High performance diamond MISFETs using CaF₂ gate insulator

A cut-off frequency of 15 GHz and a maximum frequency of oscillation of 20 GHz are realized in a 0.4-μm gate diamond metal-insulator-semiconductor field-effect transistor (MISFET). The cut-off frequency is the highest value for diamond FETs ever reported. The RF characteristics of the MISFETs are higher than those of metal-semiconductor FETs at the same gate lengths. The CaF₂ gate insulator improves the carrier mobility according to the Hall measurement system. The mobility increases in the surface conductive layer result in high RF performance. The source-gate passivation of CaF₂ results in the high DC transconductance because of the reduction of series resistances. A cut-off frequency of more than 30 GHz is expected with the gate minimization and the CaF₂ passivation of source-gate and gate-drain spacings.