Febrication And Characteristics Analysis Of ALQaN/GaN HEMTs With Transparent Gate

As the third-generation semiconductor, GaN presents lots of advantages, such aslarge band gap, high thermal conductivity, high breakdown voltage, mobility andsaturation velocity. Therefore, high electron mobility transistors (HEMTs) based onAlGaN/GaN heterostructures are widely applicated for high temperature, high powerand high frequency semiconductor devices, high brightness LED, and highperformance ultraviolet (UV) detector. Due to their high carrier concentration andoptical band gap, transparent conductive oxide thin films exhibit excellent optical andelectrical properties. In this dissertation, the properties of AlGaN/GaN HEMT deviceswith transparent gate materials of AZO (Al-doped Zinc Oxide) were studied.Firstly, the AlGaN/GaN structure and the operation mechanism of HEMT deviceswere investigated. Based on the theory, AlGaN/GaN HEMT devices with transparentAZO gate were fabricated successfully, and their properties including DCcharacteristics and Schottky characteristics were analysed. Depositing a thin Ni(10nm)layer before the AZO could effectively reduce the gate leakage current and improvethe Schottky barrier height. Besides, it gave rise to a higher saturation current.The influence of rapid thermal annealing(RTA), UV irradiation and electricalstress in the devices was also investigated in this dissertation. The results showed thatannealing at low temperature, especially300℃, would significantly improve theSchottky characteristics. But serious degradation of the devicves would occur duringannealing at400℃. UV irradiation could increase the output current, as a result of therelease of electrons captured by traps in the devices and the generation ofphoto-induced carriers. What’s important is that the devices characteristics couldnotrecover completely after the removal of UV light. Under the on-state stress, the hotelectrons in channel were captured by traps in the barrier or buffer layer, whichdepleted the two-dimensional electron gas(2DEG) and increased the conductionresistance. This degradation reduced the saturation current and peak transconductance.However, under the off-state stress, the saturation current and transconductanceincreased, and the threshold voltage negatively drifted simultaneously. This was due tothe injection of some electrons from gate into the channel, which increased theconcentration of2DEG.