The fully recessed gate GaN-on-Si MOS-channel HEMTs (MOSc-HEMT) with Back-Barrier (BB) structure gives rise to unexpected threshold voltage (V TH ) behaviors such as V TH increase with decreasing gate length (L G ) (V TH roll-up) and discrepancies between V TH values extracted from I D (V G ) (V TH_IV ) and from C GC (V G ) (V TH_CV ) characteristics. Using TCAD simulations and experimental measurements, we demonstrate that conduction band confinement, especially at gate corners, is responsible for these peculiar V TH behaviors. This band confinement is strengthened by the fully recessed gate configuration coupled with the proximity of a back-barrier.