A transistor is a sandwich of one type of semiconductor (P-type or n-type) between two layers of other types.

Transistors are classified into two types;
1. pnp transistor
pnp transistor is obtained when a n-type layer of silicon is sandwiched between two p-type silicon material.

2. npn transisitor
npn transistor is obtained when a p-type layer of silicon is sandwiched between two n-type silicon materials.

Figure below shows the schematic representations of a transistor which is equivalent of two diodes connected back to back.

The three portions of transistors are named as emitter, base and collector. The junction between emitter and base is called emitter-base junction while the junction between the collector and base is called collector-base junction.

The base is thin and tightly doped, the emitter is heavily doped and it is wider when compared to base, the width of the collector is more when compared to both base and emitter.

In order to distinguish the emitter and collector an arrow is included in the emitter. The direction of the arrow depends on the conventional flow of current when emitter base junction is forward biased.

In a pnp transistor when the emitter junction is forward biased the flow of current is from emitter to base hence, the arrow in the emitter of pnp points towards the base.

Operating regions of a transistor

A transistor can be operated in three different regions as
a) active region
b) saturation region
c) cut-off region

Active region
The transistor is said to be operated in active region when the emitter-base junction is forward biased and collector –base junction is reverse biased. The collector current is said to have two current components one is due to the forward biasing of EB junction and the other is due to reverse biasing of CB junction. The collector current component due to the reverse biasing of the collector junction is called reverse saturation current (ICO or ICBO) and it is very small in magnitude.

Saturation region

Transistor is said to be operated in saturation region when both EB junction and CB junction are forward biased as shown. When transistor is operated in saturation region IC increases rapidly for a very small change in VC.

Cut-off region

When both EB junction and CB junction are reverse biased, the transistor is said to be operated in cut-off region. In this region, the current in the transistor is very small and thus when a transistor in this region it is assumed to be in off state.