A photoconducting sampling gate which is triggered by a picosecond optical pulse and has an aperture of approximately 12 ps has been used to measure accurately the response of a high‐speed, solid‐state photodiode. The sample gate, which is made from a radiation‐damaged semiconductor, is demonstrated to have better speed, lower noise level, negligible jitter, and fewer reflections than conventional sampling oscilloscopes. In addition, it can be used over a wide temperature range by direct mounting in a variable temperature cryostat.