The edge positions of the valence band (VB) and conduction band (CB) of semiconductors at the point of zero charge can be calculated with the following equations :
EVB = X–Ee + 0.5 Eg
ECB = EVB–Eg
EVB and ECB are the edge potentials of VB and CB, respectively. X is the absolute electronegativity of the semiconductor and the value for SnO2 is 6.24 eV, Ee is the energy of the free electrons on hydrogen scale (~ 4.5 eV).
After the calculations I found the values of ECB with the negative sign !.
Please, what's the physical signfication of negative sign ?.