Commonly a Transistor has two junctions (Emitter-Base and Collector-Base Junctions), and each of these two junctions may be either Forward Biased or Reverse Biased.
Therefore, there are Four Possible configurations can be made to bias these Junctions.
different modes of operation of transistor

Accordingly transistor may operate in different conditions.

It is easy to understand the various configurations of Transistors and the region of operation by take a look on the Tubular Column.

sl.no. Biasing Condition Emitter Base Junction Collector Base Junction Operation Region
1 Forward-Reverse Forward Biased Reverse Biased Active
2 Forward-Forward Forward Biased Forward Biased Saturation
3 Reverse-Reverse Reverse Biased Reverse Biased Cut-off
4 Reverse-Forward Reverse Biased Forward Biased Inverted

1. Forward-Reverse Biasing

active mode of transistor

It is the normal biasing of a transistor in which the EMITTER-BASE Junction is Forward Biased and the COLLECTOR-BASE Junction is Reverse Biased.
This configuration is commonly called as FR biasing.
In this mode the transistor operates in Active region

2. Forward-Forward Biasing

transistor in saturation mode

In this biasing, both the emitter and collector junctions are Forward Biased.
In this mode the transistor operates in Saturation region

3. Reverse-Reverse Biasing.

transistor cutoff mode

As the name denotes both the junctions are in Reverse Biased condition.
In this biasing, the transistor has practically Zero current because the emitter doesnot emit charge carriers into the base and no charge carriers are collected by the collector expect a few thermally generated minority carriers.
Thus the transistor acts like an open Switch.

4. Reverse-Forward Biasing

In this biasing, the Emitter Junction is Reverse biased while the collector Junction is Reverse biased.
The collector is not doped to the extent as the Emitter is doped, Therefore it cannot inject the majority charge carriers to the base.
So in this region, the action of transistor is poor.

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Comparison Between Three Common Transistor Configurations

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