开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用研究

通过采集等功率的两种不同开态直流应力作用下AlGaN/GaN高电子迁移率晶体管(HEMTs)漏源电流输出特性、源区和漏区大信号寄生电阻、转移特性、阈值电压随应力时间的变化, 并使用光发射显微镜观察器件漏电流情况, 研究了开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用. 结果表明, 低电压大电流应力下器件退化很少, 高电压大电流下器件退化较明显. 高电压是HEMTs退化的主要因素, 栅漏之间高电场引起的逆压电效应对参数的永久性退化起决定性作用. 除此之外, 器件表面损坏部位的显微图像表明低电压大电流下器件失效是由于局部电流密度过高, 出现热斑导致器件损伤引起的.

Degradation induced by voltage and current for AlGaN/GaN high-electron mobility transistor under on-state stress

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 (I_DS) is 75 mA/mm. In the latter stress, V_DS is 14 V and I_DS is 150 mA/mm. The package temperatures of samples A and B are kept at 150 ℃. 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 (I_DS-V_DS) characteristics, large-signal parasitic source resistance (R_S), large-signal parasitic drain resistance (R_D), and transfer characteristics between I_DS and gate-source voltage (V_GS). The emission microscope (EMMI) is used to study the leakage current after experiment. The I_DS-V_DS characteristics of sample B are kept constant after being stressed, while that of device A shifts downward after being stressed. R_S of sample A, R_S of sample B, and R_D of sample B increase slightly, R_D of sample A increases more obviously with most change happening in the first 8 hours. I_DS-V_GS characteristics of sample B kept constant, I_DS-V_GS characteristics of sample A shift downward. The changes of threshold voltage (V_GS(th)) is obtained from the transfer characteristics, and it is similar to the changes of transfer characteristics. The V_GS(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.