PURPOSE: To prevent the injection of carrier into a semiconductor substrate by forming an impurity implantation layer to be injected with the same conductive type impurity as the substrate on the substrate under a gate insulating film.

CONSTITUTION: A capacity between a gate electrode 3 and a p type semiconductor substrate 1 through a gate insulating film 2 forms a decoupling capacity. A p type impurity implantation layer 9 is formed by an ion implantation in a boundary between the substrate 1 and the film 2. A voltage higher than 10V is necessary to form a channel at this part due to a p type impurity implantation layer 9, and the channel is not formed at all with -3∼-4V of substrate bias voltage VBB to be applied between the electrode 3 and the substrate 1. Even if the voltage VBB is varied since the channel is not formed, electrons are not implanted into the substrate 1.