| Summary: | Novel Al<sub>0.75</sub>Ga<sub>0.25</sub>N/Al<sub>x</sub>Ga<sub>1-x</sub>N/Al<sub>0.75</sub>Ga<sub>0.25</sub>N/AlN metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with symmetrically-graded widegap Al<sub>x</sub>Ga<sub>1-x</sub>N channel (x = 0.75 → 0.25 → 0.75) grown on a SiC substrate are investigated. Al<sub>2</sub>O<sub>3</sub> was devised as the gate dielectric by using a non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. Device characteristics with respect to different etch depths of the source/drain recesses were studied. For a 2-μm gate length (L<sub>G</sub>), the present widegap V-shape-channel MOS-HFET has shown improved maximum drain-source current density (I<sub>DS,max</sub>) of 299.3 A/mm at V<sub>DS</sub> = 20 V, I<sub>DS</sub> density at V<sub>GS</sub> = 0 V (I<sub>DSS0</sub>) of 153.9 mA/mm, on/off-current ratio (I<sub>on</sub>/I<sub>off</sub>) of 1.4 × 10<sup>7</sup>, extrinsic transconductance (g<sub>m</sub>,<sub>max</sub>) of 16.7 mS/mm, two-terminal off-state gate-drain breakdown voltage (BV<sub>GD</sub>) of -379 V, and three-terminal on-state drain-source breakdown voltage (BV<sub>DS</sub>) of 339 V. Besides, superior deep-UV sensing performance with high spectral responsivity (SR) of 1780 (810.2) A/W at wavelength λ = 250 (300) nm are also achieved.
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