A light-emitting device includes an active region, an n-type region, a p-type region, an n-electrode and a p-electrode. The active region is formed from a semiconductor material. The semiconductor material has a tetrahedral structure and includes an impurity. The impurity creates at least two energy levels connected with the allowed transition within a band gap of the semiconductor material. The n-type and p-type regions in contact with the active region are disposed between the n-type and p-type regions. An excitation element is configured to inject an electron from the n-type region and inject a hole from the p-type region so as to generate an electron-hole pair in the active region. The active region has a thickness no less than an atomic distance of the semiconductor and no more than 5 nm.