A Bipolar Junction Transistor (BJT) is a type of transistor that consists of three distinct layers of semiconductor material. These layers are arranged to form two pn-junctions. The three layers of a BJT are:
1. **Emitter**: This is the layer that emits charge carriers (electrons or holes) into the base region. It is heavily doped to ensure a high concentration of charge carriers. In an NPN transistor, the emitter is made of n-type material, while in a PNP transistor, it is made of p-type material.
2. **Base**: The base is the thin, middle layer that lies between the emitter and the collector. It is lightly doped compared to the emitter and collector. The base is crucial for controlling the flow of charge carriers between the emitter and collector. In an NPN transistor, the base is made of p-type material, and in a PNP transistor, it is made of n-type material.
3. **Collector**: This is the layer that collects the charge carriers from the base. It is moderately doped and is typically larger than the emitter. In an NPN transistor, the collector is made of n-type material, while in a PNP transistor, it is made of p-type material.
### Configuration of BJTs
1. **NPN Transistor**:
- Emitter: N-type
- Base: P-type
- Collector: N-type
2. **PNP Transistor**:
- Emitter: P-type
- Base: N-type
- Collector: P-type
### Functioning of a BJT
- **NPN Transistor**: When a small current flows into the base-emitter junction (forward bias), it allows a larger current to flow from the collector to the emitter. The base-emitter junction is forward-biased, while the base-collector junction is reverse-biased.
- **PNP Transistor**: When a small current flows out of the base-emitter junction (forward bias), it allows a larger current to flow from the emitter to the collector. The base-emitter junction is forward-biased, while the base-collector junction is reverse-biased.
In summary, the three layers of a BJT—emitter, base, and collector—are arranged to control and amplify electrical signals through the interaction of these layers and their doping levels.