Radiation damage caused by ion implantation is used to control the carrier lifetime in silicon‐on‐sapphire (SOS) films. Photoconductivity measurements show the relaxation time changes by several orders of magnitude and can be as short as 8 ps. The carrier mobility is found to be at least an order of magnitude higher than amorphous silicon materials with similar relaxation times. A photodetector is described that demonstrates the high‐speed capability of these high‐defect‐density films.