Modified Schottky barriers of the type Au&sngbnd;CdS : Cu were prepared by diffusing Cu to a depth of 0.1–0.2 &mgr;m into single crystals of CdS, prior to evaporating rectifying Au contacts. The electronic and photoelectronic properties of these junctions are adequately described by a simple model in which the Cu acts as an ionized acceptor, resulting in a ``humped'' potential barrier between the Au and the bulk CdS. Hole trapping by the acceptors under band‐gap illumination reduces their ionization and, consequently, the hump height. The forward current density in the dark and under illumination can be accurately described by a thermionic emission model asJ=A*T2exp {—e[VB+VH(0)]/kTexp}(eV/&bgr;kT), where the hump potentialVH(0) is light sensitive and where &bgr; (1<&bgr;<1.35) arises from a weak voltage dependence of the barrier height. Barrier height determinations, in the dark and under illumination with 5000‐Å light, by means of internal photoemission, thermal activation energy, andJ‐Vmeasurements unequivocally show that under saturating light conditions the hump in the potential barrier is reduced by 0.23±0.03 V. This barrier mechanism of photoconductive response results in steady‐state electron/photon gains in excess of 106at light intensities lower that 5×1011photons cm−2sec−1, where response times exceed 1 sec.