Understand Unijunction Transistors & their Types

Unijunction Transistor Includes:
What are unijunction transistors & their types     Traditional diodic unijunction transistor, DUJ     Programmable unijunction transistors, PUTs    

The unijunction transistor is an unusual device which is used occasional in electronic circuit designs.

To be fair, it was more widely used in the 1960s than it is today, but nevertheless, it can still be a useful electronic component within the design engineers toolkit.

Unijunction transistors, UJTs are available in two types which have slightly different properties: the traditional or diodic unijunctions, DUJs and secondly what is known as a programmable unijunction transistors or PUTs.

It is the diodic or traditional type that is most commonly seen in the explanations, but interestingly, it is the second type, the programmable unijunction that is now more widely used.

Unijunction transistor beginnings

The concept of the unijunction transistor has been known for very many years - the concept is not at all new.

In his early work in 1949, Schockley referred to the device as a filamentary transistor, and then in the 1950s it was called a double base diode.

Then in the late 1950s and 1960s, the unijunction transistor appeared in a large number of circuits, where its unique properties enabled it to provide the basis for a variety of circuit designs, but using a minimum of other electronic components.

A little later the programmable unijunction transistor, PUT was introduced. In the early 1960s General Electric developed themas a more versatile alternative to ordinary diodic unijunction transistors.

PUTs were initially used in a variety of applications, sawtooth generators, pulse generators, oscillators, and timing circuits as well as phase control and triggering circuits.

However, as integrated circuits became more common, PUTs began to be replaced by ICs that performed the same functions but with greater precision and capability.

Today, PUTs are still used in some specialized applications, such as phase control circuits and thyristor triggering circuits where they are a valuable form of electronic component.

What are unijunction transistors & what are the two types

A unijunction transistor is given its name because it only has one junction in the device in comparison to the more usual bipolar transistor which has two.

Despite being very different to the bipolar transistor, or even a FET, it is still named as a transistor, although it is more closely aligned to devices like thyristors, etc.

As there are two types of unijunction transistor, it is necessary to look at what they are and what the differences are, we will then look at each type in turn.

The two types are referred to as the original or diodic unijunction transistor which may use the abbreviation UJT or DUJ. The later type is known as the programmable unijunction transistor or PUT.

Even though the PUT is called a programmable unijunction, it is only programmable in terms of setting the trigger voltage by the use of external resistors and not any other more sophisticated technique.

When comparing a traditional or diodic unijunction transistor, DUJ with a programmable unijunction transistor, PUT, the main difference is that the trigger voltage of a PUT can be adjusted by changing the value of external resistors. This is not possible with an ordinary DUJ, for which the trigger voltage is fixed by its physical construction of the electronic component.

In an ordinary or diodic DUJ, the trigger voltage is the voltage at the emitter that is required to cause the transistor to switch from its off state to its on state. This voltage is determined by the internal geometry of the device and the doping levels of the semiconductor materials used to create it.

For a PUT, the trigger voltage is determined by the value of two external resistors. This makes PUTs far more versatile than ordinary UJTs, as they can be used in a wider variety of applications.

Interestingly it is the diodic unijunction that gives its name to this class of semiconductor electronic component even though the progrmmable unijunction has more junctions as a result of its internal structure.

Unijunction circuit symbol

Although the traditional unijunction and the programmable unijunction transistors are very similar, their usage has meant that different symbols tend to be used.

  •   Traditional unijunction transistor symbol

The circuit symbol for the traditional unijunction transistor gives an indication of its structure, showing the bar or channel and the diode junction.

In order to distinguish it from a junction FET, the arrow on the emitter terminal is shown at an angle to to the channel - JFETs and UJTs are very different devices and operate in totally different ways and therefore it is essential that they are not confused.

Often the two bases, Base 1 and Base 2 are marked on the diagram itself, but if not, then Base 1 is normally taken to the ground line and Base 2 to the supply, each via suitable resistors, etc.

As the channel is not symmetrical about the diode junction, it is important to ensure that the right base is connected to the correct part of the circuit.

  •   Programmable unijunction transistor, PUT symbol

In view of where the programmable unijunction transistor, PUT is used, the circuit symbol for it is more akin to that of a thyristor or silicon controlled resistor.

The circuit symbol for the PUT is more akin to that of a thyristor, as its physical semiconductor structure is different to that of the traditional or dodic unijunction and more like that of a thyristor.

Which type of unijunction transistor should I use

As there are two types of unijunction transistor, it is necessary to decide which type should be used at the outset of any design because they require different circuits and electronic components around them.

In order to make a choice about which type of unijunction transistor to use, it is necessary to look closely into their operation.

Both types have their advantages and disadvantages:

  •   Diodic unijunction transistors, UJTs

A summary of the advantages and disadvantages of diodic unijunction transistors when compared to PUTs is given below:

In general, diodic unijunctions, DUJs are a good choice for applications where simplicity and cost are more important than versatility. Cost tends not to be so much of an issue, but the lower availability of DUJs may be a key issue for any electronic circuit design.

  •   Programmable unijunction transistors, PUTs

A summary of the advantages and disadvantages of programmable unijunction transistors when compared to PUTs is given below:

Programmable unijunction transistors, PUTs are a good choice for circuit designs where versatility and performance are more important than simplicity and cost, although the cost differential is normally small, and availability is more of an issue, and PUTs tend to be more widely available.

Although programmable unijunction transistors tend to be more widely available, both types can still be obtained, and the traditional diodic unijunctions provide a very easy method for getting some simple electronic circuits going.

Unijunction transistors may not be as widely used as they were in the 1960s and 1970s, but nevertheless they are still able to provide a useful form of electronic component for a number of circuit designs these days.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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