Six GaN-based test structures were grown on sapphire to assess the effectiveness of scanning capacitance microscopy (SCM) in measuring n-type carrier concentrations in GaN. These structures were designed to test the carrier concentration and spatial detection limits of the technique using data obtained from secondary ion mass spectroscopy for comparison. It has been found that, where nonintentionally doped (nid) spacers are present between doped layers, SCM reliably and repeatably distinguished between different carrier concentrations for levels >= 2 x 10(17) cm(-3) and detected layers with thicknesses >= 25 nm. Where no nid spacers exist in samples, the spatial resolution improves to 10 nm. However, the exact quantitative response of SCM is seen to be highly dependent on the particular probe used, necessitating the use of calibration standards for the quantification of unknown samples. As an example of the application of such a calibration standard, we attempt to quantify the carrier concentration in the conductive layer found adjacent to the GaN/sapphire interface in nid samples. (C) 2008 American Vacuum Society.