It is shown that silicon doped with B, Al, Ga, P, As, or Sb is a fast, sensitive, and efficient detector of 10.6‐&mgr; radiation from a CO2laser. Experimental results are presented for several moderately compensated specimens. An investigation was made at temperatures from 23°–40°K of the electrical characteristics, time constant, noise, and 10.6‐&mgr; photoresponse. The results are interpreted in terms of idealized, non‐Ohmic impurity‐photoconductor models. Detectivities within a factor of three of ideal background‐limited performance were attained, and response times in the nanosecond region were found.