This work explores the effect of a constant external electric field on electron transmission through a thickness-symmetrical quantum well (d1 = d2) consisting of a single cell containing two semiconductor materials—one forming the well (ZnO) and the other forming the barrier (Zn1-xMgxO). In this work we have used the transfer matrix and Airy functions and their derivatives to obtain the transmission coefficient T as a function of incident energy and applied voltage Va. Our objective is to study the effect of the electric field on the electronic proper states (resonance states) and the transmission of electrons through our structure. Our results show that the electric field induces changes in the conduction bands of the materials that make up our system, where they are tilted relative to the absence of field. Similarly, we're seeing changes in the optical and electronic properties of the system under study, as we've discovered that the electric field applied to the structure shifts the system's electronic proper states to lower energies thanks to the Stark effect, which plays an important role in shifting electronic states. Moreover, we find that for high values of Mg thickness and concentration, the electric field has a remarkable effect on the system's eigenstates, compared with low thicknesses and concentrations.