We report on the gating in GaAs-(Al,Ga)As heterostructures using the electrical double layer of ionic liquids. We employ tetrabutylammonium bis(trifluoromethylsulfonyl)imide for the ionic liquid, which has a melting point of 92 • C. As the 'ionic liquid' is solid at room temperature, the device operation is more stable compared to when an aqueous electrolyte is used. Hysteretic behavior appeared in the variation of the resistance when the negative gate bias exceeded ∼ −3 V, which is considered to be the electrochemical window of the ionic liquid. Temporary gate degradation occurred when the positive gate bias was larger than 8 V. We attribute these characteristics to the generation of space charge and its slow dissipation in the ionic liquid. The gating ceases to function at low temperatures with a transition range of 100-200 K due to the freezing of ions.