Measurements of the sensitivity of grain‐boundary photodetectors indicate an expected minimum detectable power of better than 10−14w. Major factors governing the sensitivity are the minority carrier lifetime and the distance of the boundary from the irradiated surface. Spectral measurements show an added absorption edge response at about 1.67 &mgr; and 1.72 &mgr;. This added response could be caused by deep‐lying acceptor levels or by excitons with a long lifetime. However, the experimental evidence points to direct absorption at the grain boundary as the most likely reason for the absorption edge irregularities.