Nonpolar AlGaN/GaN Metal–Insulator–Semiconductor Heterojunction Field-Effect Transistors With a Normally Off Operation

Nonpolar AlGaN/GaN metal-insulator-semiconductor-heterojunction field-effect transistors (MIS-HFETs) with a complete normally off operation have been demonstrated for the first time. To realize the normally off operation of the MIS-HFETs, a 2-nm-thick SiN (as an insulator) and GaN-based nonpolar epitaxial layers that are free from polarization charges are employed. We have found that a thicker nonpolar AlN buffer layer achieves a GaN layer with a narrower full width at half maximum of the X-ray rocking curve and higher electron mobility. The fabricated MIS gate structure on the AlN buffer layer successfully decreases the gate leakage current and enables a more positive gate bias of up to +4 V, which is advantageous to achieve a higher drain current. Moreover, the n-GaN capping layer between gate and ohmic electrodes helps to reduce parasitic resistance and suppress current collapse. The fabricated
a
-plane MIS-HFET exhibits a threshold voltage of + 1.3 V with a high drain current of 112 mA/mm. The presented MIS-HFETs will be desirable in next-generation radio-frequency and power switching application fields.