A semiconductor device is prepared by the use of a vapor phase method and is provided with a semiconductor layer composed of boron phosphide (BP) having a band gap at room temperature of not less than 2.8 eV and not more than 3.4 eV or a boron phosphide (BP)-base mixed crystal which contains the boron phosphide (BP) and which is represented by the formula: B.sub..alpha.Al.sub..beta.Ga.sub..gamma.In.sub.1-.alpha.-.beta.-- .gamma.P.sub..delta.As.sub..epsilon.N.sub.1-.delta.-.epsilon. (0<.alpha..ltoreq.1, 0.ltoreq..beta.<1, 0.ltoreq..gamma.<1, 0<.alpha.+.beta.+.gamma..ltoreq.1, 0<.delta..ltoreq.1, 0.ltoreq..epsilon.<1, 0<.delta.+.epsilon..ltoreq.1).