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Bipolar-Junction-Transistor

 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
  1. Cut off (no collector current), useful for switch operation.
  2. In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications
  3. In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications. 


Collector Current Determination

The base-emitter voltage can be considered to be the controlling variable in determining transistor action. The collector current is related to this voltage by the Ebers-Moll relationship (sometimes labeled the Shockley equation):
where
  • T = absolute temperature
  • k = Boltzmann's constant
  • e = electron charge
The saturation current is characteristic of the particular transistor (a parameter which itself has a temperature dependence). This relationship is stable over a wide range of voltages and currents. A further useful relationship is
where can be called the current gain. The value of is not highly dependable since it depends on , and the temperature.

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