Local dC/dV spectroscopy performed in a scanning capacitance microscope is used to map. quantitatively and with high spatial resolution, lateral variations in the threshold voltage of an AlxGa1-xN/GaN heterostructure field-effect transistor epitaxial layer structure. Theoretical analysis and numerical simulations are used to quantify charge concentrations, the corresponding threshold voltage shifts, and the influence of the measurement apparatus on these results. High-resolution scanning capacitance and the associated threshold voltage images reveal round features < 150 nm in diameter within which a shift in threshold voltage of about 1.5-2 V is measured. Theoretical analysis and numerical simulations indicate that these features are consistent with the presence of charged threading dislocations with a linear charge density of similar to 10(7) e/cm(-1) that cause localized partial or full depletion of carriers from the two-dimensional electron gas. Large-scale scanning capacitance images reveal variations in contrast over areas several microns in size with corresponding threshold voltage shifts of approximately I V. These large features are postulated to arise from a combination of thickness and composition variations in the AlxGa1-xN layer.