by doping to provide the required conductance between the metallisation layers 114 12 of the secondary frame and the source respectively the construction in cross section through b b1 and c c1 is shown in fig5 and 6 respectively referring now to fig7 there is shown an assembly of groups of igbt cells arranged on a disc comprising a single wafer of silicon using the same enumeration as the preceding figures where appropriate each group of cells fills a trapezoidal secondary gate frame 114 with the length of window strips aligned to the parallel sides of the trapezium and suitably graded in length fig8 is a cross section through a a1 in fig7 i e along the mid line between two strip windows and fig9 is a cross section through b b1 in fig7 i e perpendicular to the line of the cells and extends radially outwards through the edge of the silicon disc 1 to illustrate how the voltage blocking junction s may be terminated the arrangement of the trapezoidal groups of cells enables efficient use to be made of the silicon disc 1 the primary gate frame 116 connecting all the secondary frames 114 via their removable links 115 is conveniently unified at the centre of the disc to form a gate contact area 116a conventionally a wire bond or bonds would be made in this area but it is more convenient to use a resilient contact connection to all necessary parts of the source layers 12 is made by pressing a large area.