nitride also after completion of a layer different from a silicon oxide layer the surface of the layer may be oxidized to form a surface of silicon oxide contrary to existing technique the cvd according to the invention is carried out at relatively low pressure about 300 u00b0 c since the bottom of a trench is likely to produce lattice defects therein at a high temperature the layer thus formed however has a very fine structure comparable with a layer conventionally formed at higher than 1000 u00b0 c referring to fig4 a to 4 c a third embodiment is shown the embodiment includes an extrinsic semiconductor the process is substantially identical to the preceding embodiment so that redundant descriptions will not be repeated on a silicon semiconductor are silicon oxide or silicon nitride layer 4 a polysilicon or amorphous silicon layer which is doped with phosphorus as an impurity or metalic conductive layer 5 such as of titanium chloride or tungsten and a silicon oxide layer 6 in this configuration a capacitance is constituted between the the semiconductor substrate 1 and the conductive layer 6 a fourth embodiment of the invention is shown in fig5 a to 5 c the experiment was made to form a semiconductor device in which are digged a cave in a trench to increase the capacitance.