We have performed an analysis of the influence of the probe tip geometry in scanning capacitance microscopy and spectroscopy measurements on an AlxGa1-xN/GaN heterostructure field-effect transistor (HFET) structure. The extremely small probe tip size (typical apex radius similar to10-30 nm) makes a detailed analysis essential in comparisons of dC/dV spectra with standard, large-area C/V measurements. Using three-dimensional simulation software, we have calculated dC/dV spectra for various tip geometries and have found that the nanoscale tip structure influences dC/dV spectra strongly, while the macroscopic shape has a much smaller influence on the dC/dV spectra. Thus, caution must be exercised in comparison of individual dC/dV spectra obtained over periods during which the probe tip geometry might have changed. We have analyzed these effects in detail and assessed their influence on the extraction of parameters such as threshold voltage, layer thickness, doping concentrations, and information about trap states in nitride HFETs and other electronic device structures.