What is UJT Transistor

A Unijunction Transistor (UJT) is a semiconductor switching device with three terminals: emitter (E), Base 1 (B1), and Base 2 (B2). Unlike typical transistors, it contains only one pn-junction.

Types of UJT Transistor

Original UJT (2N2646):

The classic UJT has a single pn-junction and three terminals: an emitter (E), and two bases (B1 and B2). It is used in oscillator circuits, timing circuits, and for triggering thyristors and triacs.
Programmable UJT (PUT):

The PUT is a type of transistor that can be programmed by the user to have different intrinsic standoff ratios. It is similar in operation to the standard UJT but offers more flexibility. It is often used in similar applications as the original UJT but with added versatility.

Application

• Oscillators: UJTs are often used in oscillator circuits, such as sawtooth waveform generators and relaxation oscillators. These circuits are useful in timing applications and generating clock pulses.

• Pulse Generation: UJTs are employed in pulse generation circuits where they can create sharp pulses necessary for triggering other components, such as thyristors and triacs in power control applications.

• Timing Circuits: UJT-based timing circuits are used in applications requiring precise time delays. These circuits are commonly found in delay timers, pulse generators, and timing circuits in various electronic devices.

• Phase Control: In phase control applications, UJTs can be used to control the phase of AC signals. This is particularly useful in light dimmers, motor speed controllers, and other similar applications.

• Triggering Devices: UJTs serve as triggering devices for silicon-controlled rectifiers (SCRs) and triacs. This is crucial in power control applications where precise control of switching devices is needed.

• Voltage Regulation: UJTs are sometimes used in voltage regulation circuits, where they help maintain a stable output voltage despite variations in input voltage or load conditions.

• Sawtooth Waveform Generators: UJTs can be used to generate sawtooth waveforms, which are essential in applications like function generators, television sweep circuits, and other signal processing equipment.

• Analog Signal Processing: In some analog signal processing applications, UJTs are used for their unique switching properties to shape and modify analog signals.

What is BJT Transistor

The Bipolar Junction Transistor (BJT) is a semiconductor device featuring three terminals: emitter (E), base (B), and collector (C). It operates by utilizing both electrons and electron holes as charge carriers, hence the term "bipolar." BJTs are pivotal components in electronic circuits, serving as switches and amplifiers.

Working Principle

Control of the current flowing from emitter to collector is achieved by regulating the current at the base terminal. A small current injected into the base controls a larger current flowing between emitter and collector. This characteristic enables amplification or switching functionality.

Main Differences between UJT and BJT Transistor

The Unijunction Transistor (UJT) and Bipolar Junction Transistor (BJT) are distinct types of transistors with unique structures and operational principles. The UJT is a three-terminal semiconductor switch consisting of a lightly-doped N-type silicon bar with an alloyed P-region, forming a single PN junction and featuring an emitter (E) and two base terminals (B1 and B2). It operates based on the varying emitter-base resistance, triggering a current surge when specific voltage conditions are met. This device primarily involves majority carriers and is used in applications such as thyristor triggering, oscillators, and pulse generators.

In contrast, the BJT is a three-layer, two PN-junction semiconductor device with an emitter (E), base (B), and collector (C) configuration, where the base current controls the emitter-collector current flow. It utilizes both majority and minority carriers, making it suitable for a wide range of applications, including amplifiers, switches, digital logic circuits, and temperature sensors. The fundamental difference lies in the number of PN junctions and the charge carriers involved, with the UJT having a single PN junction and relying on majority carriers, whereas the BJT has two PN junctions and uses both majority and minority carriers.

Conclusion

In summary, Unijunction Transistors (UJT) and Bipolar Junction Transistors (BJT) are both crucial components in electronic circuits, yet they serve different purposes and operate on distinct principles. The UJT, with its single PN junction and three-terminal configuration, excels in applications such as oscillators, pulse generation, and timing circuits, leveraging majority carriers to function as a reliable switching device. On the other hand, the BJT, characterized by its three-layer structure and two PN junctions, utilizes both majority and minority carriers to effectively amplify or switch currents, making it indispensable in amplifiers, switches, and digital logic circuits. Understanding the unique features and applications of each type of transistor allows for more efficient and effective design and implementation of electronic systems.