A unijunction transistor (UJT) and a bipolar junction transistor (BJT) are both types of semiconductor devices used in electronic circuits, but they have distinct differences in terms of structure, operation, and applications. Let's explore these differences:
Unijunction Transistor (UJT): A UJT is a three-terminal semiconductor device that consists of a bar of N-type material with a P-type layer diffused into its center. The two ends of the N-type material serve as the base terminals (B1 and B2), and the P-type region acts as the emitter terminal (E). This unique structure is responsible for its name "uni-junction" as it has only one junction (P-N junction) inside the device.
Bipolar Junction Transistor (BJT): A BJT, on the other hand, is a three-terminal semiconductor device with two P-N junctions. It has three layers: the emitter (P-type), base (N-type), and collector (N-type) regions. BJTs come in two types: NPN (N-type emitter, P-type base, and N-type collector) and PNP (P-type emitter, N-type base, and P-type collector).
Unijunction Transistor (UJT): The operation of a UJT is primarily based on the electrical characteristics of the N-type material and the P-type region. When a voltage is applied to B1 terminal with respect to B2 terminal, it forward biases the P-N junction between B1 and the P-type region. As the voltage increases, eventually, the UJT reaches a point called the "peak point." At this peak point, the device conducts and allows current flow between the emitter and B1 terminal.
Bipolar Junction Transistor (BJT): The operation of a BJT depends on the flow of majority charge carriers (electrons for NPN and holes for PNP) between the emitter and the base regions. When a small current is allowed to flow from the base to the emitter (or vice versa), it controls a much larger current flow from the collector to the emitter (controlled by the base current). BJTs can operate in both the active region (amplification) and the cutoff/saturation region (switching).
Unijunction Transistor (UJT): UJTs are primarily used as relaxation oscillators, pulse generators, and timing circuits. They find applications in trigger circuits, switching devices, and in some cases as voltage-controlled oscillators.
Bipolar Junction Transistor (BJT): BJTs are widely used in a variety of electronic circuits. NPN and PNP transistors are commonly employed in amplification, switching, signal modulation, digital logic circuits, and power control applications. They are the fundamental building blocks of many electronic devices, such as audio amplifiers, radio receivers, digital logic gates, and voltage regulators.
In summary, a UJT is a three-terminal device with one P-N junction, mainly used in relaxation oscillator circuits, while a BJT is also a three-terminal device but has two P-N junctions and finds extensive use in amplification and switching applications.