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Silicon crystals with phosphorus concentrations up to 1.5Ã1019 atoms per cubic centimetre have been prepared by the Czochralski technique. The phosphorus was added either in the form of ammonium orthophosphate or by means of a silicon-phosphorus alloy containing approximately 1Â¿% phosphorus. It is shown that the use of such an alloy as the doping agent results in improved control of the phosphorus concentration compared with the results obtained with ammonium orthophosphate. Aluminium-wire-type alloyed diodes were prepared using silicon with phosphorus concentrations from 2Ã1016 to 1.5Ã1019 atoms per cubic centimetre. The breakdown voltage of these diodes was found to be dependent on the phosphorus concentration up to 3Ã1018 atoms per cubic centimetre. At concentrations greater than this, the breakdown voltage was found to be sensitive to the alloying conditions. The mechanism of the formation of the p-n junction in silicon with phosphorus concentrations greater than 3Ã1018 atoms per cubic centimetre is discussed.