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Widegap-channel Al 0.65 Ga 0.35 N/Al 0.3 Ga 0.7 N/AlN/SiC metal-oxide-semiconductor heterostructure field-effect transistors (MOS-HFETs) with ultrasonic spray pyrolysis deposition (USPD) grown Al 2 O 3 gate-oxide demonstrating enhancement-mode (E-mode) operation are investigated for the first time. The E-mode operation was achieved by using fluorine ions (F -) implantation. In comparison, conventional Schottky-gate device (sample A) and MOS-HFET (sample B) showing depletion-mode (D-mode) operation were fabricated on the same epitaxial structure. The device characteristics with respect to different gate-to-drain spacings (L GD ) of 6 µm and 14 µm have also been studied. The present E-mode Al 0.65 Ga 0.35 N/Al 0.3 Ga 0.7 N/AlN MOS-HFET (sample C) with L GD = 6 (14) µm has demonstrated improved maximum drain-source current density (I DS,max ) of 206.3 (163.5) mA/mm at V DS = 20 V, maximum extrinsic transconductance (g m,max ) of 32.9 (22.0) mS/mm, on/off-current ratio (I on /I off ) of 3.7 × 10 9 (1.8 × 10 9 ), two-terminal off-state gate-drain breakdown voltage (BV GD ) of −370 (−475) V, and three-terminal on-state drain-source breakdown voltage (BV DS ) of 330 395 V. INDEX TERMS Widegap AlGaN channel, MOS-HFET, enhancement-mode, Al 2 O 3 , non-vacuum ultrasonic spray pyrolysis deposition.

3/in-situ SiN passivation and Al2O3 gate dielectric were investigated. 20 nm thick high-k Al2O3 was deposited using a non-vacuum ultrasonic spray pyrolysis deposition method. Comparative studies between an in-situ SiN-passivated Schottky-gate HFET (sample A) and a composite Al2O3/SiN-passivated MOS-HFET were made. Electrical and deep-UV sensing characteristics for devices with different gate-drain separations (LGD) of 6 and 14 μm were also studied. Improved device performance has been obtained for the present sample B (A) with LGD = 6/14 μm separately, including maximum drain-source current density (IDS, max) of 634.4/463.1 (421.8/301.1) mA/mm, maximum extrinsic transconductance (gm, max) of 25.2/17.9 (19.1/15.2) mS/mm, on/off-current ratio (Ion/Ioff) of 7.4 × 107/5.4 × 107 (4.5 × 105/5.4 × 104), two-terminal off-state gate-drain breakdown voltage (BVGD) of -420/-480 (-320/-390) V, and three-terminal on-state drain-source breakdown voltage of 310/380 (220/300) V at 300 K. Superior spectral responsivity of 885.6 A/W under 250-nm deep-UV radiation has also been achieved for the present MOS-HFET.

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