Unijunction Transistor: working, application, characteristics

What is Unijunction Transistor(UJT)?

The Unijunction Transistor, often abbreviated as UJT, is a three-terminal semiconductor switching device. Known for its unique characteristic - 'negative resistance', it's largely been used in triggering circuits, namely for Thyristors, oscillators and wave-shaping circuits.

ujt pin diagram

The UJT is distinctive from other common transistors like the Bipolar Junction Transistor (BJT) or Field Effect Transistor (FET), by virtue of its structure and operation. A UJT is made of an N-type silicon rod, having a P-type material alloyed to its bar at one end – thus forming a single PN junction. This effectively leads to two bases (B1 and B2) and one emitter (E).

UJT Construction

When we apply a voltage between the two bases, an equilibrium is eventually reached which can be disturbed by providing a small pulse to the emitter. An interesting fact about the UJT is that, unlike the conventional transistors that are current controlled, the UJT is voltage controlled, meaning that a change in emitter voltage can be utilized to regulate the current.

Types of Bipolar Junction Transistor

Bipolar Junction Transistors (BJTs) are a type of transistor that uses both electron and hole charge carriers. They are named bipolar because their operation involves both electrons and holes. There are two types of Bipolar Junction Transistors:

NPN Transistor

This type of BJT is structured as an n-type semiconductor sandwiched between two p-type semiconductors. In NPN transistors, electrons serve as majority charge carriers that facilitate current flow from the collector terminal through to the emitter terminal when sufficient voltage is applied at the base-emitter junction.

PNP Transistor

Unlike its NPN counterpart, PNP transistors consist of a central n-type semiconductor flanked by two p-type semiconductors. Herein lies another key difference: PNP transistors rely on holes as majority charge carriers which allow current flow towards the collector when adequate voltage is applied across the base-emitter junction.

Unijunction Transistor Working Principle

Unijunction Transistor (UJT) has a unique structure and working principle. Let's take a ride through it:

BJT Symbol

A UJT, as already mentioned, has only one junction and three terminals, two bases (B1 and B2) and one emitter (E). The UJT is a three-layer, two pn-junction device. The two bases are connected to an n-type silicon bar, and a heavily doped p-type material is diffused into this bar to create the emitter. The resistance path between B1 and B2 (called RB1 and RB2), with the emitter junction in between, forms a voltage divider circuit. 

UJT Simplified Equivalent Circuit

In the off state (before reaching the "peak voltage" or Vp), the emitter junction is reversed biased and no current flows. As the emitter-base1 (E-B1) voltage is increased, a moment comes (at Vp) where the junction can't withstand the electric field, and the avalanche process starts. This state is called the "negative resistance" state because beyond this point, an increase in current leads to a drop in the voltage (V_EB1). 

UJT Circuit Characteristics

When the emitter current increases, more charge carriers are injected toward B1. Since these carriers will reduce the resistance of RB1, the voltage drop across RB1 decreases, resulting in the emitter voltage (V_EB1) also dropping. This situation represents the unique "negative resistance" phenomenon. 

UJTs are typically used as relaxational oscillators, pulse generators, sawtooth generators, and timing circuits due to these properties. The key to these applications is the unique unijunction structure and its voltage-controlled triggering.

UJT Characteristics

Unijunction Transistors (UJTs) stand out in the realm of semiconductor devices with their unique properties and functionality. Most significantly, they exhibit 'negative resistance,' a particular behavior where the voltage across the device drops as current through it rises beyond a point called the peakpoint. UJTs also rely on voltage control rather than current control, which simplifies circuit designs for some applications.

UJTs get their name from their singular junction structure comprising of three terminals: one emitter and two bases. This unique construction results in their ability to swiftly switch from high to low resistance, making them superb at generating sharp pulses. Their typical usage includes pulse generators and oscillator designs. Furthermore, the low-cost and favorable electrical characteristics have made UJTs common in many basic electronic applications.

Application of UJT

Typically known as the humble workhorses of semiconductor devices, Unijunction Transistors find their way into a range of applications because of their unique properties. Let me walk you through some exciting uses:

1. Pulse Generators: The principal application of the UJT is in generating pulses. They can convert a slowly varying input voltage into an output of a series of sharp voltage pulses. The Pulse Generation Circuit may be simple, but the inconsistent pulse it generates? Pure poetry!

2. Relaxation Oscillators: The UJT is widely used in relaxation oscillators because of its unusual "negative resistance" behavior. It helps generate a non-sinusoidal waveform (usually a sawtooth waveform).

3. Timing Circuits: The UJT can be used in timing circuits where it works in tandem with a capacitor. A classic watch ticking away wouldn't have the same beat without a little UJT magic!

4. Firing Circuits for SCRs: The sharp pulse generating capability makes the UJT perfect for triggering SCRs (Silicon Controlled Rectifiers) in firing circuits. It gives the kick that wakes up the SCRs from their slumber.

5. Phase Control Circuits: Embarking on a slightly more technical journey, our UJT taxicab drives us to phase control circuits as well. They can be used to control the load power in AC circuits.

6. Intruder Alarm Systems: Because the UJT is able to produce an oscillating output even under low voltage conditions, it's perfect for powering that siren in intruder alarm circuits.

7. Voltage Regulators: To top it all off, UJTs also find use in voltage regulation circuits.