PURPOSE: To accurately form two P-N junctions in a one conductivity type semiconductor substrate in less number of steps by selectively opening a window in a polysilicon layer on the substrate, then overetching for a predetermined time, and then diffusing an opposite conductivity type impurity to the substrate and one conductivity type impurity in the substrate exposed from the window.

CONSTITUTION: A polysilicon layer 15 is formed in a thickness of approx. 6,000 on one conductivity type such as an N-type semiconductor substrate 14, a photoresist film 16 is formed on the layer 15, a window is selectively opened in the film 16 to form a window 17 in the film 16. Then, the layer 15 exposed from the window 17 of the film 16 is etched by an anisotropic etching method. After the plasma etching of the layer 15 is advanced so that so-called a just etching is finished, the plasma etching is extended for a predetermined time and continued. The lower portion of the inner wall of the window 18 formed in the layer 15 is overetched in a tapered shape.