Threshold voltage control of non-recessed GaN MOS HEMTs and recessed GaN MOS FETs by AlxGa1−xN back barrier

Threshold voltage (Vth) control of a GaN MOS transistor by AlxGa1−xN back barrier was systematically studied. Non-recessed GaN MOS HEMTs and recessed GaN MOS FETs with an AlxGa1−xN (x = 0%, 3%, 5%, 8%) back barrier layer were fabricated on the same 6 inch GaN-on-Si wafers and characterized. Al2O3 gate-insulator thickness was changed from 20 to 100 nm to evaluate the Al2O3/GaN interface fixed charge. The Vth of the GaN MOS transistor increases with increasing in Al content x of the AlxGa1−xN back barrier layer and decreasing in Al2O3 thickness. The dependences of Vth on Al content x and Al2O3 thickness were different between non-recessed and recessed transistor. The different dependences of Vth on Al content x and Al2O3 thickness were explained well by the newly introduced surface donor model, which compensate not only the surface polarization charge but also the surface electric field.