View source for Unijunction transistor

{{Short description|Type of transistor}}
{{Infobox electronic component
|name              = Unijunction Transistor
|image             = Image:unijunction transistors.jpg
|image_size        = 110px
|caption           = Unijunction transistors
|type              = [[Active device|active]]
|working_principle =
|invented          = [[General Electric]] (1953)
|first_produced    =
|symbol            = [[File:IEEE 315-1975 (1993) 8.6.8.svg|100px]] [[File:IEEE 315-1975 (1993) 8.6.9.svg|100px]]<br />UJT N and P symbol<ref>https://saliterman.umn.edu/sites/saliterman.dl.umn.edu/files/general/solid_state_power_switching.pdf Page 12</ref>
|pins              =  B2, B1, emitter
}}
A '''unijunction transistor''' ('''UJT''') is a three-lead [[Electronics|electronic]] [[semiconductor]] device with only one [[p-n junction|junction]]. It acts exclusively as an electrically controlled switch.

The UJT is not used as a linear amplifier. It is used in free-running oscillators, synchronized or triggered oscillators, and pulse generation circuits at low to moderate frequencies (hundreds of kilohertz). It is widely used in the triggering circuits for [[silicon controlled rectifier]]s. In the 1960s, the low cost per unit, combined with its unique characteristic, warranted its use in a wide variety of applications like oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies.<ref name=Cleary64>J. F. Cleary (ed.), ''General Electric Transistor Manual'', General Electric, 1964 Chapter 13 "Unijunction Transistor Circuits"</ref> The original unijunction transistor types are now considered obsolete, but a later multi-layer device, the [[programmable unijunction transistor]], is still widely available.

==Types==
[[File:UJT caratteristica.png|thumb|Graph of UJT characteristic curve, emitter-base1 voltage as a function of emitter current, showing current-controlled negative resistance (downward-sloping region)]]
There are three types of unijunction transistor:
# The original unijunction transistor, or UJT, is a simple device that is essentially a bar of [[n-type semiconductor]] material into which p-type material has been diffused somewhere along its length, fixing the device parameter <math>\eta</math> (the "intrinsic stand-off ratio"). The 2N2646 model is the most commonly used version of the UJT.
# The complementary unijunction transistor, or CUJT, is a bar of [[p-type semiconductor]] material into which n-type material has been diffused somewhere along its length, defining the device parameter <math>\eta</math>. The 2N6114 model is one version of the CUJT.
# The programmable unijunction transistor, or PUT, is a multi-junction device that, with two external resistors, displays similar characteristics to the UJT. It is a close cousin to the [[thyristor]] and like the thyristor consists of four p-n layers. It has an [[anode]] and a [[cathode]] connected to the first and the last layer respectively, and a [[gate]] connected to one of the inner layers. PUTs are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter <math>\eta</math>, they behave like a conventional UJT. The 2N6027, 2N6028<ref>2N6027, 2N6028 data sheet by ON Semiconductor, at [http://www.farnell.com/datasheets/112532.pdf farnell.com]</ref> and BRY39 models are examples of such devices.

==Applications==
Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple [[oscillator]]s to be built using just one active device. For example, they were used for [[relaxation oscillator]]s in variable-rate strobe lights.<ref>
{{cite journal
 | journal = Popular Science
 | title = A Repeating Flash You Can Build
 | author = Ronald M. Benrey
 | volume = 185
 | issue = 4
 | pages = 132–136
 | date = October 1964
 | url = https://books.google.com/books?id=1yUDAAAAMBAJ&pg=PA132
 }}</ref>
Later, as [[integrated circuit]]s became more popular, oscillators such as the [[555 timer IC]] became more commonly used.

In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger [[thyristor]]s ([[silicon controlled rectifier]]s (SCR), [[TRIAC]]s, etc.). A DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. This application is important for large AC current control.

UJTs can also be used to measure magnetic flux. The [[Hall effect]] modulates the voltage at the PN junction. This affects the frequency of UJT relaxation oscillators.<ref>{{cite journal
|last1=Agrawal
|first1=S. L.
|last2=Saha
|first2=D. P.
|last3=Swami
|first3=R.
|last4=Singh
|first4=R. P.
|title=Digital magnetic fluxmeter using unijunction transistor probe
|journal=International Journal of Electronics
|date=23 April 1987
|volume=63
|issue=6
|pages=905–910
|doi=10.1080/00207218708939196
}}</ref> This only works with UJTs. PUTs do not exhibit this phenomenon.

== Construction ==
{{Disputed section|date=October 2021}}
[[Image:UJT struttura.png|thumb|right|130px|Structure of a p-type UJT]]
[[Image:Unijuction transistor KT117 (open).jpg|thumb|130px|UJT die: the larger contact in the centre of the crystal is the emitter, the smaller one is B<sub>1</sub>; B<sub>2</sub> is at the bottom of the crystal]]
The UJT has three terminals: an emitter (E) and two bases (B<sub>1</sub> and B<sub>2</sub>) and so is sometimes known a "double-base diode". The base is formed by a lightly [[Doping (semiconductor)|doped]] [[N-type semiconductor|n-type]] bar of silicon. Two ohmic contacts B<sub>1</sub> and B<sub>2</sub> are attached at its ends. The emitter is of heavily-doped [[P-type semiconductor|p-type]] material. The single PN junction between the emitter and the base gives the device its name. The resistance between B1 and B2 when the emitter is open-circuit is called ''interbase resistance''. The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is not symmetrical, because a symmetrical unit does not provide optimum electrical characteristics for most of the applications.

If no potential difference exists between its emitter and either of its base leads, there is an extremely small [[Electric current|current]] from B<sub>1</sub> to B<sub>2</sub>. On the other hand, if an adequately large voltage relative to its base leads, known as the ''trigger voltage'', is applied to its emitter, then a very large current from its emitter joins the current from B<sub>1</sub> to B<sub>2</sub>, which creates a larger B<sub>2</sub> output current.

The [[schematic diagram]] symbol for a unijunction transistor represents the emitter lead with an arrow, showing the direction of [[conventional current]] when the emitter-base junction is conducting a current. A complementary UJT uses a p-type base and an n-type emitter, and operates the same as the n-type base device but with all voltage polarities reversed.

The structure of a UJT is similar to that of an N-channel [[JFET]], but p-type (gate) material surrounds the N-type (channel) material in a JFET, and the gate surface is larger than the emitter junction of UJT. A UJT is operated with the emitter junction forward-biased while the JFET is normally operated with the gate junction reverse-biased. The UJT is a current-controlled [[negative resistance]] device.

==Device operation==
The device has a unique characteristic in that when it is triggered, its emitter current increases regeneratively until it is restricted by the emitter power supply. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.

The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes [[conductivity modulation]], which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.

==Invention==
The unijunction transistor was invented as a byproduct of research on [[germanium]] tetrode transistors at [[General Electric]].<ref>{{cite web|url=http://www.semiconductormuseum.com/Transistors/GE/OralHistories/Suran/Suran_Index.htm|title=Transistor Museum Oral History Suran Index GE Unijunction Transistors|author=Jack Ward|date=2005|work=SemiconductorMuseum.com|access-date=April 10, 2017}}</ref> It was patented in 1953. Commercially, silicon devices were manufactured.<ref>{{cite web|url=https://sites.google.com/site/transistorhistory/Home/us-semiconductor-manufacturers/general-electric-history|title=General Electric History - Transistor History|work=Google.com|access-date=April 10, 2017}}</ref> A common part number is 2N2646.

==References==
{{reflist}}

{{Commons category|Unijunction transistors}}


{{Electronic component}}

[[Category:Transistor types]]
[[Category:General Electric inventions]]
[[Category:1953 in technology]]