Solid-state detectors offer high sensitivity, stability and resolution and are frequently the dosimeter of choice for on-line dosimetry and small field therapies such as stereotactic radiosurgery. The departure from tissue equivalence of many solid-state devices, including diodes and MOSFETs, has to be carefully considered at lower energies and for Compton scattered radiation where the strongly Z-dependent photoelectric effect is significant. A modification of Burlin cavity theory is proposed that treats primary and scatter photon spectra separately and this has been applied to determine the correction factors for diode detector measurements of 6 and 15 MV linear accelerator beams. Uncorrected, an unshielded diode overestimates the dose at depth by as much as 15% for the 6 MV beam. The model predicts the effect to within 1% for both energies offering a basis for the correction of diodes for use in routine dosimetry.