A semiconductor device having a high degree of reliability is provided. A
second object of the invention is to provide a semiconductor device of
high yield. The semiconductor includes a silicon substrate, a gate
dielectric film formed on one main surface of the silicon substrate, a
gate electrode formed by being stacked on the gate dielectric film and a
diffusion layer containing arsenic and phosphorus. Both of the
concentration of the highest concentration portion of arsenic and the
concentration of the highest concentration portion of phosphorus are each
at 10.sup.26 atoms/m.sup.3 or more and 10.sup.27 atoms/m.sup.3 or less,
and the depth of the highest concentration portion of phosphorus from the
surface of the silicon substrate is less than the depth of the highest
concentration portion of arsenic.