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× 10¹⁸ and 2 × 10¹⁷ 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_d) remained in the subthreshold regime owing to limited gate voltage swing, the I_d ON/OFF ratio exceeded five orders of magnitude.