The invention provides a process for the production of flexible to semi-rigid polyurethane shaped articles with a compacted edge zone of given hardness and a significantly more flexible cell core, so-called integral foams, in which non-combustible mixtures of fluoroalkanes which comprise 1,1,1,3,3-pentafluorobutane are employed as physical blowing agents. By the formation of the impurity diffusion region, the depletion layer is extended toward the base region so as to improve the breakdown voltage. In a second state, the potential supply circuit responds to a second control signal at a second potential level, and supplies the first and second reference cells respectively with potentials corresponding to first data via the first signal line and first data via the second signal line. Subsequent etching steps form corresponding trenches in the underlying substrate. The decision feedback equalizer serves to cancel both inter-codeword interference or bleed-over between codewords, and intra-codeword chip interference or smearing of the energy within the chips of a respective codeword.