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