The AlAs/GaAs fractional superlattice in an AlGaAs/GaAs modulation‐doped structure, which is used for quantum wire transistors and electron wave interference transistors, is investigated from the viewpoint of the electronic state. A threshold voltage (Vth) difference of 0.27 V between the quantum wire and electron wave interference transistor indicates a periodic potential amplitude of 10 meV.Vthof both these transistors and the flatband voltage (Vfb) calculated from the gate capacitance are obtained at various temperatures between 4.2 and 100 K. The density of states (DOS) is determined from the temperature dependence forVthof both transistors andVfb. A DOS below the bottom of the well in the periodic potential is one dimensional, but a DOS between the bottom of the well and the top of the barrier in the periodic potential is two dimensional.