Bipolar Junction Transistors (BJTs): Definition, Functions & Applications

A bipolar junction transistor is a three-layer semiconductor device that contains 2 p-n junctions to amplify the signal. A transistor is a three-terminal device whose output current, power, and voltage are controlled by the input current. Transistors can be classified as – bipolar junction transistors and field effect transistors. Bipolar Junction Transistors (BJTs) are used for the transfer of an input signal from a lower resistance circuit to a higher resistance circuit. The transistors have three terminals with two p-n junctions used to increase the range of the signal. The BJT is used as a linear amplifier to amplify an electrical signal and also as an electronic switch.

Key Terms: Bipolar Junction Transistor, Transistors, Semiconductors, P-N Junction, Voltage, Current, Diode, Resistance, Circuit, Signal

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What is Bipolar Junction Transistor?

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Bipolar Junction Transistors are responsible for the transfer of an input signal from a lower resistance circuit to a higher resistance circuit.

These transistors are controlled by the current. A signal of small amplitude applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Bipolar Junction Transistor requires an external source of DC power supply to carry out the amplification process.

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Operation of Bipolar Junction Transistors

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Bipolar Junction Transistor has three operating regions:

Active Region

In the active region of BJT, it acts as an amplifier.

Saturation Region

The transistor is on and operates as a switch so that the collector current is equal to the saturation current in the saturation region.

Cut-off Region

The transistor is off and the collector current is equal to zero in the cut-off region.

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Types of Bipolar Junction Transistors

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Bipolar Junction transistors are of two types:

PNP Bipolar Junction Transistor

In PNP Bipolar Junction Transistor, the n-type semiconductor is sandwiched between two p-type semiconductors.

• The n-type semiconductor acts as a base.
• The two p-type semiconductors in PNP BJTs act as emitter and collector respectively.
• Through the emitter, the current enters the transistor making the emitter-base junction forward biased and the collector-base junction reverse biased.

PNP Bipolar Junction Transistor

NPN Bipolar Junction Transistor

In NPN Bipolar Junction Transistor, a p-type semiconductor is sandwiched between the two n-type semiconductors.

• The p-type semiconductor acts as a base.
• The two n-type semiconductors act as emitter and collector respectively.

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Function of Bipolar Junction Transistors 

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Bipolar Junction Transistors are responsible for the transfer of an input signal from a lower resistance circuit to a higher resistance circuit.

• BJT is a transistor with three terminals with two p-n junctions used to increase the range of the signal. 
• Bipolar Junction Transistors have 3 doped semiconductor regions which are called base, collector and emitter and they are separated by a 2 p-n junction.
• The transistor operates as an amplifier in the active region of the Bipolar Junction Transistor. 

Bipolar Junction Transistor Notes PDF Download

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Applications of Bipolar Junction Transistors 

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Some of the applications of Bipolar Junction Transistors are – 

• Bipolar Junction Transistor is used as a switch, an amplifier, switch, filter, and oscillator.
• Demodulator, also known as Detector, uses Bipolar Junction Transistor.
• Bipolar Junction Transistor is used in logic and switching circuits.
• Bipolar Junction Transistor is also used for clipping circuits so as to shape the waves.

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Bipolar Junction Transistors: Voltage, Charge Control and Current

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Collector emitter current controls the base-emitter current. Bipolar Junction Transistor can be connected with an electric circuit in three ways as Bipolar Junction Transistor has three terminals:

• Common Emitter Configuration: It has both voltage and current gain
• Common Collector Configuration: It has no voltage gain but has a current gain
• Common Base Configuration: It has no current gain but has a voltage gain

Characteristics of Bipolar Junction Transistor

The characteristics of several transistor configurations are mentioned below:

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Things to Remember

• Bipolar Junction Transistor has three operating regions – Active region, Saturation Region, and Cut-off region.
• There are two types of Bipolar Junction Transistors – PNP Bipolar Junction Transistor and NPN Bipolar Junction Transistor.
• Bipolar Junction Transistor can be connected with an electric circuit in three ways.
• BJTs have three terminals: Common Emitter Configuration, common collector configuration, and common base configuration.
• BJT is mainly used to increase the range of the signal. 

Read More:

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Previous Year Questions

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