1,071,632. Semi-conductor controlled rectifiers. SIEMENS-SCHUCKERTWERKE A.G. Oct. 21, 1964 [Oct. 26, 1963], No. 43016/64. Heading H1K. A four-layer semi-conductor controlled rectifier has one of its terminal layers embedded in that intermediate layer of opposite conductivity type which bears the control electrode. The concentration of uncompensated impurity centres in this intermediate layer decreases from its outer surface in a direction towards its junction with the other intermediate layer. The rectifier is so constructed that the concentration of uncompensated impurity centres in those parts of the intermediate layer immediately adjacent the embedded terminal layer is not less than 5 x 10<SP>16</SP> centres, cm.<SP>-3</SP> and that every current path between the control electrode and the electrode on the embedded terminal layer passes through a part of the intermediate layer in which part the concentration of uncompensated impurity centres is not greater than 10<SP>17</SP> centres, cm.<SP>-3</SP>. In the embodiment described an N-type silicon wafer is subjected to overall diffusion of gallium and aluminium to form a 70 Á P-type surface layer. An annular gold/ antimony electrode 17 is alloyed to this surface layer to produce a 23 Á N-type embedded zone 15. The doping level at the surface of the diffused surface layer is about 3 x 10<SP>18</SP> centres. cm.<SP>-3</SP> and is about 4 x 10<SP>17</SP> centres, cm.-<SP>3</SP> at the level of the PN junction formed by the embedded layer. A gold-boron is alloyed within the annular electrode to form the control electrode 18 and a similar foil alloyed to the other major face of the wafer to form the third electrode 16. The edge portions of the wafer are mechanically removed (e.g. by sand blasting) to form the four layer structure. That part of the diffused surface layer between the annular and disc electrodes 17, 18 is removed by sand blasting to a depth of 30-40 Áat which depth the doping level is about 10<SP>15</SP> centres. cm.-<SP>3</SP>. Thus every current path between electrodes 17 and 18 must pass through a region in which the doping level is no higher than about 10<SP>15</SP> centres, cm.<SP>-3</SP>. The surface of the recess formed is given a light after-etch to remove surface disturbances of the crystal lattice. In a variant, the surface conductivity of the intermediate layer may be substantially metallic.