<P> Small changes in the voltage applied across the base--emitter terminals cause the current between the emitter and the collector to change significantly . This effect can be used to amplify the input voltage or current . BJTs can be thought of as voltage - controlled current sources, but are more simply characterized as current - controlled current sources, or current amplifiers, due to the low impedance at the base . </P> <P> Early transistors were made from germanium but most modern BJTs are made from silicon . A significant minority are also now made from gallium arsenide, especially for very high speed applications (see HBT, below). </P> <P> NPN is one of the two types of bipolar transistors, consisting of a layer of P - doped semiconductor (the "base") between two N - doped layers . A small current entering the base is amplified to produce a large collector and emitter current . That is, when there is a positive potential difference measured from the base of an NPN transistor to its emitter (that is, when the base is high relative to the emitter), as well as a positive potential difference measured from the collector to the emitter, the transistor becomes active . In this "on" state, current flows from the collector to the emitter of the transistor . Most of the current is carried by electrons moving from emitter to collector as minority carriers in the P - type base region . To allow for greater current and faster operation, most bipolar transistors used today are NPN because electron mobility is higher than hole mobility . </P> <P> A mnemonic device for the NPN transistor symbol is "not pointing in", based on the arrows in the symbol and the letters in the name . </P>

Describe the components of current in npn transistor
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