Editing Bipolar junction transistor (section)

== Function ==
{{Technical|section|date=July 2012}}

BJTs exist as PNP and NPN types, based on the doping types of the three main terminal regions.  An NPN transistor comprises two semiconductor junctions that share a thin p-doped region, and a PNP transistor comprises two semiconductor junctions that share a thin n-doped region. N-type means doped with [[Impurity|impurities]] (such as [[phosphorus]] or [[arsenic]]) that provide mobile electrons, while p-type means doped with impurities (such as [[boron]]) that provide holes that readily accept electrons.

[[File:NPN BJT Basic Operation (Active) jP.svg|thumb|center|NPN BJT with forward-biased B–E junction and reverse-biased B–C junction]]

Charge flow in a BJT is due to [[diffusion]] of [[Charge carriers in semiconductors|charge carriers]] (electrons and holes) across a junction between two regions of different charge carrier concentration. The regions of a BJT are called ''emitter'', ''base'', and ''collector''.{{efn|See [[Point-contact transistor]] for the historical origin of these names.}} A discrete transistor has three [[Lead (electronics)|leads]] for connection to these regions. Typically, the emitter region is heavily doped compared to the other two layers, and the collector is doped more lightly (typically ten times lighter<ref name="hu">{{cite book |author=Chenming Calvin Hu |url=http://people.eecs.berkeley.edu/~hu/Book-Chapters-and-Lecture-Slides-download.html |title=Modern Semiconductor Devices for Integrated Circuits |date=2010 }}</ref>) than the base. By design, most of the BJT collector current is due to the flow of charge carriers injected from a heavily doped emitter into the base where they are [[minority carrier]]s (electrons in NPNs, holes in PNPs) that diffuse toward the collector, so BJTs are classified as ''minority-carrier devices''.

In typical operation, the base–emitter junction is [[p–n junction#Forward bias|forward biased]], which means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base–collector junction is [[p–n junction#Reverse bias|reverse biased]]. When forward bias is applied to the base–emitter junction, the equilibrium between the thermally generated carriers and the repelling electric field of the emitter [[depletion region]] is disturbed. This allows thermally excited carriers (electrons in NPNs, holes in PNPs) to inject from the emitter into the base region. These carriers create a [[diffusion current]] through the base from the region of high concentration near the emitter toward the region of low concentration near the collector.

To minimize the fraction of carriers that [[Carrier generation and recombination|recombine]] before reaching the collector–base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority-carrier lifetime. Having a lightly doped base ensures recombination rates are low. In particular, the thickness of the base must be much less than the [[Fick's law#Example solution in one dimension: diffusion length|diffusion length]] of the carriers. The collector–base junction is reverse-biased, and so negligible carrier injection occurs from the collector to the base, but carriers that are injected into the base from the emitter, and diffuse to reach the collector–base depletion region, are swept into the collector by the electric field in the depletion region. The thin ''shared'' base and asymmetric collector–emitter doping are what differentiates a bipolar transistor from two ''separate'' diodes connected in series.

=== Voltage, current, and charge control ===
The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is the usual exponential current–voltage curve of a p–n junction (diode).<ref name="Horowitz 1989">{{cite book |last1=Horowitz |first1=Paul |author-link1=Paul Horowitz |last2=Hill |first2=Winfield |author-link2=Winfield Hill |title=The Art of Electronics |edition=2nd |date=1989 |publisher=Cambridge University Press |isbn=978-0-521-37095-0 |url=https://books.google.com/books?id=bkOMDgwFA28C |access-date=June 22, 2023 }}</ref>

The explanation for collector current is the concentration gradient of minority carriers in the base region.<ref name="Horowitz 1989" /><ref>{{cite book |title=Semiconductor Device Physics and Simulation |first1=Juin Jei |last1=Liou |first2=Jiann S. |last2=Yuan |publisher=Springer |date=1998 |isbn=978-0-306-45724-1 |url=https://books.google.com/books?id=y343FTN1TU0C&q=charge-controlled+bjt+physics&pg=PA166 }}</ref><ref>{{cite book | title = Transistor Manual |author=General Electric |edition=6th |date=1962 |page=12 |bibcode = 1964trma.book.....C }} "If the principle of space charge neutrality is used in the analysis of the transistor, it is evident that the collector current is controlled by means of the positive charge (hole concentration) in the base region. ... When a transistor is used at higher frequencies, the fundamental limitation is the time it takes the carriers to diffuse across the base region..." (same in 4th and 5th editions).</ref> Due to [[low-level injection]] (in which there are many fewer excess carriers than normal majority carriers) the [[Ambipolar diffusion|ambipolar transport]] rates (in which the excess majority and minority carriers flow at the same rate) is in effect determined by the excess minority carriers.

Detailed [[transistor models]] of transistor action, such as the [[Gummel–Poon model]], account for the distribution of this charge explicitly to explain transistor behavior more exactly.<ref>{{cite book |title=Semiconductor Device Modeling with Spice |first1=Paolo |last1=Antognetti |first2=Giuseppe |last2=Massobrio |publisher=McGraw–Hill Professional |date=1993 |isbn=978-0-07-134955-0 |url=https://books.google.com/books?id=5IBYU9xrGaIC&q=gummel-poon+charge+model&pg=PA96 }}</ref> The charge-control view easily handles [[phototransistor]]s, where minority carriers in the base region are created by the absorption of [[photon]]s, and handles the dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, because base charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in circuit design and analysis.

In [[analog circuit]] design, the current-control view is sometimes used because it is approximately linear. That is, the collector current is approximately <math>\beta_\text{F}</math> times the base current. Some basic circuits can be designed by assuming that the base–emitter voltage is approximately constant and that collector current is β times the base current. However, to accurately and reliably design production BJT circuits, the voltage-control model (e.g. the [[Ebers–Moll model]]) is required.<ref name="Horowitz 1989" /> The voltage-control model requires an exponential function to be taken into account, but when it is linearized such that the transistor can be modeled as a transconductance, as in the Ebers–Moll model, design for circuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is often preferred. For [[translinear circuit]]s, in which the exponential I–V curve is key to the operation, the transistors are usually modeled as voltage-controlled current sources whose [[transconductance]] is proportional to their collector current. In general, transistor-level circuit analysis is performed using [[SPICE]] or a comparable analog-circuit simulator, so mathematical model complexity is usually not of much concern to the designer, but a simplified view of the characteristics allows designs to be created following a logical process.

=== Turn-on, turn-off, and storage delay ===
{{Main|Baker clamp}}

Bipolar transistors, and particularly power transistors, have long base-storage times when they are driven into saturation; the base storage limits turn-off time in switching applications. A [[Baker clamp]] can prevent the transistor from heavily saturating, which reduces the amount of charge stored in the base and thus improves switching time.<!-- at a cost of increased V<sub>CE,sat</sub> -->

=== Transistor characteristics: alpha (''α'') and beta (''β'') <span class="anchor" id="Alpha"></span><span class="anchor" id="Beta"></span><span class="anchor" id="AlphaBeta"></span> ===
The proportion of carriers able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. A thin and lightly doped base region means that most of the minority carriers that are injected into the base will diffuse to the collector and not recombine.

==== Common-emitter current gain ====
The ''[[common-emitter]] current gain'' is represented by {{mvar|β}}<sub>F</sub> or the [[Two-port network#h-parameters|{{mvar|h}}-parameter]] {{mvar|h}}<sub>FE</sub>; it is approximately the ratio of the collector's direct current to the base's direct current in forward-active region. (The F subscript is used to indicate the forward-active mode of operation.) It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications. Both injection efficiency and recombination in the base reduce the BJT gain.

==== Common-base current gain ====
Another useful characteristic is the ''[[common-base]] current gain'', {{mvar|α}}<sub>F</sub>. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.980 and 0.998. It is less than unity due to recombination of charge carriers as they cross the base region.

Alpha and beta are related by the following identities:
: <math>\begin{align}
  \alpha_\text{F} &= \frac{I_\text{C}}{I_\text{E}}, & \beta_\text{F} &= \frac{I_\text{C}}{I_\text{B}}, \\
  \alpha_\text{F} &= \frac{\beta_\text{F}}{1 + \beta_\text{F}} & \iff \beta_\text{F} &= \frac{\alpha_\text{F}}{1 - \alpha_\text{F}}.
\end{align}</math>

Beta is a convenient figure of merit to describe the performance of a bipolar transistor, but is not a fundamental physical property of the device. Bipolar transistors can be considered voltage-controlled devices (fundamentally the collector current is controlled by the base–emitter voltage; the base current could be considered a defect and is controlled by the characteristics of the base–emitter junction and recombination in the base). In many designs beta is assumed high enough so that base current has a negligible effect on the circuit. In some circuits (generally switching circuits), sufficient base current is supplied so that even the lowest beta value a particular device may have will still allow the required collector current to flow.