20 30 and others not shown are piled and can be soldered together using a solder preform 40 disposed there between a light beam 41 42 incident on the periphery of the piled thyristors is reflected by the mirror formed by metallization 29 of notch 28 toward the exposed junction area of the adjacent lower thyristor the metallizations that are conventionally used to form anode and or cathode metallizations generally constitute suitable reflectors for example metallizations formed from aluminum gold titanium and nickel whether alone or possibly sandwiched in alloys with silicon etc can be used preferably notch 28 38 is concave the curvature of the notch is preferably selected so that the light beam is not only reflected toward the lower surface but focussed on the photosensitive area as well in the field of semiconductors where single elements are formed from a large surface silicon wafer which is sawn off into single elements concave notches can be conventionally formed as follows prior to sawing the areas corresponding to the thyristor limits grooves which will subsequently constitute the notches are chemically etched if the notches are sufficiently wide and deep metallization 29 39 contacts both the p type layers 24 34 and substrates 21 31 respectively in practice this.