Undoped β-Ga2O3 Layer Thickness Effect on the Performance of MOSFETs Grown on a Sapphire Substrate

β-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) with various unintentionally doped (UID) layer thicknesses (50, 100, and 200 nm) of (2̅01) beneath the Si tetraethoxysilane (TEOS)-doped film were grown by metal–organic chemical vapor deposition on a (0001) sapphire substrate. The UID layer thickness causes the MOSFET turn-on current to increase from 1.7 to 700 μA/mm, RON to decrease from 1.13 MΩ·mm to 3.8 kΩ·mm, and breakdown voltage to decrease from 910 to 240 V. Through X-ray photoelectron spectroscopy (XPS), we ascribe this increased turn-on current and reduced breakdown voltage to oxygen vacancies.