nd wherein theforming is by CVD with a gas comprising Si.sub.x L.sub.2x, L is an amino group, X is 1 or 2, the gate layer comprises a P.sup.+ region and an N.sup.+ region, and the P.sup.+ and N.sup.+ regions are separated by a region which is on an isolation region ofthe substrate having a width of at most 0.4 microns.2.