The backgating (sidegating) effect in III-V MESFET's devices is analyzed through the modelisation of a Metal (Schottky barrier)-N (channel)-SI (Semi Insulating)-N+ (back-gate contact) structure. Numerical and analytical results, using the drift-diffusion charge transport model, show that along the applied voltage range associated with backgating: (i) quasi space charge neutrality across most of the bulk SI layer and (ii) quasi Boltzmann equilibrium for the free electron across the reverse biased N (channel)-SI contact prevail for GaAs (SI) or InP (SI). The circumstances under which a negative bias applied on the backgate (N+) contact will either develop across the reverse biased N-SI contact (strong backgating) or across the SI layer (negligible backgating) are described by means of a simple analytical relation as a function of the deep level parameters values. The electric field dependence of the carrier mobility (Gunn effect) produces a backgating effect with a threshold voltage. The presence of a low lifetime, buffer layer, at the N-SI interface is shown to strongly reduce it. (C) 2013 Elsevier Ltd. All rights reserved.