Heterojunction field effect transistors (HFET) for high-frequency and high-power electronics have been an area of active research in recent years as a key enabling technology for applications ranging from wireless communications to power distribution. III-Nitride semiconductors are a leading candidate for fulfilling the material requirements of these devices based on the combination of large bandgap energy, high thermal conductivity, high electron mobility and saturated electron velocity. While III-Nitride HFETs have demonstrated remarkable advances, serious materials related limitations still exist, primarily related to charge states and trapping effects at the semiconductor surface. Several groups have investigated solutions such as the deposition of dielectric passivation layers and asymmetric field-plate gate geometries for controlling the influence of trap states near the metal/semiconductor FET interface. We have demonstrated a metal-oxide semiconductor FET (MOSFET) with a substantially unpinned interface which is capable of establishing substantial charge accumulation under the gate. These III-Nitride MOSFETs may be designed to operate in either depletion mode or enhancement mode. GaN/InGaN heterojunction MOSFETs exhibit enhancement mode peak transconductance at gate voltages Vg>+5V, corresponding to energy greater than the bandgap of the underlying semiconductor which provides strong evidence of an unpinned MOS interface. Additionally III-Nitride MOSFETs eliminate the need for field plate gate structures as the electric field geometry in the gate-drain region changes limiting the tunneling of charge to unfilled surface states. In depletion mode, low-rf dispersion InGaN/GaN MOSFETs exhibit excellent microwave with ft = 8GHz for optically defined gates dimensions.
We review the history of compound semiconductor MOSFET development and overlaying these developments with recent advances in the III-Nitride materials and device research. Differences in chemistry of III-Nitrides relative to all other compound semiconductors and the epitaxial deposition of gate-oxides such as Gadolinium Gallium Oxide (GGO), opens the possibility for dramatically improved devices at microwave and mm-wave frequencies as well as power MOSFET rectifiers. Initial III-Nitride MOSFETs results are presented as well as a quaternary thermodynamic framework for the stability of gate-oxide on nitride semiconductors. We also identify key materials related research challenges expected to impact the ongoing development of III-Nitride MOSFETs.
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The authors thank their student and faculty collaborators in the NCSU Photonics and Atomic Resolution Electron Microscopy Labs, most importantly J. Matthews for his expert assistance. One author (DB) thanks collaborators at OSEMI and Univ. of Michigan. This project has been supported by ORAU Powe Faculty Enhancement Award (MJ), SRC Exploratory Development Grant (MJ and DB) and the Navy SBIR Program (Randy Lasiter, Program Manager).
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Johnson, M.A.L., Barlage, D.W. & Braddock, D. Prospect for III-Nitride Heterojunction MOSFET Structures and Devices. MRS Online Proceedings Library 829, 338–349 (2004). https://doi.org/10.1557/PROC-829-B7.7