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|bipolar transistor |
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.
A transistor made from a sandwich of n- and p-type semiconductor material: either npn or pnp. The middle section is known as the "base" and the other two as the "collector" and "emitter". When used as an amplifying element, the base to emitter junction is in a "forward-biased" (conducting) condition, and the base to collector junction is "reverse-biased" or non-conducting. Small changes in the base to emitter current (the input signal) cause either holes (for pnp devices) or free electrons (for npn) to enter the base from the emitter. The attracting voltage of the collector causes the majority of these charges to cross into and be collected by the collector, resulting in amplification.
Contrast field effect transistor.