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A survey evaluation of the quality of commercial silicon used in the preparation of p-n junction devices has been made, with emphasis on the detection and characterization of micro-inhomogeneities. The examination techniques included transmission electron microscopy, special etching techniques with optical microscopy, and microresistivity determination. These investigations have shown that almost all silicon produced by current commercial techniques has an appreciable concentration of microdefects, including precipitates and microcrystallographic defects, which are too small to be detected by conventional techniques. Furthermore the impurity distribution is inhomogeneous on a microscopic scale. These microinhomogeneities are frequently present in sufficient concentration to be more influential in determining the quality of the finished device than the material properties usually specified. Some suggestions are made concerning the establishment of standards of acceptability for the major types of microdefects and for developing simple, rapid evaluation procedures.