PICT:0999296/C1/1 Doped semi-conductor material is prepared by deposition on to the same semi-conductor material from a mixture of a gaseous compound of the semi-conductor, the dopant and a carrier gas, part of the stream of carrier gas being diverted before mixing with the semi-conductor compound, to pass over the doping material in solid form. In the Figure, part of the hydrogen stream from source 9 is passed over solid phosphorus pentachloride in chamber 10 maintained at a constant temperature by bath 11. Another part of the hydrogen stream is passed through a thermally decomposable source 6 of silicon such as silico-chloroform, and the two streams are reassociated and passed into reaction chamber 20 which contains electrically heated silicon rods 24 upon which silicon containing phosphorus is deposited. By control of rates of flow and the temperature of the solid deposit source, the impurity concentration can be accurately controlled. Antimony trichloride, arsenic pentachloride, decaborane are described as alternative sources of dopant.
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