We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.
Electrical

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

What is a Unijunction Transistor?

By C. Greason
Updated: May 17, 2024
Views: 8,749
Share

A unijunction transistor is a three-terminal, solid-state semiconductor device used to control or trigger other electronic devices in low-power, low-voltage circuits. The device has a single p-n junction, called the emitter junction, between its two base terminals. Unijunction transistors were popularly used by electronic hobbyists in the 1970s to build simple oscillator circuits. By the early 21st century, these transistors were commonly used in relaxation oscillator circuits to control the discharge of a capacitor and to trigger a thyristor in alternating current (AC) control circuits.

Previously known as a filamentary transistor or a double-base diode, the unijunction transistor belongs in the thyristor family, a type of power-switching, solid-state semi-conductor device. A unijunction transistor has a high impedance when it is off, which means it strongly resists current flow when it is off. When it is on, it has a low impedance, which means it minimally resists current flowing through it. Thus, a the device can control the flow of current in a circuit.

When its Base 1, or B1, terminal is grounded and a voltage is applied to its Base 2, or B2, terminal, a current is produced on the N side of the emitter junction. As more voltage is applied between B1 and B2, the semi-conductor material causes conductivity between the emitter and B1 to increase, so it is easier for current to flow. When the conductivity between the emitter and B1 increases, the emitter voltage naturally goes back down, which can shut the transistor off. This on-and-off effect produces the desired oscillation, or trigger.

A result of these oscillations is that a unijunction transistor can be used in a triggering circuit along with a capacitor. The capacitor absorbs current until the emitter voltage on the transistor reaches a certain level, at which time the transistor turns on. The transistor increases conductivity to ground when it is turned on, so current can flow from the capacitor to ground. The result is a triangle wave oscillation.

A programmable unijunction transistor, or PUT, is a special kind of unijunction transistor. A PUT is a p-n-p-n semiconductor device with a gate contact made to the n base instead of the p base. A PUT produces an oscillation much like a standard unijunction transistor, except that its emitter voltage is a different ratio of the B2 voltage. A programmable unijunction device can be used in a resonant circuit to adjust the circuit’s oscillating frequency.

Share
About Mechanics is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
Share
https://www.aboutmechanics.com/what-is-a-unijunction-transistor.htm
Copy this link
About Mechanics, in your inbox

Our latest articles, guides, and more, delivered daily.

About Mechanics, in your inbox

Our latest articles, guides, and more, delivered daily.