The interface-alloy technique has been used to produce heterojunctions between GaAs and InSb. X-ray and Kossel line patterns show that, despite the relatively large 14 percent lattice mismatch between the semiconductors, these heterojunctions are single-crystal. Photocurrent and I–V measurements are explained by a model for the heterojunction band structure in which the salient feature is a region of the order of 60 Å long which has a linearly-graded energy gap joining the GaAs to the InSb. In addition, interface states ‘fix’ the location of the GaAs bands at the heterojunction interface such that, at room temperature, the conduction band extrapolates to a value approximately 0.93 eV above the Fermi level. The photocurrent occurs via hot carriers generated in the graded-gap region which traverse this region (with a mean free path of approx. 20 Å) to the heterojunction barrier maximum. As predicted by the model, incident monochromatic radiation of energy smaller than the GaAs bandgap produces a photocurrent which varies exponentially with photon energy as I 0 exp[ C( hν − E g )], where C is a positive parameter which decreases for increasing reverse bias on the heterojunction, and I 0 is the extrapolated response at the GaAs bandgap, which is independent of applied bias. The forward current of units fabricated with n-type GaAs varies as exp( qV ηkT ) , and, except at lower temperatures where tunneling becomes important, the values of η as a function of the GaAs impurity doping concentration can be explained in terms of the increase in the heterojunction barrier height with voltage, since part of the depletion layer potential is across the graded-gap region. Similarly, the voltage dependence of the reverse current is quantitatively explained by this model. The results of the C–V measurements are consistent with the I–V and photocurrent measurements evaluated in terms of the graded-gap heterojunction model.