The power gain and noise figure of two kinds of diode structures in a 0.25-/spl mu/m CMOS process is investigated by using the two-port GSG measurement in radio-frequency region (/spl sim/GHz). The power gain is degraded by ESD device with larger layout area and more serious at higher operating frequency. The noise figure is increased by ESD device with larger layout area and also is more serious at higher frequency. After the comparison on the power gain and MM ESD level between the poly-gated diode and the STI diode, the poly-gated diode is more suitable for ESD protection in RF circuits.