Differences in detector response between measured small fields, f_clin, and wider reference fields, f_msr, can be overcome by using correction factors k^{fclin, fmsr} _Qclin,Qmsr or by designing detectors with field-size invariant responses. The changing response in small fields is caused by perturbations of the electron fluence within the detector sensitive volume. For solid-state detectors, it has recently been suggested that these perturbations might be caused by the non-water-equivalent effective atomic numbers Z of detector materials, rather than by their non-water-like densities. Using the EGSnrc Monte Carlo code we have analyzed the response of a PTW 60017 diode detector in a 6 MV beam, calculating the k^{0.5,4 cm}_{Q0.5,4 cm} correction factor from computed doses absorbed by water and by the detector sensitive volume in 0.5 × 0.5 and 4 × 4 cm² fields. In addition to the ‘real’ detector, fully modelled according to the manufacturer’s blue-prints, we calculated doses and k0.5,4 cm Q0.5,4 cm factors for a ‘Z → water’ detector variant in which mass stopping-powers and microscopic interaction coefficients were set to those of water while preserving real material densities, and for a ‘density → 1’ variant in which densities were set to 1 g cm⁻³, leaving mass stopping-powers and interaction coefficients at real levels. k^{0.5,4 cm}_{Q0.5,4 cm} equalled 0.910 ± 0.005 (2 standard deviations) for the real detector, was insignificantly different at 0.912 ± 0.005 for the ‘Z → H₂O’ variant, but equalled 1.012 ± 0.006 for the ‘density → 1’ variant. For the 60017 diode in a 6 MV beam, then, k^{0.5,4 cm}_{Q0.5,4 cm} was determined primarily by the detector’s density rather than its atomic composition. Further calculations showed this remained the case in a 15 MV beam. Interestingly, the sensitive volume electron fluence was perturbed more by detector atomic composition than by density; however, the density-dependent perturbation varied with field-size, whereas the Z-dependent perturbation was relatively constant, little affecting k^{0.5,4 cm} _{Q0.5,4 cm}.