In a method of treating a semiconductor element which at least includes a semiconductor, a threshold voltage of the semiconductor element is changed by irradiating the semiconductor with light with a wavelength longer than an absorption edge wavelength of the semiconductor. The areal density of in-gap states in the semiconductor is 1013 cm−2eV−1 or less. The band gap may be 2 eV or greater. The semiconductor may include at least one selected from the group consisting of In, Ga, Zn and Sn. The semiconductor may be one selected from the group consisting of amorphous In—Ga—Zn—O (IGZO), amorphous In—Zn—O (IZO) and amorphous Zn—Sn—O (ZTO). The light irradiation may induce the threshold voltage shift in the semiconductor element, the shift being of the opposite sign to the threshold voltage shift caused by manufacturing process history, time-dependent change, electrical stress or thermal stress.