u give the clue: bipolar junction transistor (BJT)

A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes.

Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-carrier' devices.

An NPN transistor can be considered as two diodes with a shared anode region. In typical operation

the emitter–base junction is forward biased
the base–collector junction is reverse biased

In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction

the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced
allowing thermally excited electrons to inject into the base region
These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector
The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.

The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. To ensure this

the thickness of the base is much less than the diffusion length of the electrons

The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction.

A BJT consists of three differently doped semiconductor regions

the emitter region
the base region
the collector region

These regions are, respectively, pn type and p type in a PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled:

emitter (E)
base (B)
collector (C)

The diagram shows the schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E,

V B E

must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs, but can be different depending on the current level selected for the application and the type of transistor.

$I_E = I_B + I_C\,$

In the diagram, the arrows representing current point in the direction of the electric or conventional current—the flow of electrons is in the opposite direction of the arrows since electrons carry negative electric charge. The ratio of the collector current to the base current is called the DC current gain. This gain is usually quite large and is often 100 or more.

As the applied collector–base voltage (

V B C

) varies, the collector–base depletion region varies in size. An increase in the collector–base voltage, for example, causes a greater reverse bias across the collector–base junction, increasing the collector–base depletion region width, and decreasing the width of the base. This variation in base width often is called the "Early effect" after its discoverer James M. Early.

Narrowing of the base width has two consequences:

There is a lesser chance for recombination within the "smaller" base region.
The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases.

Both factors increase the collector or "output" current of the transistor in response to an increase in the collector–base voltage.

In the forward active region the Early effect modifies the collector current ( $i C$ ) and the forward common emitter current gain ( $β F$ ) as given by the following equations:^{[citation needed]}

$i_\mathrm{C} = I_\mathrm{S} e^{\frac{v_\mathrm{BE}}{V_\mathrm{T}}} \left(1 + \frac{V_\mathrm{CB}}{V_\mathrm{A}}\right)$