Electric contact in transistors and related devices between two different types of material called math.p-type and math.n-type semiconductors. These materials are pure semiconductor materials, such as silicon, to which impurities have been added. Materials of math.p-type contain “holes” (vacancies formerly occupied by electrons) that behave like positively charged particles, whereas math.n-type materials contain free electrons. Electric current flows more easily across a math.p-math.n junction in one direction than in the other. If the positive pole of a battery is connected to the math.p-side of the junction, and the negative pole to the math.n-side, charge flows across the junction. If the battery is connected in the opposite direction, very little charge can flow. The math.p-math.n junction forms the basis for computer chips, solar cells, and other electronic devices.

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