[en] Amorphous silicon-zinc-tin-oxide (a-SZTO) has been fabricated by radio frequency (RF) magnetron sputtering at room temperature. Direct investigation on the change of the band gap of a-SZTO depending on Si contents was performed simply by combining the results of x-ray photoelectron spectroscopy (XPS), Kelvin probe microscopy, and UV-VIS optical spectrometer. The chemical properties, such as metal cation binding energy and oxygen vacancies related O 1s peak were measured by using XPS. We report on the electrical performance and the thermal stability of SZTO TFTs related with energy band gap analysis depending on Si contents. The activation energy and the density of states were calculated by applying temperature stress (TS). As Si contents increased, we have observed E_C-E_F was increased. The relative position of the E_C shifts far from the E_F because the shallow and/or deep states, such as ionized oxygen vacancies were decreased. It was confirmed that sample 3 with high Si content had the best stability (ΔV_TH of 0.21 V) on thermal stress. This investigation on the variation of energy band gap and the stability of a-SZTO has represented possible step towards not only the control of band gap but also controllable achievement of electrical property and stability using optimized Si contents. - Highlights: • Direct Investigation on the change of the band gap of a-SZTO thin film depending on Si contents. • As Si contents increased, oxygen vacancies were decreased and enhanced thermal stability. • Activation energy falling rates were increased depending on Si contents. • Density of states were decreased depending on Si contents.