Editing Bipolar junction transistor (section)

== Structure ==
[[File:NPN BJT (Planar) Cross-section.svg|frame|left|Simplified cross section of a planar ''NPN'' bipolar junction transistor]]

BJTs consists of three differently doped semiconductor regions: the ''emitter'' region, the ''base'' region and the ''collector'' region. These regions are, respectively, ''p'' type, ''n'' type and ''p'' type in a PNP transistor, and ''n'' type, ''p'' type and ''n'' type in an NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: ''emitter'' (E), ''base'' (B) and ''collector'' (C).

The ''base'' is physically located between the ''emitter'' and the ''collector'' and is made from lightly doped, high-resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape without being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross-section view of a BJT indicates that the collector–base junction has a much larger area than the emitter–base junction.

The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter.

[[File:IPRS BANEASA 2N2222.jpg|thumb|Die of a 2N2222 NPN transistor: the NPN materials are made in layers with the collector at the bottom.  Bond wires connect metalization on the base to the left lead, and emitter to the right.  The collector is connected to the can with a third external lead..]]

The low-performance "lateral" bipolar transistors sometimes used in bipolar and MOS integrated circuits are sometimes designed symmetrically, that is, with no difference between forward and backward operation.

Small changes in the voltage applied across the base–emitter terminals cause the current between the ''emitter'' and the ''collector'' to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled [[current source]]s, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base.

Early transistors were made from [[germanium]] but most modern BJTs are made from [[silicon]]. A significant minority are also now made from [[gallium arsenide]], especially for very high speed applications (see HBT, below).

The [[heterojunction bipolar transistor]] (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred [[Hertz|GHz]]. It is common in modern ultrafast circuits, mostly RF systems.<ref>{{cite book |editor1-first=D.V. |editor1-last=Morgan |editor2-first=Robin H. |editor2-last=Williams |title=Physics and Technology of Heterojunction Devices |date=1991 |publisher=Institution of Electrical Engineers (Peter Peregrinus Ltd.) |location=London |isbn=978-0-86341-204-2 |url=https://books.google.com/books?id=C98iH7UDtzwC&q=%22SIGe+heterojunction%22&pg=PA210 }}</ref><ref name="Ashburn">{{cite book |last=Ashburn |first=Peter |title=SiGe Heterojunction Bipolar Transistors |date=2003 |pages=Chapter 10 |publisher=Wiley |location=New York |isbn=978-0-470-84838-8 |url=http://worldcat.org/isbn/0470848383 |no-pp=true }}</ref>

[[File:Diagrama de Transistor NPN.svg|thumb|upright=0.5|Symbol for NPN bipolar transistor with current flow direction]]

Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by [[epitaxy]] techniques like [[Metalorganic vapour phase epitaxy|MOCVD]] and [[Molecular beam epitaxy|MBE]].