Simulation study on comparison between metal gate and polysilicon gate forsub-quarter-micron MOSFET's

Two-dimensional (2-D) process and device simulation is used to make investi gation into the effectiveness of the depletion-free metal gate for a sub-qu arter-micron MOSFET as compared with surface channel polysilicon gate MOSFE T's which greatly suffer from the gate depletion effect. The results reveal that the subthreshold characteristic for the metal gate NMOSFET is conside rably degraded since the depletion-free merit is covered up by an undesirab le influence of the buried channel structure, which is indispensable to obt ain an appropriate threshold voltage for the midgap gate. Consequently, the drivability of the metal gate MOSFET is comparable to that of the heavily doped polysilicon gate MOSFET under commonly used conditions, and further t he metal gate structure is of disadvantage against the reduction of the sup ply voltage.