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. |
Comments
Post a Comment