Normally-Off Ga2O3 MOSFETs With Unintentionally Nitrogen-Doped Channel Layer Grown by Plasma-Assisted Molecular Beam Epitaxy

We found that nitrogen (N) and silicon (Si) were unintentionally co-doped in Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE), and that the ratio between N and Si could be controlled by O/Ga flux ratio during the growth. Note that N and Si act as a deep acceptor (theoretically predicted) and a shallow donor (experimentally confirmed) in Ga2O3, respectively. Taking advantage of this unique characteristic of PAMBE-grown Ga2O3, normally off operation of Ga2O3 metal–oxide–semiconductor field-effect transistors (MOSFETs) was demonstrated. The unintentionally-doped Ga2O3 channel layer of the MOSFETs had N and Si concentrations of $1\times 10^{18}$ and $2\times 10^{17}$ cm−3, respectively, and thus it was considered to behave as a p -type material. The MOSFETs showed a turn-on threshold gate voltage of larger than +8 V, implying formation of an inversion channel in the N-doped Ga2O3 layer. Although the ON-state drain current ( ${I}_{\mathrm{d}}$ ) remained in the subthreshold regime owing to limited gate voltage swing, the ${I}_{\mathrm{d}}$ ON/OFF ratio exceeded five orders of magnitude.