The AC impedance spectra of pure and doped ZnO ceramics on a Nyquist plot (Z″vs Z′) interpreted using equivalent circuit model are shown in Fig. 4. It is obvious that all specimens show only one incomplete and depressed semi-circle which is derived from the grain boundary barrier, suggesting the appearance of non-Debye type relaxation resulting from the heterogeneity of the barriers of the specimens [22]. The incomplete semicircle reveals that the relaxation frequency decreases to lower frequency which could be attributed to the development of porosity (Fig. 2) in the ceramics [23], and the measurement of this low frequency was not possible in this experiments. Moreover, the resistance of grain boundary (Rgb) decreased sharply with the increase concentration of Al dopant.