The ability to externally control the properties of magnetic materials would be highly desirable both from fundamental and technological point of views. In this respect, dilute magnetic semiconductor (DMS), in which a fraction of atoms of the nonmagnetic semiconductor host is replaced by magnetic ions, have recently attracted broad interest for their potential application in spintronics. In this work, we focused on transition metal (TM) (Co, Mn and Cu) doped Zinc oxide (ZnO) because room temperature ferromagnetism was both theoretically predicted and experimentally observed. However, the origin of such ferromagnetism, in particular whether it is a signature of a true DMS behaviour (long range magnetic interaction between the doping ions) or it arises from the formation of secondary phases, segregation or clustering is still under debate. Measuring the dependence of the magnetic properties on the carrier concentration can clarify the underlying physics. The samples were characterized by resistivity, Hall effect, magnetoresistance, Seebeck effect, synchrotron X-ray adsorption spectra (XAS) and magnetic dichroism (XMD) while modulating the carrier density by electric field. The insulating-gate field-effect transistor structures are realized in ZnO/Strontium Titanate (SrTiO3) heterostructures by pulsed laser deposition. These devices offers the capability to modulate the carrier density of a probe accessible (light, AFM tip, ...) channel, by more than 5 orders of magnitude (from ≈1015 to ≈1020 e-/cm3, estimated by Hall effect measurements under FE). The Co and Mn films measured by DC SQUID magnetometer result ferromagnetic and anomalous Hall effect was observed at low temperature but nor ferromagnetic nor antiferromagnetic signal was detectable in the XMD spectra. Cu doped films are insulating and nonmagnetic. Photo Emission Electron Microscopy (x-PEEM) and magnetic force microscopy (MFM) showed that the sample are homogeneus and no clustering of TM were detected. A large effect of the magnetic ions, strongly dependent on the carrier concentration, was observed on the transport properties and this effect according can be explained by a giant s-d exchange leading to spin splitting of the s-type conduction band. Since the filling of such band can be modified by field effect a electric field control of the spin polarization can be achieved.