Deep level transient spectroscopy measurements were performed on three non-intentionally doped n-type ZnO samples grown by different techniques in order to investigate the electronic properties of E3 electron trap. The ionization energy and the capture cross-section are found respectively at 0.275 eV from the conduction band and 2.3×10-16 cm2 with no electric field dependence. This center is present irrespective of the synthesis method. In view of its physical properties and recent works published in the literature, its physical origin is discussed. Based mainly on its insensibility to the macroscopic electric field, the best candidates turn out to be dual defects with opposite charges on adjacent sites, like the dual vacancy VO–VZn.