A bipolar junction transistor (BJT) is a current-controlled three-terminal semiconductor device that consists of two p-n junctions formed by sandwiching either p-type or n-type semiconductor material between a pair of opposite-type semiconductors. Both electrons and holes, participate in the current-conduction process, hence the name bipolar. The BJT has a very low input resistance and is ideal to be use in amplifiers, oscillators, electronic switches, power converters, temperature sensors, clipping circuits, digital circuits, etc.

Basic Construction of BJT

The two pn-junctions in a BJT can be considered as a combination of two back-to-back connected diodes. Based on the physical arrangement of the p-type and n-type semiconductor materials, there are two types of bipolar junction transistor

• PNP Transistor
• NPN Transistor

A PNP transistor consists of two p-type semiconductors which are separated by a thin layer of n-type material. An NPN transistor consists of two n-type semiconductor materials which are separated by a thin layer of p-type semiconductor. 

A BJT (NPN or PNP) comprises of three regions of doped semiconductors - Emitter, Base, and Collector.

• Emitter: The emitter is a heavily doped region responsible for supplying charge carriers (electrons in NPN or holes in PNP) in the transistor. The emitter-base junction is always forward-biased so that it can supply large numbers of majority charge carriers to the Base. 
• Base: The base is the lightly doped middle, thin region of the transistor. The base-emitter region is always forward-biased so it offers a very low resistive path for the emitter circuit. The base-collector region is always reverse-biased so it offers a high resistive path for the collector circuit.
• Collector: It collects the charge carriers at the other side of the transistor. The doping level of a collector lies between the doping level of the emitter and the base. The area of the collector is larger than the emitter and the base.

The arrow in the circuit symbol of the PNP and NPN transistor is always present on the emitter terminal and indicates the direction of “conventional current flow” between the base terminal and the emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly as for the standard diode symbol.

Modes of Operation of BJT

When an external voltage is applied to a transistor, it is said to be biased. A BJT has different modes of operation depending on the bias condition (forward or reverse) of its emitter-base junction (EBJ) and the collector-base junction (CBJ).

• Cut-Off Mode: When both the junctions of the transistor are in reverse bias, no current flows through the device, and the transistor is said to be in cut-off mode. The transistor is in OFF mode and acts like an open switch.
• Active Mode: In this mode, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased. This mode is used for amplification of current. 
• Reverse Active Mode: In this mode, the emitter-base junction is reverse-biased and the collector-base junction is forward-biased. This mode has very limited application but is conceptually important. 
• Saturation Mode: When both the junctions of the transistor are in forward bias, current flows through the device, and the transistor is said to be in saturation mode. The transistor is in ON mode and acts like a closed switch.

Working Principle of NPN Transistor

For an NPN transistor, the emitter-base junction is forward-biased by the DC source V_EB, and the collector-base junction is reverse-biased by the DC source V_CB. This reduces the width of the depletion region in the emitter-base junction and increases the width of the depletion region in the collector-base junction.

• When the emitter-base junction is forward biased, the majority of carriers from the n-type region, i.e., electrons start flowing towards the p-type base region as shown in the figure. This flow of charge carriers constitutes the emitter current, I_E. 
• Since the p-type base region is thin and lightly doped, it has very few majority carriers i.e., holes in that region. The electrons that are injected from the emitter to the base get recombined with the holes of the base region. This constitutes the base current, I_B. 
• A large number of remaining electrons cross the base region and reach the collector region where they are attracted by the positive potential of V_CB. This forms the collector current, I_C. 

In an NPN transistor, since the electron component is much larger than the hole component, the emitter current will be dominated by the electron component. The emitter current is the sum of the base current and collector current.

Working Principle of PNP Transistor

For a PNP transistor, the emitter-base junction is forward-biased by the DC source V_EB, and the collector-base junction is reverse-biased by the DC source V_CB. This reduces the width of the depletion region in the emitter-base junction and increases the width of the depletion region in the collector-base junction.

• When the emitter-base junction is forward biased, the majority of carriers from the p-type region, i.e., holes start flowing towards the n-type base region as shown in the figure. This flow of charge carriers constitutes the emitter current, I_E. 
• Since the n-type base region is thin and lightly doped, it has very few majority carriers i.e., electrons in that region. The holes that are injected from the emitter to the base get recombined with the electrons of the base region. This constitutes the base current, I_B. 
• A large number of remaining holes cross the base region and reach the collector region where they are attracted by the negative potential of V_CB. This forms the collector current, I_C. 

In a PNP transistor, since the hole component is much larger than the electron component, the emitter current will be dominated by the hole component. 

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