A type of transistor made of three layers of semiconductors. Each layer has been treated so that the layer in the middle (called the base) has an inherent electric charge, while the layers around it (the emitter and collector) have an inherent opposite charge. A bipolar transistor with a negative base is designated PNP, and one with a positive base is designated NPN. When subjected to current flow, the base acts like a gate, enhancing or inhibiting the current flow from the emitter to the collector. Bipolar transistors are used primarily to amplify or switch current signals, and are common in audio equipment. Compare field effect transistor.