The spectral photoresponse of Si and GaAs photodiodes near the band edge has been measured with reverse bias up to nearly 100 V at room temperature. The measurement shows that the reverse voltage V raises the photoresponse &Dgr;I, especially in the long‐wavelength region, and hence also shifts the spectrum toward longer wavelength. At a fixed wavelength, &Dgr;Iis proportional toVn, where,ndecreases as the photon energy increases. For the step‐type Si junction,nranges from ⅕ to ½ as the photon energy changes from 1.6 (&agr;−1=10 &mgr;) to 1.15 eV (&agr;−1=1000 &mgr;); this voltage dependence may mainly result from the widening of the junction depletion‐layer by the reverse bias. For GaAs photodiodes, the exponentnchanges from ⅙ to 1.3 as the photon energy varies from 1.44 (&agr;−1=1.6 &mgr;) to 1.36 eV (&agr;−1=1000 &mgr;). The strong voltage‐dependence of the photoresponse of a GaAs junction, which behaves like ap‐i‐njunction electrically, in the long wavelength region may result from the photoexcitation of electrons or holes to shallow levels in the pseudointrinsic region followed by some strongly field‐dependent ionization process, such as tunneling, etc.