Side-gating of GaAs MESFETs on semi-insulating substrates is studied. Strong side-gating took place for negative side-gate potential at side-gate breakdown voltage V//T. Side-gating involved white light emission from source edge, long-range nature, light sensitivity, sensitivity to surface passivation conditions and linear dependence of V//T on side-gate separation. These observations cannot be explained by the previous SCLC model. A new model is proposed which includes avalanche injection due to field concentration by surface state filling.