Depletion-mode N-polar AlN-based high electron mobility transistors on sapphire with improved on/off ratios

We report N-polar AlN-based high electron mobility transistors (HEMTs) with a GaN channel thickness of 5.2 nm on N-polar AlN on sapphire. The threshold voltage is around −3.0 to −2.4 V, with saturation currents over 240 mA/mm and on/off ratios as high as 104, much higher than those of previously reported N-polar AlN-based HEMTs. The improved on/off ratio achieved in this work is attributed to the use of an abrupt AlN/GaN heterostructure with a dedicated AlN transition layer, together with improved gate leakage. Both the high-frequency performance and the on-resistance of ≈20 Ω mm are limited by the 2000 Ω/□ sheet resistance of the channel layer.