Ultra-wide bandgap AlGaN channel MISFET with polarization engineered ohmics

Abstract

We report on the first ultra-wide bandgap Al_0.75Ga_0.25N channel metal-insulator-semiconductor field-effect transistor (MISFET) with heterostructure engineered ohmic contacts. The large breakdown field of AlN (12 MV/cm) and the superior device figures of merit make wider bandgap AlGaN attractive for the next-generation RF power amplifiers and switches [1]. However, a critical challenge preventing advancement in high composition AlGaN-based devices is the high resistance of ohmic contacts, due to the large ionization energy of dopants and the low electron affinity of AlN, both of which increase tunneling barrier for electrons. In this work, we use reverse polarization-graded n++ AlGaN contact layers to achieve a record low contact resistance (Rc) of 0.3 Ω.mm to 75 nm thick n-Al_0.75Ga_0.25N channel, translating in a specific contact resistance (ρsp) of 1.9×10^-6 Ω.cm². We then demonstrate the first ultra-wide bandgap Al_0.75Ga_0.25N channel MISFET with gate-recessed structure, employing polarization-graded contacts and Atomic Layer Deposited Al₂O₃ as the gate-dielectric.