Comprehensive TCAD Analysis of Threshold Voltage on GaN-on-Si MOS-Channel Fully Recessed Gate HEMTs

Unexpected threshold voltage ( ${V}_{\text {TH}}$ ) behaviors are experimentally observed in gallium nitride (GaN)-on-Si metal-oxide-semiconductor-channel high electron mobility transistors (MOSc-HEMTs) with fully recessed gate and back-barrier (BB): 1) ${V}_{\text {TH}}$ increases with decreasing gate length ( ${L}_{G}$ ) ( ${V}_{\text {TH}}$ roll-up); 2) discrepancies between ${V}_{\text {TH}}$ values extracted from drain current characteristics ${I}_{\text {D}}$ ( ${V}_{\text {G}}$ ) ( ${V}_{\text {TH}\_\text{IV}}$ ) and from gate-to-channel capacitance characteristics ${C}_{\text {GC}}{(}{V}_{\text {G}}$ ) ( ${V}_{\text {TH}\_\text{CV}}$ ); and 3) significant dependence of ${V}_{\text {TH}\_\text{CV}}$ with frequency. Using TCAD simulations and experimental measurements, it is demonstrated that conduction band confinement, especially at gate corners, is responsible for these peculiar ${V}_{\text {TH}}$ behaviors. This band confinement is strengthened by the fully recessed gate configuration coupled with the proximity of a BB. It depends on recessed gate topology and BB efficiency.