integral with the surface of the p type strips 13 the lateral faces of rails 15 are covered with an insulating layer 19 formed from silica in the present embodiment as will be gathered from the detailed description of the process hereinafter an advantage of this structure is that it can be produced by successive self alignment methods which obviates any problem of mask repositioning the n layer 14 having a very limited diffusion depth ensures a good ohmic contact between the polycrystalline silicon rail 15 and the type n layer 11 in order to ensure a satisfactory conductivity towards the source metallization 17 fig3 to 7 relate to certain of the successive production steps of a device according to the invention in a first step on the basis of an upper type n layer 11 designated n to stress that it has a low doping level is formed a polycrystalline silicon layer 20 on which is formed a masking layer 21 in the form of close together strips e g a silica layer or a silicon nitride layer the polycrystalline silicon layer 20 is broken down into three sublayers having different doping levels lower part 22 and upper part 24 are weakly doped or undoped and the median part 23 is strongly or highly doped this is.