The measured photomultiplication and excess noise characteristics of two 4H-SiC p-i-n diodes, with i-region widths of 0.105 mum and 0.285 mum, were modelled using a nonlocal multiplication model to determine the ionization threshold energies and the impact ionization coefficients of 4H-SiC. The modelled ionization coefficients accurately predicted the breakdown voltage of a 0.485 mum p-i-n structure. The breakdown voltage of 4H-SiC calculated using these parameters is approximately 7.2 and 4.6 times higher than that of similar Si and Al0.8Ga0.2As structures respectively. Dead space, the minimum distance required for carriers to equilibrate in the electric field, has to be taken into account to accurately predict the breakdown voltage in thin devices.