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The density and kind of charge carriers in semiconductors are controlled by the density and nature of added impurities. The charge carriers (electrons and holes) move about in the crystal simultaneously by diffusion and drift. Rectification occurs at p-n boundaries because of the difference in the carrier types on either side. Junction transistors, capable of amplification, consist of two junctions placed close to each other with a thin middle region or base layer. As a switch, the junction transistor is capable of switching "on" to voltage of the order of 10 millivolts and slope resistances of a few ohms. In transmission applications experimental junction transistors have been built to give useful power output at 250 mc and with maximum frequency of oscillation in the range from 500 to 1,000 mc. Reliability experience to date has been encouraging; some systems are operating with replacement rate as low as one per 150,000 transistor hours.