Photocurrent measurements using sub‐band‐gap light on low dopedn‐type gallium arsenide are described. Measurements of photocurrentIas a function of depletion widthWand wavelength &lgr; were made to determine minority carrier diffusion lengthsLof order 1 &mgr;m. For long wavelengths (&lgr;≳0.879 &mgr;m)I(W) plots could be used to measureLfor carrier concentrationsn?2×1015cm−3. For larger values ofn, whenW≳L, the method was inapplicable owing to nonlinearities in the plots. The mean change in absorption coefficient, &Dgr;&agr;, at &lgr; = 0.8907 &mgr;m was determined from the plots as a function of electric field and was in qualitative agreement with calculations of the expected variation due to the Franz–Keldysch effect. It is shown that the measurements ofI(&lgr;) may be used to determineLforn<5×1015cm−3provided accurate absorption coefficient data are available. A correction for the Franz–Keldysch effect is necessary for 2×1015<n<5×1015cm−3. For 5×1015<n<1016cm−3the method is inapplicable, whenW≳L, owing to nonlinearities caused by the Franz–Keldysch effect.