The metal-oxide semiconductor technology has attracted much attention because it is currently used in novel applications such as flexible displays, biosensing, memories or RF tags. However, the main bottleneck for commercialization of these devices is the understanding of the Density of States (DOS) within the gap of the metal-oxide semiconductors, which is the key for the development of analytical models to enable the circuits design. Moreover, determining the DOS in semiconductors is a complex work because this requires to know the contributions of the main interfaces in electronic devices such as capacitors and field-effect transistors. In this study, the modeling of the DOS through the fitting of the electrical characteristics in field-effect devices is presented. The transfer and output characteristics are fitted in Zinc Oxide thin-film transistors (ZnO TFTs), along with the capacitance - voltage curves in Metal-Insulator-Semiconductor (MIS) capacitors using ZnO as active layer. The ZnO semiconductor devices were fabricated by Ultrasonic Spray Pyrolysis at high frequency on polyethylene terephthalate plastic substrates. We propose an accurate estimation of the DOS through the calculation of equivalent DOS in similar field-effect devices of same dielectric-semiconductor interface, since this interface plays an important role to induce the channel in TFTs and to form the accumulation and depletion regions in MIS capacitors. The results show the agreement between the experimental data and the calculated electrical characteristics of the ZnO field-effect devices. Different aspects were considered and discussed to model the interfaces such as additional contributions from fixed oxide charge, interface charge density and contact resistance.