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

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.

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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