PURPOSE: To lower the dielectric resistance of a semiconductor element once and elevate it again, to set Zener voltage at desired one and to improve the yield of products and reduce cost by elevating the temperature of heat treatment even when Zener voltage exceeds a desired value on the formation of a P-N junction.

CONSTITUTION: A polysilicon layer 5, in which boron is doped previously in high concentration, is laminated and formed on a substrate 3 and an oxide film 4 through a window hole 4a. When the substrate is thermally treated at a temperature of T1, boron is diffused, and boron concentration in a P-N junction section lowers and Zener voltage Vz rises. When the substrate is thermally treated at a temperature of T2 higher than T1, the degree of solid solution of boron in an impurity diffusion region 6 increases, boron in quantity more than that on heat treatment at T1 intrudes into the impurity diffusion region 6, boron concentration in the P-N junction section increases, and Zener voltage Vz suddenly drops. Boron further continues to deeply diffuse through heat treatment at the temperature T2, and Zener voltage Vz rises again. Desired Zener voltage Vz0 is obtained by utilizing the re-rise.