On the semiconductor monocrystal (mainly n-type silicon wafers), the diffusion P-type impurity, the shielding effect of the silicon oxide film on the surface of silicon wafer, the PN Junction formed on the N-type Si monocrystal only selectively diffusion part. Therefore, there is no need to adjust the corrosion of the PN Junction area of drugs. Because the semiconductor surface was made flat, hence the name. Also, the PN binding surface, because of the oxidation film covering, so the public is considered to be a good stability and long life type. Initially, for the used semiconductor materials are formed by epitaxial method, so the planar type is called the epitaxial planar type. In the case of planar diodes, it seems that the model used for large current rectifier is very few, and the small current switching model is a lot.