Editing Bipolar junction transistor (section)

== Regions of operation ==
<!-- "Emitter-coupled logic" links here. -->
{| class="wikitable floatright"
|-
! rowspan=2 | Junction <br />type
! rowspan=2 | Applied <br />voltages
! colspan=2 | Junction bias
! rowspan=2 | Mode
|-
! B–E
! B–C
|-
| rowspan=4 | NPN
| E &lt; B &lt; C || Forward || Reverse || Forward-active
|-
| E &lt; B &gt; C || Forward || Forward || Saturation
|-
| E &gt; B &lt; C || Reverse || Reverse || Cut-off
|-
| E &gt; B &gt; C || Reverse || Forward || Reverse-active
|-
| rowspan=4 | PNP
| E &lt; B &lt; C || Reverse || Forward || Reverse-active
|-
| E &lt; B &gt; C || Reverse || Reverse || Cut-off
|-
| E &gt; B &lt; C || Forward || Forward || Saturation
|-
| E &gt; B &gt; C || Forward || Reverse || Forward-active
|}

Bipolar transistors have four distinct regions of operation, defined by BJT junction biases:<ref>{{cite web |author=JIMBLOM |title=Transistors: Operation Modes |url=https://learn.sparkfun.com/tutorials/transistors/operation-modes |publisher=[[SparkFun Electronics]] |access-date=June 22, 2023 }}</ref><ref>{{cite web |title=Lecture 18 Outline: The Bipolar Junction Transistor (II) – Regimes of Operation |url=http://web.mit.edu/6.012/www/SP07-L18.pdf |date=Spring 2007 |access-date=June 22, 2023 }}</ref>
; Forward-active (or simply ''active'') : The base–emitter junction is forward biased and the base–collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, β<sub>F</sub>, in forward-active mode. If this is the case, the collector–emitter current is approximately  [[Proportionality (mathematics)|proportional]] to the base current, but many times larger, for small base current variations.
; Reverse-active (or ''inverse-active'' or ''inverted'') : By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Because most BJTs are designed to maximize current gain in forward-active mode, the β<sub>F</sub> in inverted mode is several times smaller (2–3 times for the ordinary germanium transistor). This transistor mode is seldom used, usually being considered only for failsafe conditions and some types of [[Transistor–transistor logic#Implementation|bipolar logic]]. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.
; Saturation : With both junctions forward biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.
; Cut-off : In cut-off, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.

{{Multiple image
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| image1 = Input characteristic common-base silicon transistor-en.svg
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| caption1 = Input characteristics
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| caption2 = Output characteristics 
| footer = Input and output characteristics for a common-base silicon transistor amplifier.
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Although these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than a few hundred millivolts) biases. For example, in the typical grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the "off" state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of the forward active region can be regarded as the cutoff region.

=== 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.