HfO₂ as gate insulator on N-polar GaN–AlGaN heterostructures

To further increase the operation frequency of GaN high electron mobility transistors (HEMTs) with high power gain in the mm-wave band (30–300 GHz), channel thickness must be scaled. High-k dielectrics such as HfO₂ can be inserted as gate insulator to reduce the leakage, while maintaining good gate control. In this work, we investigated HfO₂ on N-polar HEMT structure by performing frequency dependent capacitance–voltage measurements on metal-oxide-semiconductor capacitors (MOSCAPs). The impact of annealing on the quality of HfO₂ was studied. Moreover, we compared the CV characteristics of MOSCAPs with HfO₂ deposited on the GaN channel with that deposited on the AlGaN cap layer. We showed that HfO₂ deposition on AlGaN cap leads to Fermi level pinning, which prevents the channel being pinched off by applying gate voltage. On the other hand, well-behaved CV profiles were achieved by depositing HfO₂ directly on GaN or SiN dielectric. (paper)