In NPN transistor, when emitter junction is forward biased and collector junction is reverse biased, the transistor will operate in
Active region
Transistor Operating Regions Explained
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. NPN transistors are a common type, consisting of three layers of semiconductor material: two N-type regions separated by a P-type region. These three regions are called the emitter (E), base (B), and collector (C). There are two junctions within a transistor: the emitter-base (EB) junction and the collector-base (CB) junction.
NPN Transistor Biasing and Operation
The way these two junctions are biased (either forward biased or reverse biased) determines the operating region or mode of the transistor. Each region has distinct characteristics and applications:
- Forward Biased: When the positive terminal of a voltage source is connected to the P-type material and the negative terminal to the N-type material, the junction's depletion region narrows, allowing current to flow easily.
- Reverse Biased: When the positive terminal of a voltage source is connected to the N-type material and the negative terminal to the P-type material, the junction's depletion region widens, resisting current flow significantly.
Active Region Operation
When an NPN transistor's emitter junction (the junction between the emitter and the base) is forward biased, and its collector junction (the junction between the collector and the base) is reverse biased, the transistor operates in the active region. This is the most crucial operating region for amplification purposes.
- Emitter Junction (Forward Biased): This biasing allows electrons from the N-type emitter to inject into the P-type base.
- Collector Junction (Reverse Biased): This biasing creates a strong electric field that sweeps the injected electrons from the base into the N-type collector, allowing a large collector current to flow, which is controlled by a small base current.
In the active region, a small change in the base current causes a large change in the collector current, demonstrating the transistor's ability to amplify signals. This mode is ideal for analog amplification applications like audio amplifiers.
Summary of Transistor Operating Regions
The table below summarizes the biasing conditions for each of the primary operating regions of a bipolar junction transistor (BJT), including NPN transistors:
| Operating Region | Emitter-Base (EB) Junction Biasing | Collector-Base (CB) Junction Biasing | Typical Application |
|---|---|---|---|
| Active Region | Forward Biased | Reverse Biased | Amplification |
| Cut-off Region | Reverse Biased | Reverse Biased | Open Switch (OFF) |
| Saturation Region | Forward Biased | Forward Biased | Closed Switch (ON) |
| Inverted (Reverse Active) Region | Reverse Biased | Forward Biased | Seldom used, low gain switching |
Therefore, when an NPN transistor has its emitter junction forward biased and collector junction reverse biased, it is indeed operating in the active region, making it suitable for signal amplification.
Important Questions from Bipolar Junction Transistors
Additional heat is dissipated from power transistor by using
When emitter-base junction of a transistor is reverse-biased, the collector current
Emitter follower is used for:
In the common-base configuration, the collector current is given by:
The circuit which produces the best stabilisation of an operating point is _______.
