Thyristor

1,240,510. Semi-conductor devices. ALLMANNA SVENSKA ELEKTRISKA A.B. 23 Dec., 1968 [28 Dec., 1967], No. 60982/68, Heading H1K. In a thyristor having four layers of alternately opposite conductivity types, the two outer, or emitter layers 2, 5, have a comparatively low resistivity and the two inner, or base layers 3, 4, have a comparatively high resistivity. The base layer 4 is much thinner in the region around the ignition electrode 10 than elsewhere, this being achieved by providing it with a depression 41 in the ignition area which is substantially circular. The emitter 5 is arranged to have an opening in this area, allowing direct contact to the base layer 4, the opening being of smaller diameter than the depression so that the emitter 5 contacts the bottom of the depression. As the distance between the emitter 5 and base 3 is small here ignition is rapid. The relatively thick base layer 4 outside the ignition region, contacted by contacts 15, 151 protects against dV/dt ignition, the capacitor 16 providing a low resistivity current path across the emitter junction at steeply increasing forward blocking voltages while electrode 10 is negatively biased. n-Type silicon is used as the basic material for the thyristor, diffusion with gallium providing the p-type regions, the nemitter layer 5 being formed by alloying a gold antimony alloy to the surface of base region 4. A further embodiment is described, Fig. 2, not shown, of a similar device operated by light, in which the ignition electrode is absent, and replaced in its function by a diode lamp. As an alternative to reducing the thickness of the base layer 4 in the ignition region, gold can be diffused into the surface of the device in such a quantity as to reduce the minority carrier lifetime everywhere except in the ignition area, a phosphorus layer having been provided here to provide a "getter" preventing the gold from having effect.