Voltage and current degrade the AlGaN/GaN high electron mobility transistors (HEMTs) under on-state stress. To determine which one dominates the degradation, two on-state stresses which have equal power are exerted on AlGaN/GaN HEMTs: high voltage and low current on sample A, low voltage and high current on sample B. In the former stress, drain-source voltage (V DS) is 28 V, drain-source current (IDS) is 75 mA/mm. In the latter stress, V DS is 14 V and IDS is 150 mA/mm. The package temperatures of samples A and B are kept at 150 ◦C. The samples are measured every 24 hours, with an extra measurement at the 8th hour in the first 24 hours (note that the time refers to the stressing time). There is an interval of 4 hours between the stressing and the measurement. The device parameters include drain-source current-voltage (IDS-V DS) characteristics, large-signal parasitic source resistance (RS), large-signal parasitic drain resistance (RD), and transfer characteristics between IDS and gate-source voltage (VGS). The emission microscope (EMMI) is used to study the leakage current after experiment. The IDS-VDS characteristics of sample B are kept constant after being stressed, while that of device A shifts downward after being stressed. RS of sample A, RS of sample B, and RD of sample B increase slightly, RD of sample A increases more obviously with most change happening in the first 8 hours. IDS-VGS characteristics of sample B kept constant, IDS-VGS characteristics of sample A shift downward. The changes of threshold voltage (VGS(th)) is obtained from the transfer characteristics, and it is similar to the changes of transfer characteristics. The VGS(th) magnitude (absolute value) of sample A decreases obviously while that of sample B decreases slightly. The measurements show that the device under low voltage and high current stress degrades little and the device under high voltage and low current stress degrades more obviously. The EMMI images show that the leakage of sample A is greater than that of sample B. The analyses of the parameter change, experiment setting and EMMI image indicate that the voltage, rather than the current, dominates the degradation for AlGaN/GaN HEMTs. The influences of hot electron effect, gate electron injection, and self-heating are recoverable, and they vanish in the interval between the stressing and the measurements. The permanent degradation of device parameter is caused by the inverse piezoelectric effect induced by high electrical field between the gate and the drain. Besides, it is found that sudden failure without precursor is easy to happen to the device under low voltage and high current stress. The microscope image of damaged area shows that the failure is due to hot spot induced by high current density.
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