The 2D electron gas (2DEG) confinement on high electron mobility transistor (HEMT) heterostructures with a thin undoped GaN channel layer on the top of a grain-boundary-free AlN nucleation layer is studied. This is the first time demonstration of a buffer-free epi-structure grown with metal–organic chemical vapor deposition with thin GaN channel thicknesses, ranging from 250 to 150 nm, without any degradation of the structural quality and 2DEG properties. The HEMTs with a gate length of 70 nm exhibit good DC characteristics with peak transconductances of 500 mS mm⁻¹ and maximum saturated drain currents above 1 A mm⁻¹. A thinner GaN channel layer improves 2DEG confinement because of the enhanced effectiveness of the AlN nucleation layer acting as a back-barrier. An excellent drain-induced barrier lowering of only 20 mV V⁻¹ at a V_DS of 25 V and an outstanding critical electric field of 0.95 MV cm⁻¹ are demonstrated. Good large-signal performance at 28 GHz with output power levels of 2.0 and 3.2 W mm⁻¹ and associated power-added efficiencies of 56% and 40% are obtained at a V_DS of 15 and 25 V, respectively. These results demonstrate the potential of sub-100 nm gate length HEMTs on a buffer-free GaN-on-SiC heterostructure.