Electro‐optic sampling is performed on a high‐speed silicon multiplexer integrated circuit using a gain‐switched semiconductor laser and an external probe tip fabricated from GaAs. An approximate electrostatic model is used to calculate the dependence of the electro‐optic modulation on the height of the probe tip above the circuit, the geometry of the circuit, and the dielectric constants of the probe tip material and the passivation layer (if any) on the circuit. The measured variation of the electro‐optic modulation with probe tip height is in good agreement with the prediction of the model.