Normally-Off, Quasi-Vertical GaN FinFETs on Sapphire Substrates With Fin Widths Structured by i-Line Lithography in the 100 nm Range

This work reports on the fabrication of vertical GaN FinFETs on a sapphire substrate. The FinFET concept allows for realizing normally-off transistors with dense multi-channel structures. However, using time-consuming e-beam lithography represents an obstacle to effective device production at a wafer level. Therefore, this work investigates the possibility of creating thin fins at submicrometer dimensions below a width of 200 nm using i-line lithography. Starting with broad fins and thinning them with a wet etch resulted in an undesired shortening of the length of the fins, necessitating structuring the fins with widths significantly below the used wavelength. Our process proves capable of achieving fin widths in the range of 100 nm. The FinFETs consist of at least 100 fins and exhibit a maximum averaged threshold voltage ( ${V}_{\text {th}}$ ) of ( $0.90~\pm ~0.10$ ) V. ${V}_{\text {th}}$ displays the expected parabolic decrease with increasing fin width, becoming negative at around 400 nm. The contributions of the accumulation channel and the bulk fin are distinguished in the transconductance profile. Small-signal RF characterization of a device featuring a fin width of 370 nm shows maximum values of ${{f}_{\text {t}}} = {{3}.3}~{\text {GHz}}$ and ${{f}_{\max }} =$ 5.2 GHz.