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Although a thin AlN spacer layer between the AlGaN barrier and GaN channel layers effectively increases electron mobility and sheet carrier concentration in a two-dimensional electron gas, the very wide bandgap AlN makes ohmic contacts difficult to form. We overcame this problem using Si ion implantation to attain contact resistance below 2:5 Â 10 À6 cm 2 . Samples without ion implantation had poor ohmic properties. Inserting the thin AlN spacer layer dramatically improved the drain current of high-electron mobility transistors.

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A channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. Si ion implantation doping technique was utilized to achieve sufficiently low resistive source/drain contacts. The fabricated AlGaN channel HEMTs with the field plate structure demonstrated good pinch-off operation with sufficiently high drain current density of 0.5 A/mm without noticeable current collapse. The obtained maximum breakdown voltages was 1700 V in the AlGaN channel HEMT with the gate-drain distance of 10 μm. These remarkable results indicate that AlGaN channel HEMTs could become future strong candidates for not only high-frequency devices such as low noise amplifiers but also high-power devices such as switching applications.