A method of preparing semiconductor p-n-p-n switching devices

Four-layer p-n-p-n semiconductor structures have been prepared in germanium by combining rate-growth and alloying mechanisms. Single crystals of germanium are grown containing both donor and acceptor impurities, with the former in excess. Small filaments are cut from the crystals and one end of each filament is melted back into the form of a globule, using a specially designed furnace. By suitably controlling the rates of cooling in the furnace, the segregation constants of the impurities can be utilized to cause an n-p-n structure to form. A further acceptor impurity is added to the resolidified end of the filament, and the end is then remelted under slightly different conditions. Owing to the overall predominance of the acceptor impurity, a further p-type region is formed. All four layers are of good crystal structure and the three junctions have the same geometrical shape. The resulting elements are mounted as two-terminal devices and operate as switches which trigger into the `on¿ condition when the applied voltage exceeds a certain critical value.