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A normally-off GaN double-implanted vertical MOSFET (DMOSFET) with an atomic layer deposition (ALD)-Al2O3 gate dielectric film on a free-standing GaN substrate fabricated by triple ion implantation is presented. The DMOSFET was formed with Si ion implanted source regions in a Mg ion implanted p-type base with N ion implanted termination regions. A maximum drain current of 115 mA/mm, maximum transconductance of 19 mS/mm at a drain voltage of 15 V, and a threshold voltage of 3.6 V were obtained for the fabricated DMOSFET with a gate length of 0.4 μm with an estimated p-type base Mg surface concentration of 5 × 1018 cm−3. The difference between calculated and measured Vths could be due to the activation ratio of ion-implanted Mg as well as Fermi level pinning and the interface state density. On-resistance of 9.3 mΩ·cm2 estimated from the linear region was also attained. Blocking voltage at off-state was 213 V. The fully ion implanted GaN DMOSFET is a promising candidate for future high-voltage and high-power applications.

A high breakdown voltage of 5.0 kV has been achieved for the first time in vertical GaN p-n junction diodes by using our newly developed guardring structures. A resistance device was inserted between the main diode portion and the guard-ring portion in a ring-shaped p-n diode to generate a voltage drop over the resistance device by leakage current flowing through the guard-ring portion under negatively biased conditions before breakdown. The voltage at the outer mesa edge of the guard-ring portion, where the electric field intensity is highest and the destructive breakdown usually occurs, is decreased by the voltage drop, so the electric field concentration in the portion is reduced. By adopting this structure, the breakdown voltage (V B ) is raised by about 200 V. Combined with a low measured on-resistance (R on ) of 1.25 mΩ cm 2 , Baliga's figure of merit ðV 2 B =R on Þ was as high as 20 GW/cm 2 .

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