High ON/OFF and high FoM of <italic>f</italic>$_{\rm~max}$$\times$BV$\times$<italic>L</italic>$_g$ InAlN/GaN HEMTs by using polycrystalline-AlN capnowendneedtodelete

In this paper, we demonstrate the excellent performance of InAlN/GaN high electron mobility transistors (HEMTs) for high-voltage radio frequency (RF) application. By using a composite cap layer structure of GaN cap layer and polycrystalline (PC)-AlN cap layer, the gate leakage is effectively suppressed, resulting in a 0.5-$\upmu$m InAlN/GaN HEMTs achieving a high current on/off ratio (ON/OFF) of 7 $\times$ 10$^7$ as well as a record breakdown voltage (BV) of 270 V. The device yields a high peak transconductance (G$_m$) ofłinebreak 350 mS/mm and a low sub-threshold swing (SS) of 80 mV/decade while virtually free of drain induced barrier lowering (DIBL) effect (about 2.5 mV/V). Besides, the InAlN/GaN HEMTs with composite cap layer structure exhibit a current gain cut-off frequency/ maximum oscillation frequency (f$_T$/f$_{\rm~max}$) of 22/64 GHz. To the best of our knowledge, the device has the highest figure-of-merit (FoM = f$_{\rm~max}$ $\times$ BV $\times$ L$_g$) of 8.6 THz$\cdot$V$\cdot$$\upmu$m compared to the InAlN/GaN HEMTs reported in the past. These results demonstrate that the design of the composite cap layer structure extends the RF InAlN/GaN HEMTs for high voltage applications.