similarly, improved cut-off frequency (f(T)), maximum oscillation frequency (f(max)) land device output power (P-out) values were also observed on the un-stressed HEMTs. The Si3N4 passivation completely suppressed the I-D collapse in un-stressed or fresh-HEMTs which leads to 70% improvement in f(T) and 60% improvement in the device P-out. The Si3N4 passivation did not completely suppress I-D collapse in the quiescent-bias stressed-HEMTs. This may be due to the generation of additional surface-related traps in the HEMTs by quiescent-bias-stresses. (C) 2010 Elsevier Ltd. All rights reserved.