A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector). The device can be characterized as a current amplifier, having many applications for amplification and switching A transistor in a circuit will be in one of three conditions Cut off (no collector current), useful for switch operation. In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications.
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