Inconsistencies in the concentrations of unintentional donor impurities and free charge carriers in GaN/AlGaN layer stacks hosting a two-dimensional electron gas (2DEG) can be attributed to the measurement procedure and solely depend on the way in which the free charge carrier concentration is extracted. Particularly, when the 2DEG acts as the bottom electrode in capacitance versus voltage measurements, unphysically low concentrations of free charges are calculated. This originates from the depletion of the 2DEG and the accompanying disappearance of the bottom electrode. It is shown that, for the case of a defined (non-vanishing) bottom electrode, the levels of donor impurities and resulting free charges consistently match.