Transistor Operation

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 V_BE 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

Base-Emitter Junction Details

1. A base emitter voltage V_BE of about 0.6 v will "turn on" the base-emitter diode and that voltage changes very little, < +/- 0.1v throughout the active range of the transistor which may change base current by a factor of 10 or more.
2. An increase in base-emitter voltage V_BE by about 60 mV will increase the collector current I_C by about a factor of 10.
3. The effective AC series resistance of the emitter is about 25/I_C ohms.
4. The base-emitter voltage V_BE is temperature dependent, decreasing about 2.1 mV/C
5. The base-emitter voltage V_BE varies slightly with the collector-emitter voltage V_CE at constant collector current I_C : ΔV_BE ≈ -0.001ΔV_CE.

Transistor Action

More about transistor regions

Collector Current

Normal transistor action results in a collector-to-emitter current which is about 99% of the total current. The usual symbols used to express the transistor current relationships are shown.

The proportionality β can take values in the range 20 to 200 and is not a constant even for a given transistor. It increases for larger emitter currents because the larger number of electrons injected into the base exceeds the available holes for recombination so the fraction which recombine to produce base current delines even further.

Use of the Current Gain

Any circuit that depends on a specific value of the current gain β is a bad circuit because that value varies for a given transistor as well as between different transistors of the same type.