Bipolar Junction Transistor

What is Bipolar Junction Transistor?

A bipolar junction transistor is a BJT. A bipolar junction transistor is a three terminal semiconductor device that acts as a static switch or amplifier. And BJT is also called transistor. The name BJT, stands for Transistor, is derived from two words 'transfer' and 'resistor'. Because these two words can describe the operation of a BJT, which is the transfer of a signal from a low-resistance circuit to a high-resistance circuit. Basically a bipolar transistor is a 3 terminal semiconductor device. It consists of two types of semiconductor material, p-type (positive type) and n-type (negative type), and current flows through these regions.

Construction of Bipolar Junction Transistor

A bipolar junction transistor is formed by sandwiching a p-type or n-type semiconductor between two layers of n-type or p-type semiconductor as shown in the figure. A metal contact is connected to each of the three layers that act as the terminals of the transistor. The three semiconductor layers of a bipolar junction transistor are named emitter (E), base (B) and collector (C). The emitter is the region that emits the charge carriers, the base is the region that acts as a conductor, and the collector is the region that collects the charge carriers. The emitter is a medium-sized and highly doped region, the base is the smallest and lightly doped region, and the collector is the largest in size and lightly doped region. A BJT consists of two pn-junctions, an emitter-base junction and a collector-base junction. Therefore, a BJT is a two-junction, three-terminal semiconductor device.

BJT is has three terminals like base terminal, emitter terminal and collector terminal. The emitter of a BJT has an arrow symbol and the direction of the arrow indicates the direction of current flow.

Emitter Terminal

The emitter terminal is the charge carrier in a PNP transistor – holes and electrons are emitted in an NPN transistor. The emitter is the most heavily doped of the three terminals. The base terminal is always reverse-biased with respect to the other terminals to produce a large majority carrier. The emitter-base junction is forward-biased for PNP and NPN transistors. The emitter terminal provides electrons and holes respectively towards the EB-junction in NPN and PNP transistors.

Collector Terminal

The collector is the opposite side of the emitter terminal and collects the emitted charge carriers. So this terminal is heavily doped. The CB-junction is kept reverse-biased across both transistors. The collector terminal of the transistor collects the electrons emitted by the emitter terminal in an NPN transistor, while it collects the holes emitted by the emitter terminal in a PNP transistor.

base Terminal

The base terminal is the middle part between the collector and emitter terminals and forms two PN junctions between them. The base of a BJT is the most lightly doped section. Therefore, the base controls the charge carrier current through the emitter and collector terminals.

Types of BJT

NPN Bipolar Junction Transistor
PNP Bipolar Junction Transistor

NPN Bipolar Junction Transistor

An NPN bipolar junction transistor is made by sandwiching a layer of p-type semiconductor between two layers of n-type semiconductor. In case of NPN BJT, two n-type semiconductor layers act as emitter and collector respectively. A p-type semiconductor layer acts as a base.

PNP Bipolar Junction Transistor

A PNP bipolar junction transistor consists of a layer of n-type semiconductor sandwiched between two layers of p-type semiconductor. In a PNP transistor, two p-type semiconductor layers act as emitter and collector, respectively, and an n-type semiconductor layer acts as the base of the transistor.

Operation of Bipolar Junction Transistor

Basically bipolar junction transistors operate in three regions, first active region, second saturation region and third cut off region. As we have already discussed that BJT consists of two PN-junctions namely emitter-base (EB) junction and collector-base (CB) junction.

active region

When one junction of a BJT is forward-biased and the other is reverse-biased, the BJT is said to be conducted in the active region. In the active region, the bipolar junction transistor acts as an amplifier. When a small base current flows, we get a 'β' amplified collector current. current gain of the transistor. The magnitude of the collector current can be expressed by the following mathematical expression. And active region is also called linear region which is between cutoff and saturation region of BJT. The amplification operation of this transistor takes place in this region.

saturation region

When both PN junctions of a bipolar junction transistor are forward-biased, in this case, the BJT operates in the saturation region. A transistor operating in saturation region acts as an off switch and conducts current. In the saturation region, maximum current flows between emitter and collector (saturation current 'ISAT').

Ic = Isat

where Ic is the collector current

Iset is the BJT saturation current

Cut off region

When both PN junctions are reverse-biased, the BJT is said to operate in the cut-off region and conduct no current. Hence, the bipolar junction transistor acts as an open switch in the cut-off region. Cutoff region operation is the opposite of BJT operation in the saturation region. In the saturation region, the collector current is the highest, while in the cutoff region no collector current flows. The cutoff region can be obtained when the voltage from the base to the emitter junction is less than 0.7 volts.

Ic=0

where Ic is the collector current

Configuration of Bipolar Junction Transistor

A bipolar transistor can be connected in three configurations. they are,

Common Emitter Configuration - This configuration includes both voltage and current gains.
Common Collector Configuration - The configuration has no voltage gain but current gain.
Common Base Configuration - This has no current gain but voltage gain.

Applications of Bipolar Junction Transistor

Bipolar junction transistors are widely used in many electronic devices and systems.
A bipolar junction transistor is used as a switch, as an amplifier, as a filter and as an oscillator.
A bipolar junction transistor is used as a solid-state switch.
Bipolar junction transistors are used to realize logic gates etc.
A bipolar junction transistor is used as a detector for demodulation of the modulated signal.
Bipolar junction transistors are used for clamping circuits.