Traditional or Diodic Unijunction Transistor, UJT

The diodic unijunction transistor, DUJ was the first form of unijunction transistor to be introduced and it provides the basis for many simple oscillator and timing circuits.

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

The diodic form of unijunction, DUJ was the first type of unijunction to be introduced, and it is based around a simple bar structure that has a single PN junction - this gives rise to the name.

The DUJ is very simple and easy to use, requiring no setting of the trigger voltages, and hence the circuits using them can have an exceedingly low electronic component count.

Diodic unijunctions are less widely available these days when compared to their "programmable" cousins, and therfore care needs to be taken in ensuring that the correct type of component is obtained when using this type.

One of the most common examples of the diodic unijunction transistor available from some sources today is the 2N4820, although other types will also be seen including 2N1671, 2N2646 or the 2N2647.

The main thing is to make sure that if you need a diodic unijunction, then you buy the correct type and not one of the more common programmable unijuntion transistor or PUT.

Diodic unijunction circuit symbol

The two different types of unijunction transistor tend to be represented by different circuit symbols.

The circuit symbol for the DUJ is normally represented by one that is quite close to that of a field effect transistor, but it can be differentiated by the fact that it has an angled arrow onto what may be thought of as the channel.

Circuit symbol for a diodic unijunction transistor, UJT or DUJ — Circuit symbol for a diodic unijunction transistor

The unijunction circuit symbol may or may not have the various electrode marked, if not, then they can be assumed to be those that are marked on the circuit symbol shown.

Concept of the diodic unijunction transistor.

The diodic unijunction transistor gains its name from the fact that it consists of a "channel" of semiconductor into which a PN junction diode is fabricated.

As a result of its structure, it was also known as a filamentary transistor as it was built around a filament of semiconductor into which the diode was fabricated.

Basic structural concept of a unijunction transistor

The diagram shows the outline or basic structure of a unijunction. As can be seen from the diagram, the device has three terminals. One is names the emitter and the others are base one and base 2.

The emitter is connected to the junction and the two base connections are connected to the main conducting semiconductor "channel."

Conventionally, the grounded base terminal is referred to as Base 1and the other terminal as Base 2.

Also, the the emitter junction is typically positioned along the "channel" so that it is closer to terminal Base 2 terminal than the Base 1 terminal.

In terms of its operation the device can be explained using its equivalent circuit.

Unijunction transistor equivalent circuit showing the base resistance and the dynamic resistance — Unijunction transistor equivalent circuit

The equivalent circuit shows the diode connected to the emitter contact and the channel of semiconductor.

The equivalent circuit also shows three resistors. R_B1 and R_B2 are simply the body resistance of the N-type "channel" material. Their relative values depend upon the position of the diode junction along the body material.

The shunt resistor R_D arises from the carrier injection when the emitter is under high forward bas and it plays a key role in the overall characteristics of the device.

When the emitter junction is unbiassed, R_D is infinite, but when the junction is exposed to a strong forward bias, then holes are injected into the channel area and they drift towards Base 1 because of the field applied between the two base electrodes.

When the hole concentration is higher than the background doping concentration, then it is said that a high level injection occurs.

The excess holes draw an equal number of excess electrons from Base 1 in order to restore the charge neutrality. It is this change or modulation of the conductivity resulting from the excess electrons that gives rise to the resistance termed R_D.

The emitter base I/V characteristics are unique to this type of unijunction transistor and they are shown in the I-V plot below.

IV characteristic of a diodic unijunction transistor, DUJ or UJT — I-V characteristic of a diodic unijunction transistor, DUJ

The emitter base voltage and current curve follows exactly that of a typical PN junction diod when it is in its forward bias condition - initially no current folows and as the voltage across the junction increases, a small amount of current flows.

When the potential across the diode reaches the turn on voltage (around 0.6 to 0.7 volts for silicon), the current significantly increases and the diode junction is said to be "turned on."

At the point of the middle of the curve with a fixed positive V_BB, the voltage at the cathode electrode of the PN junction is V_C and can be designated by V_C.

If the emitter voltage V_E is below this, then the diode is reverse biassed and the emitter current is small and negative.

If the value of V_E is increased, then the emitter current becomes positive. However the region before significant carrier injection occurs is refered to as the cutoff region.

As the voltage increases, so does the carrier injection and the level of R_D falls.

A smaller level of V_C actually increases the level of the forward bias of the diode giving a positive feedback effect where the level of carrier injection rises higher. The overall effect of this is that a negative resistance region is created within the IV characteristic.

After the the value V_V is reached the characteristic returns to that off a normal diode and the curve enters what is referred to as the saturation region.

Physical structure of unijunction devices

Although it is easy to visualise the structure of the unijunction as shown above for an explanation, this may not be realisable in terms of the easy and cost effective fabrication techniques that are used.

Typically devices are fabricated using successive deposition layers from the top surface. Contacts may be added on the underside, but it is not normally easy to add contacts in, for example, in two planes that are at right angles.

Most unijunction transistors are made from n-type material because of the carrier mobility is higher for n-type - electrons have a much greater mobility than holes. However it is possible to fabricate devices using p-type material.

Also virtually all devices use silicon as the base semiconductor, although germanium was explored in the early days.

The level of doping normally used means that the resistivity of the material used is around 100 Ω cm and this generally means that the resistance between the two base contacts is often between about 5 &10 kΩ without any carrier injection from the emitter.

Typical device semiconductor structures of a diodic unijunction transistor, DUJ or UJT — Typical device semiconductor structures of a diodic unijunction transistor

Although the physical semiconductor structure looks slightly similar to that of a junction FET, the voltages used and dimensions of the structure, etc mean hat the depletion region never extends across the channel to cut off the channel between the two bases.

How does a traditional unijunction work in a circuit

Despite the name, a unijunction transistors works in a totally different manner to normal transistors when used in an electronic circuit design.

Unijunctions have a low current gain and and as a result they cannot be used for voltage or current amplification. Instead they are used for a form of on-off switching.

one common circuit design for diodic unijunction transistors is a simple relaxation oscillator.

in this circuit, the output voltage rises as the capacitor is charged until it reaches the switching voltage, at which point the unijunction switches and the voltage falls relatively rapidly, making the period a little longer than the charge up time of the capacitor.

Simple circuit of a unijunction transistor relaxation oscillator

Although diodic unijunctions may be seen in other circuit designs, the simple relaxation oscillator is by far the most widely seen.

Diodic unijunction transistors can be used as the basis of very simple and straightforward circuits and they are most often seen in use in simple relaxation oscillator circuits, they can be used in other circuits.

The diodic unijunction was superseeded by the programmable unijunction transistor or PUT, but despite this, the diodic version is possible the most widely seen and described.