NPN Transistor

The most often used bipolar junction transistor is an NPN transistor, which is made by sandwiching two N-type semiconductors between a P-type semiconductor. The collector, emitter, and base are the three terminals of an NPN transistor. The NPN transistor functions similarly to two diodes in a PN junction that are connected back-to-back. The collector-base junction and base-emitter junction are the names given to these back-to-back PN junction diodes. With respect to the NPN transistor's three terminals, the Emitter is an area that provides charge carriers to the Collector through the Base region. The majority of the charge carriers released by the emitter are collected by the Collector area. The Base region initiates and regulates the current flowing from the Emitter to the Collector.

Table of Contents

• What is a NPN Transistor
• Construction of NPN Transistor
• NPN Transistor Symbol
• Working Principle of an NPN Transistor
• Operating Mode of Transistor
• Applications of NPN Transistor
• Some Drawbacks of NPN Transistors
• Conclusion

What is a NPN Transistor

A popular kind of bipolar junction transistor is an NPN transistor, which has two N-type semiconductor layers on either side of a P-type semiconductor layer. It can serve as an electronic switch or an amplifier. The diagram illustrates the connection between voltage sources: A load resistance (R_L) restricts the maximum current flow by connecting the collector to the positive terminal of  V_CC. With base resistance RB, the base terminal is linked to the positive terminal of the base supply voltage V_B. The maximal base current is constrained by the base resistance. When the transistor is turned on, a lower base current flowing into the base terminal drives a larger collector current.

NPN Transistor
Image used courtesy of electronicsTutorial 

What is a NPN Transistor

In contrast to the base, the emitter and collector layers are broader. The emitter has a lot of doping. As a result, a lot of charge carriers can be injected into the base. In contrast to the other two sections, the base is extremely thin and only slightly doped. The majority of the charge carriers that are released by the emitter are sent to the collector. Charge carriers from the base layer are collected by the collector, which is mildly doped.

NPN Transistor Symbol

The graphic below depicts the NPN transistor's symbol. The arrowhead displays the collector current (I_C), base current (I_B), and emitter current (I_E) in their typical directions.

NPN Transistor Symbol

Working Principle of an NPN Transistor

The supply voltage V_EE causes the base-emitter junction to be forward biased, whereas the supply voltage V_CC causes the collector-base junction to be reverse biased.

Operation of NPN Transistor
Image used courtesy of electrical4u 

The N-type semiconductor (Emitter) is linked to the supply source's negative terminal (V_EE) under forward bias conditions. Similarly, the N-type semiconductor (Collector) is linked to the positive terminal of the supply source (V_CC) in a reverse bias situation. Compared to the depletion zone of the collector-base junction, the emitter-base region's depletion region is thinner. It should be noted that the depletion region acts as a barrier that prevents current passage and is characterized by the absence of mobile charge carriers. Electrons make up the majority of the charge carriers in an N-type emitter. Electrons thus begin to move from an N-type emitter to a P-type base. Additionally, current will begin to flow via the emitter-base junction due to electrons. Emitter current IE is the name given to this current. The base, a narrow, weakly doped P-type semiconductor with few recombination holes, is where electrons migrate. As a result, very few electrons recombine, and the majority avoid the base. Base current, or I_B, is the current that flows through the circuit as a result of recombination. In comparison to the emitter current, the base current is quite low. It usually accounts for 2–5% of the emitter current overall. In a collector-base junction, the majority of electrons flow via the collector region after passing through the depletion zone. The collector current, or I_C, is the current passing through the leftover electrons. In comparison to the base current, the collector current is high.

Operating Mode of Transistor

Cut-off Mode: Both junctions are in reverse bias while in cut-off mode. The transistor operates as an open circuit in this mode. Additionally, it will prevent the device's electricity from flowing.

Saturation Mode: Both junctions are connected in forward bias when a transistor is in a saturation state. When the base-emitter voltage is high, the transistor operates as a closed circuit, with current flowing from collector to emitter.

Active Mode: The collector-base junction is reverse biased and the base-emitter junction is forward biased in this transistor mode. The transistor functions as a current amplifier in this mode.

The amount of current that flows between the emitter and collector is proportional to the base current.

Applications of NPN Transistor

• Voltage Regulation: NPN transistors can be used in certain configurations to provide voltage recommendations, which will increase the yields of a stable power supply.
• Current Regulation: By monitoring applications in current sources and steady current circuits, NPN transistors can be used to regulate or control the flow of current in a circuit.
• Amplification: In electronic circuits, NPN transistors are typically used to increase signals. They are crucial components of sound enhancers, which are devices that help power speakers with feeble information streams.
• Amplitude modulation, a technique used in basic correspondence, involves NPN transistors in circuits with many changes (AM).
• Switching: In sophisticated circuits, NPN transistors serve as electronic switches. It is possible to allow a massive current to flow between the producer and the gatherer by managing the base current.

Some Drawbacks of NPN Transistors

Though NPN transistors are cost-effective and smaller in size, they have some drawbacks also:

• Sign Reversal: The resultant signal is frequently inverted with respect to the information signal, which in some applications may necessitate additional hardware for signal correction.
• Heat Generation: Extra-intensity scattering systems may be needed in high-power applications due to the heat generated by NPN semiconductors during operation.
• Temperature Sensitivity: NPN semiconductors' properties are sensitive to temperature variations, which may impact how they behave.
• Design Complexity: Complex circuit layouts and biasing strategies may be needed for high-level applications, which can be challenging for novices.

Conclusion

NPN transistors have a big and diverse influence on electronic circuits. The current flowing from the emitter to the collector is increased when the electric field eliminates most of the electrons that are able to pass the emitter-base junction across the collector-base junction.The base current controls the flow of electrons from the emitter through the base and into the collector, which is how the NPN transistor works. Nevertheless, despite several limitations, this invention was an innovative strength in electronics.