The aim of the research project presented in this thesis is to investigate the effects of electrostatic gating on the magnetic properties of carrier-mediated ferromagnetic Ga1-xMnxAs diluted magnetic semiconductors. (Ga,Mn)As can be regarded as a prototype material because of its strong spin-orbit coupling and its crystalline properties which can be described within a simple band structure model. Compressively strained (Ga,Mn)As epilayer with more complex in-plane competing cubic and uniaxial magnetic anisotropies is of particular interest since a small variation of these competing anisotropy fields provide a means for the manipulation of its magnetization via external electric field. An all-semiconductor epitaxial p-n junction field-effect transistor (FET) based on low-doped Ga0.975Mn0.025As was fabricated. It has an in-built n-GaAs back-gate, which, in addition to being a normal gate, enhances the gating effects, especially in the depletion of the epilayer, by decreasing the effective channel thickness by means of a depletion region. A shift in the Curie temperature of ~2 K and enhanced anisotropic magnetoresistance (AMR) (which at saturation reaches ~30%) is achieved with a depletion of a few volts. Persistent magnetization switchings with short electric field pulses are also observed. The magnitude of the switching field is found to decrease with increasing depletion of the (Ga,Mn)As layer. By employing the k . p semiconductor theory approach (performed by our collaborators in Institute of Physics, ASCR, Prague), including strong spin-orbit coupling effects in the host semiconductor valence band, a change in sign of Kc at hole density of approximately 1.5x1020 cm-3 is observed. Below this density, the [110]/[1⁻10] magnetization directions are favoured, consistent with experimental data. A double-gated FET, with an ionic-gel top-gate coupled with a p-n junction back-gate based on the same material, was also employed in an attempt to achieve larger effects through gating. It reaffirms the results obtained and demonstrates enhanced gating effects on the magnetic properties of (Ga,Mn)As.