Implications of gate design on RF performance of sub-100 nm strained-Si/SiGe nMODFETs

The effects of gate structure design on RF performance of strained-Si/SiGe nMODFETs are studied using device simulation and experiments. It is found that while gate resistance only affects fmax, fringing gate capacitance can have a significant impact on both fT and fmax, indicating that the physical gate structure has to be optimized for any specific application. The experiments suggest that low-ic material is needed as sidewall spacer (if any) and passivation for reducing fringing gate capacitance. Furthermore, the simulations show that if low gate resistance can be achieved by using a multi-finger geometry, a rectangular-shaped gate should be used in order to reduce fringing gate capacitance. If not, a T-gate should be used to reduce gate resistance for high fmax.