High frequency characterization of gate resistance in RF MOSFETs

The gate resistances (R/sub g/) of MOSEETs with various geometries have been characterized at various bias conditions at high frequency (HF). The results show that R/sub g/ decreases when either channel length (L/sub f/) or per-finger-width (W/sub f/) increases before reaching a critical L/sub f/ or W/sub f/, and then starts to increase as L/sub f/ or W/sub f/ continues to increase. The irregular geometry dependence of R/sub g/ is caused by the combined distributed effects in both the gate and channel at HF. Stronger contribution from the distributed channel to the effective R/sub g/ is observed in the saturation region of devices with longer channel length (L/sub f/) at lower gate bias (V/sub gs/). The results show that an optimized design of the per-finger-width is necessary for an rf MOSFET to achieve the lowest effective R/sub g/, which is desirable in rf applications.