Molecular layer epitaxy (MLE) of GaAs related compounds and Si with SiO2 deposition has been developed to realize the tera-hertz operating devices. At sufficiently low process temperature, lower than in the conventional growth method, device quality epitaxial layers were achieved by MLE. In GaAs, 100 Angstrom scale static induction transistors are fabricated with MLE. These transistors operated in a mixed ballistic-tunneling mode or in the pure tunneling mode. Basic research on MLE in the field of surface science and material science is performed, from the point of view of device application of this technology.