Complex impedance patterns of aluminum nitride (AlN) doped with various ions of different ionic sizes and valences have been examined at temperatures between 400° and 950°C. The patterns showed distinct differences between samples doped with ions of different sizes. Grain and grain boundary contributions to the resistance and capacitance were compared. It was found that the grain boundary: grain resistance ratio increases with density. Samples doped with large cations have higher effective dielectric constants at the grain boundary than samples doped with small cations or undoped samples, indicating a thinner grain‐boundary la