A photoconductive grounded coplanar waveguide (GCPW) millimeter-wave switch controlled by a single infrared light emitting diode (LED) is presented. By converting a straight GCPW transmission line into an alternating steppedimpedance line structure, the proposed device shows an on-state insertion loss of less than 2.9 dB and an off-state isolation of more than 15 dB over a wide frequency range of 12-30 GHz, with a particularly good RF performance in the K band (18-27 GHz). The required optical power and electrical power of the switch are only 199 mW and 364 mW, repectively. This device also demonstrates an absorptive characteristic with a low reflection coefficient of less than -13.5 dB within this frequency range for both the on and off states. In addition, by employing through-holes for illumination of a silicon superstrate, the proposed device can be fabricated with rapid and low-cost laser-etching and can be easily integrated with other microwave and millimeter-wave circuits.