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We proposed and fabricated normally off GaN MOSFETs with an epitaxially regrown n+ GaN source/drain after a short period of dry etching on a sapphire substrate. The regrown S/D MOSFET after dry etching (MOSFET A) exhibited enhanced performance in terms of current drivability and access resistance compared with the same MOSFET without the surface etching before the regrowth (MOSFET B). While MOSFET A has a saturation drain current of 10 mA/mm at V G = 8 V, a field-effect mobility of 22 cm2 V-1 s-1, and a series resistance R SD of 0.57 kΩ, MOSFET B has 3 mA/mm, 12 cm2 V-1 s-1, and 0.93 kΩ, respectively. The electrical characteristic of MOSFET A was also much more improved than that of MOSFET B at low temperatures. Mobility degradation at low temperatures was related to the effect of impurity scattering caused by crystal defects generated during the metal organic chemical vapor deposition (MOCVD) growth.

A vertical-type mesa-gate GaN metal-oxide semiconductor field-effect transistor (MOSFET) has been fabricated. The mesa-gate structure can be easily achieved by a single deep etch to the n + -GaN which is the drain of the device, whereas the trench-gate structure, the commonly used structure for the vertical-type MOSFETs, requires an additional etching process to define the gate region. The mesa-gate GaN MOSFET exhibited a normally-off operation with the threshold voltage of 3 V, a normalised drain current of ∼ 55 mA/mm and a high on/off current ratio of 10 8 .Introduction: GaN is a promising material for next-generation powerdevice applications, owing to its superior material properties, such as high breakdown electric field (up to 3 MV/cm), high saturation velocity (∼3 × 10 7 cm/s at 150 kV/cm) and good thermal conductivity [1, 2]. Furthermore, a polarisation-induced high two-dimensional electron gas (2DEG) density of ∼1 × 10 13 cm −2 is formed at the AlGaN/GaN heterointerface, which enables high-current operation in lateral-type AlGaN/GaN heterostructure field-effect transistors (HFETs) [3][4][5].For power-switching applications, a normally-off operation is required to minimise the static power consumption and to simplify the driving circuit [6]. However, most of the lateral-type AlGaN/GaN HFETs exhibit normally-on operation, owing to the existence of the 2DEG under the gate, which makes it very difficult to achieve a normally-off operation; hence, additional techniques, such as the recessed-gate [7], the p-GaN gate [8] and the F − treated gate [9] are required. In general, vertical-type devices offer many advantages such as low-specific on-resistance (R on A), high breakdown voltage (V B ) and smaller size. In addition, the vertical-type devices are attractive in easily obtaining normally-off operation with high-current capability. Recently, trench-gate GaN vertical metal-oxide semiconductor field-effect transistors (MOSFETs) [10][11][12], which are similar to the device geometry of the Si vertical power MOSFET [13], were fabricated with surface treatment on the etched GaN surface. They successfully demonstrated normally-off operation with relatively high device performances in terms of current capability and I ON /I OFF ratio.In this Letter, we propose and fabricate a mesa-gate-type GaN vertical MOSFET that requires only a simple single-etching process to define all device geometries and hence simplifies the fabrication process, whereas the commonly used trench-gate-type MOSFET requires an additional etching process to define the gate geometry.

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