Editing Bipolar junction transistor (section)

=== Active-mode transistors in circuits ===
[[File:NPN BJT - Structure & circuit.svg|frame|right|Structure and use of NPN transistor; arrow according to schematic]]

The diagram shows a schematic representation of an NPN transistor connected to two voltage sources. (The same description applies to a PNP transistor with reversed directions of current flow and applied voltage.)  This applied voltage causes the lower p–n junction to become forward biased, allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by ''V''<sub>CE</sub>) will cause the majority of these electrons to cross the upper p–n junction into the collector to form the collector current ''I''<sub>C</sub>. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, ''I''<sub>B</sub>. As shown in the diagram, the emitter current, ''I''<sub>E</sub>, is the total transistor current, which is the sum of the other terminal currents, (i.e. ''I''<sub>E</sub>&nbsp;=&nbsp;''I''<sub>B</sub>&nbsp;+&nbsp;''I''<sub>C</sub>).

In the diagram, the arrows representing current point in the direction of conventional current&nbsp;– the flow of electrons is in the opposite direction of the arrows because electrons carry negative [[electric charge]]. In active mode, the ratio of the collector current to the base current is called the ''DC current gain''. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see [[op-amp]]). The value of this gain for DC signals is referred to as <math>h_{\text{FE}}</math>, and the value of this gain for small signals is referred to as <math>h_{\text{fe}}</math>. That is, when a small change in the currents occurs, and sufficient time has passed for the new condition to reach a steady state <math>h_{\text{fe}}</math> is the ratio of the change in collector current to the change in base current. The symbol <math>\beta</math> is used for both <math>h_{\text{FE}}</math> and <math>h_{\text{fe}}</math>.<ref name="Horowitz 1989" />{{Rp|62–66}}

The emitter current is related to <math>V_{\text{BE}}</math> exponentially. At [[room temperature]], an increase in <math>V_{\text{BE}}</math> by approximately 60&nbsp;mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.