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 Ga
₂
O
₃
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 Ga
₂
O
₃
, respectively. Taking advantage of this unique characteristic of PAMBEgrown Ga
₂
O
₃
, normally off operation of Ga
₂
O
₃
metal- oxide-semiconductor field-effect transistors (MOSFETs) was demonstrated. The unintentionally-doped Ga
₂
O
₃
channel layer of the MOSFETs had N and Si concentrations of 1 × 10
¹⁸
and 2 × 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 Ga
₂
O
₃
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.