AlGaN (2.5%) fully vertical FinFETs: Influence of <i>m</i> - and <i>a</i> -plane substrate alignments

We demonstrate the first fully vertical single-fin AlGaN FinFETs (Al = 2.5%) on ammonothermal n+-GaN substrates, featuring a gate length of Lg = 200 nm and excellent gate control. This work presents a systematic study of the impact of fin orientation along the a- and m-crystallographic planes on the electrical performance of AlGaN vertical FinFETs. We report that threshold voltage is maximized at fin width, Wfin = 100 nm, with VT = 1.9 V (a-plane) and 1.75 V (m-plane). The highest ON-current density taken at an overdrive voltage VOV = 1.5 V and peak extrinsic transconductance are achieved at Wfin = 200 nm, with JON = 4.4 kA/cm2 (a-plane) and 4.1 kA/cm2 (m-plane), and gme,peak = 4.1 kS/cm2 (a-plane) and 3.6 kS/cm2 (m-plane). The minimum specific on-resistance of FinFETs with Wfin = 200 nm, extracted at VOV = 1.5 V, is as follows: RON,sp = 0.85 mΩ cm2 (a-plane) and 0.97 mΩ cm2 (m-plane). The single-fin device area, including current spreading in the drift layer, is used for normalization. Among all variants, a-plane devices with narrower fins delivered the superior electrical performance. These results establish the critical role of crystallographic alignment in optimizing vertical AlGaN device performance and represent a significant step toward scalable vertical AlGaN power transistors.