Bipolar Junction Transistors
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The bipolar junction transistor, or BJT, is a three-lead device made by stacking three layers of doped semiconductor material with alternating p- and n-doping. Hence, there are NPN or PNP type transistors. While the details of the electronic behavior of these transistors differs from that seen for the FET, the general approach for circuit analysis remains the same. Solutions are found for an operating point based on device characteristics, that is, the “large signal behavior,” then, if necessary, linearized models are used to look at small changes from that operating point. While the FET can be considered a voltage-controlled device, the BJT is considered current-controlled.