BJT or Bipolar Junction Transistor is a type of transistor that is bipolar and has a junction. Bipolar means that it uses both types of charge carriers i.e. electrons and holes. While the junction refers to the boundary between two different semiconductor materials usually known as the PN junction. A BJT is made of three alternating layers of P-type and N-type semiconductor materials having two PN junctions. It has 3 terminals i.e. Emitter, Base, and Collector. Each terminal is connected to each layer of the transistor.
The base is the middle layer sandwiched between Emitter and Collector. The base is the most lightly doped layer of all. The emitter and collector are both heavily doped with the emitter comparatively more heavily doped than the collector.
Features of Bipolar Transistor
- The ability to control the current flowing between the collector and emitter terminals by applying a voltage to the base terminal.
- It can be used in both amplifying and switching applications
- High current and voltage handling capability
- Low input impedance, high output impedance
- It can be used in both Common Emitter and Common Base configurations.
- Have a high input impedance when used in Common Base configuration.
- Have a low input impedance when used in Common Emitter configuration.
Types of Bipolar Junction Transistors
- PNP Transistor
- NPN Transistor
NPN Transistor
An NPN transistor is a type of bipolar junction transistor (BJT) that consists of two n-type semiconductor layers separated by a thin p-type layer. The n-type layers are called the collector and emitter, and the p-type layer is called the base.
PNP Transistor
A PNP transistor is a type of bipolar junction transistor (BJT) that consists of two p-type semiconductor layers separated by a thin n-type layer. The p-type layers are called the collector and emitter, and the n-type layer is called the base.
Bipolar Junction Transistor Symbol
Working of Bipolar Junction Transistor
The working of a BJT (Bipolar Junction Transistor) is based on the flow of current through its three layers of semiconductor material: the base, collector, and emitter.
In an NPN transistor, when a positive voltage is applied to the base terminal, it causes the base-emitter junction to become forward-biased, which allows a small current to flow through the base terminal and into the base-emitter junction. This in turn causes a larger current to flow from the collector to the emitter terminals, because the base-emitter junction is now acting as a switch, allowing current to flow from the collector to the emitter.
In PNP transistor works in the opposite way of NPN transistor, but the principle is the same. A small current applied to the base terminal causes a larger current to flow between the collector and emitter terminals.
Applications of BJT
- Amplifiers
- Oscillators
- Digital logic gates
- Switching
- Power supply regulators