The full form of PUJT is Programmable Unijunction Transistor. It is also termed as PUT in most applications. A programmable unijunction transistor is a three-lead electronic semiconductor device which has similar characteristics of a transistor, except that its behavior can be controlled using external components, meaning it can be programmed. In this article, we will provide you details of PUT technology and its applications.

Explaining PUJT

The definition of PUJT contains three different terms, namely "programmable, unijunction, and transistor.” Let’s find out what each of them refers to.

• Transistor

Transistors are active devices made from different semiconductor materials and have three terminals. They can act as either an insulator or a conductor by the application of a small signal voltage. The ability of a transistor to change between these two states makes it suitable for two basic functions: switching (in digital electronics) or amplification (in analog electronics). There are two basic types of transistor construction: PNP and NPN. This configuration describes the physical arrangement of the P-type and N-type semiconductor materials from which it is made. Usually, there are two p-n junctions in a transistor.

• Unijunction

The Unijunction (UJT) consists of an n-type silicon semiconductor bar with an electrical terminal on each end which is known as the base terminal. Near the base terminal, one p-n junction is constructed by a p-type emitter and n-type silicon material. The third terminal is connected to the p-type material. This device has three terminals and only one p-n junction so it is called Unijunction Transistor.

• Programmable

The term programmable refers to a condition when a device can be instructed to perform in a certain way using its capacity. The PUT is called programmable because its trigger voltage can be programmed or decided by the designer using external resistors.

Construction of PUJT

• Programmable Unijunction Transistor (PUT) has a four-layered construction just like the thyristors and has three terminals named anode (A), cathode (K), and gate (G).
• PUT is called unijunction because its characteristics and parameters match with the UJT. It is called programmable because the parameters like intrinsic standoff ratio, peak voltage, etc can be programmed with the help of two external resistors.
• PUT has a four-layered construction. The topmost P-layer is called the anode (A). The N-layer next to it is called the gate (G). The P-layer next to the gate is left alone. The bottom-most N-layer is called the cathode (K). The anode of the PUT is connected to a positive voltage and the cathode is connected to the ground.
• The gate is connected to the junction of the two external resistors (R1 and R2) which forms a voltage divider network. The value of these two resistors is programmable which determines the intrinsic standoff ratio and peak voltage of the PUT.

Important Terms

• Intrinsic Standoff Ratio

It is the ratio of the external resistor R1 to the sum of R1 and R2. It helps us to predict how much voltage will be dropped across the gate and cathode for a given voltage. The intrinsic standoff ratio can be expressed using the equation: η = R1/ (R1+R2)

• Peak Voltage

It is the limiting voltage between the anode and cathode voltage after which the PUT jumps into the negative resistance region.

• The Operation of PUJT

When the anode voltage is less than the gate voltage the anode gate junction becomes reverse-biased, and the PUT is in the off state. When the anode voltage exceeds the gate voltage the anode-cathode junction becomes forward biased, thus PUT is turned ON. Programmable Unijunction Transistor has two states:

• A conduction state: When there is a current between A and K and the voltage drop is small
• A cut-off state: When the current from A (anode) to K (cathode) is very small.

PUT enters the active mode from the cut-off state when the current between A and K is very small. The voltage between A and K must be raised to an extent depending on the value of the voltage at the gate. Only until the voltage at ‘A’ reaches the specific value, this transistor will turn on and remain in this state until the current through the transistor has decreased in value. This is accomplished by reducing the voltage between A and K or by reducing the voltage between G and K.

Application of PUJT

• The PUJT has wide applications and the relaxation oscillator is the most essential one. The oscillator is the circuit that converts direct current (DC) from a power supply to an AC signal without using any power inverters.
• The name relaxation oscillator signifies that the time interval of this oscillator starts by charging the capacitor and the time interval is terminated by discharging the capacitor.
• In the PUJT, when the positive terminal of the capacitor is connected to the supply, it starts charging through the resistor (R. As the capacitor is charging, the voltage across the capacitor starts increasing and ultimately reaches peak voltage (Vp) when the PUT goes into the negative resistance mode. Due to this, a low resistance path is created through the anode to the cathode and by using this path, the capacitor starts discharging.
• Consequently, the voltage across the capacitor decreases and the PUT reverses its initial condition. Thus, the capacitor starts charging again and this cycle continues.

Conclusion

The unijunction transistor was invented by General Electric as a byproduct of research on germanium tetrode transistors. It is similar to UJT function wise, but has the advanced feature that it is programmable. It finds its applications in many fields of electronics that deal with advanced semiconductor technology.

Frequently Asked Questions (FAQs)

Q1. What is the application of PUT in electronics? Ans. PUT is popularly used for Relaxation Oscillators in electronics applications. Q2. Why is the PUJT called a programmable device? Ans. The PUJT is similar to the unijunction transistor (UJT), but its intrinsic standoff ratio can be set by two external resistors. Hence, it is called programmable. Q3. How many layers and terminals does a Programmable Unijunction Transistor have? Ans. A Programmable Unijunction Transistor has four layers and three terminals.