Investigation of anomalous positive bias temperature instability in GaN MIS-HEMTs with PEALD SiNx gate dielectrics

In this work, a “turn-around” behavior in the threshold voltage (VTH) shift of GaN metal–insulator–semiconductor (MIS) high electron mobility transistors on Si substrate under positive gate bias stress is observed with increasing temperature, and the physical mechanism is systematically analyzed. At room and low temperatures, the positive VTH shift dominated by the electron trapping and tunneling effects is well known. However, the VTH exhibits an unexpected negative shift at high temperatures, and its underlying mechanism remains unclear. This work attributes the phenomenon to the dissociation of Si–H bonds and the generation of positive charges within the plasma-enhanced atomic layer deposition SiNx, confirmed by Fourier transform infrared spectroscopy. Meanwhile, a physical model involving hydrogen migration and release is adopted to explain the abnormal negative VTH shift. The activation energy of the negative VTH shift is estimated to be 0.32 eV. The identified mechanism of VTH shift can offer a pathway to improve the reliability of MIS devices.