The surface potential of GaN:Si, of fundamental interest for knowledge of the electrostatic potential and electric field strength, is determined for Si doping in the device relevant range from 6x10(17) cm(-3) to 2.3x10(19) cm(-3), in layers grown by low-pressure metal-organic vapor-phase epitaxy. Comparing the Si doping concentration, measured by secondary ion mass spectrometry, to the free electron concentration, measured by Hall effect, shows that all Si doping atoms are activated. We used the sheet resistance of the samples with different thicknesses measured by eddy current, a nondestructive, contactless method, to determine the depleted region. From the width of the depletion layer, which is dependent on the doping concentration, we obtained the GaN:Si surface potential on the basis of the depletion approximation. The surface potential decreases with increasing carrier concentration from about 1.5 eV to 0.2 eV. A qualitative explanation based on the theoretical description of two surface states, an occupied surface state at 1.7 eV and an empty state 0.6 eV below the conduction band edge, is given.