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.