A unijunction transistor (UJT) is an electronic

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 device that has only one junction

P-n junction

A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...

. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type

N-type semiconductor

N-type semiconductors are a type of extrinsic semiconductor where the dopant atoms are capable of providing extra conduction electrons to the host material . This creates an excess of negative electron charge carriers....

 bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type

P-type semiconductor

A P-type semiconductor is obtained by carrying out a process of doping: that is, adding a certain type of atoms to the semiconductor in order to increase the number of free charge carriers ....

 and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance.

There are three types of unijunction transistors:

• The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor
  Semiconductor
  A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
  
   material into which P type material has been diffused somewhere along its length, defining the device parameter . The 2N2646 is the most commonly used version of the UJT.

• The complementary unijunction transistor, or CUJT, that is a bar of P type semiconductor
  Semiconductor
  A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
  
   material into which N type material has been diffused somewhere along its length, defining the device parameter . The 2N6114 is one version of the CUJT.

• The programmable unijunction transistor, or PUT, is a close cousin to the thyristor
  Thyristor
  A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
  
  . Like the thyristor it consists of four P-N layers and has an anode
  Anode
  An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
  
   and a cathode
  Cathode
  A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
  
   connected to the first and the last layer, and a gate
  Gate
  A gate is a point of entry to a space enclosed by walls, or a moderately sized opening in a fence. Gates may prevent or control entry or exit, or they may be merely decorative. Other terms for gate include yett and port...
  
   connected to one of the inner layers. They 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 , they behave like a conventional UJT. The 2N6027 is an example of such a device.

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

Negative resistance

Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...

 at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.

Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple oscillators to be built using just one active device. For example, they were used for relaxation oscillator

Relaxation oscillator

A relaxation oscillator is an oscillator based upon the behavior of a physical system's return to equilibrium after being disturbed. That is, a dynamical system within the oscillator continuously dissipates its internal energy...

s in variable-rate strobe lights.
Later, as integrated circuits became more popular, oscillators such as the 555 timer IC

555 timer IC

The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation and oscillator applications. The part is still in widespread use, thanks to its ease of use, low price and good stability. , it was estimated that 1 billion units are manufactured every year.- Design :The IC...

 became more commonly used.

In addition to its use as the active device in relaxation oscillator

Relaxation oscillator

A relaxation oscillator is an oscillator based upon the behavior of a physical system's return to equilibrium after being disturbed. That is, a dynamical system within the oscillator continuously dissipates its internal energy...

s, one of the most important applications of UJTs or PUTs is to trigger thyristor

Thyristor

A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...

s (SCR, TRIAC

TRIAC

TRIAC, from Triode for Alternating Current, is a genericized tradename for an electronic component that can conduct current in either direction when it is triggered , and is formally called a bidirectional triode thyristor or bilateral triode thyristor.TRIACs belong to the thyristor family and are...

, etc.). In fact, 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

Hall effect

The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

modulates the voltage at the PN junction. This affects the frequency of UJT relaxation oscillators. This only works with UJTs. PUTs do not exhibit this phenomenon.

External links

• Diagrams and description of unijunction transistors from American Microsemiconductor