The realization of depletion‐mode planar and both enhancement‐ and depletion‐mode tri‐gate high electron mobility transistors (HEMTs) based on Al0.2Ga0.8N/AlN/GaN heterostructures grown on silicon substrates using an ultrathin 3C‐SiC transition layer is presented. This substrate configuration simplifies heterostructure growth compared to SiC and thick 3C‐SiC substrates. The threshold voltage of the tri‐gate devices strongly depends on the AlGaN/GaN body (fin) width. A transition from depletion‐mode to enhancement‐mode operation occurred at 110 nm body width for tri‐gate devices without Si3N4 passivation, and is expected to occur at 75 nm for devices with Si3N4 passivation. Threshold voltages of −0.25/0.35 V were achieved at fin widths of 82/100 nm with/without Si3N4 passivation, respectively, for the tri‐gate HEMTs in comparison to −3.5 V for the planar HEMTs. Cutoff frequencies fT of 45 GHz and maximum frequencies of oscillation fmax of 50 GHz have been measured for our best 100‐nm tri‐gate enhancement‐mode HEMTs, while depletion‐mode planar HEMTs with the same gate length showed 110 GHz for both fT and fmax. This demonstrates the applicability of the developed substrate configuration for high‐performance device applications.