What is NPN Transistor? Working, Applications, and Benefits

What is NPN Transistor? . An NPN transistor is one of the two main types of bipolar junction transistors (BJT), widely used in electronic circuits. It consists of two n-type semiconductors sandwiching a p-type semiconductor. The NPN is primarily used for switching and amplifying electrical signals. Its ability to control current flow makes it essential in numerous electronic devices.

In simpler terms, an NPN allows current to flow from the collector to the emitter when a small current is applied to the base terminal. Due to its reliability and efficiency, it finds applications in power regulation, signal processing, and electronic control systems.

Read More : What is PNP Transistor?

NPN Transistor

NPN transistors are a type of bipolar transistor with three layers that are used for signal amplification. It is a device that is controlled by the current. A negative-positive-negative transistor is denoted by the abbreviation NPN. A p-type semiconductor is fused between two n-type semiconductor materials in this configuration.

It is divided into three sections: emitter, base, and collector. In an NPN transistor, the flow of electrons is what causes it to conduct.

In the above figure, we can see an arrow pointing outwards from the emitter terminal. This indicates the direction of the flow of current through the device.

Symbol of NPN:

The following diagram depicts the NPN transistor’s symbolic representation:

Construction of NPN Transistor

The NPN transistor is built in two ways.

NPN transistors are formed when a p-type semiconductor material (either Silicon or Germanium) is fused between two n-type semiconductor materials, as we already know.

The NPN transistor is made up of a number of different components.

It is divided into three sections: emitter, base, and collector.

The emitter-base junction is the region that connects the emitter and the base region. The collector-base junction, on the other hand, is the point where the base and collector regions meet. It functions as two PN junction diodes due to the presence of two junctions in between three regions.

The levels of doping in each of the three regions are different. The emitter region has a lot of doping, while the base region also has a lot of doping. And the collector region’s doping level is moderate, falling somewhere between the emitter and the base region. Its inverse is the PNP transistor, which has a P-region sandwiched between two N-Type regions.

It’s worth noting that the emitter and collector regions cannot be switched around. The reason for this is that the collector region’s thickness is slightly greater than the emitter region. So that more energy can be dissipated.

Emitter:

One section which supplies charge carriers is called the emitter. To supply a large number of charge carriers, the emitter is always in forward biased when compared to the base.

Base:

The middle section of the transistor, which forms the two PN-junctions between emitter and collector, is called the base. The base-emitter junction is forward biased which allows a low resistance for the emitter circuit. The base-collector junction is reverse biased which provides a high resistance in the collector circuit.

Collector:

The section other than emitter which collects the charges is called the collector. The collector is always reverse biased.

Identification of the transistor can be done using the resistance of the diodes present inherently inside the NPN transistor. Also we can take the knowledge we know of the diodes to analyse the internal diodes of the transistor.

Emitter-Base terminal: There is a diode in between the emitter-base terminals so those two terminals should function as a normal diode and conduct in only one direction.

Collector-Base terminal: There is a diode similar to the collector-base terminals. These terminals again should act as the terminals of a normal diode, and conduct only in one direction.

Emitter-Collector terminals: The Emitter-Collector terminals are not connected internally and hence will not conduct in either direction.

A table showing the resistance values across the different terminals of the transistor is given below.

Between Transistor Terminals		Resistance Values
Collector	Emitter	R_high
Collector	Base	R_high
Emitter	Collector	R_high
Emitter	Base	R_high
Base	Collector	R_low
Base	Emitter	R_low

Read More : BJT vs. FET Transistors

Working of NPN Transistor

When there is no applied bias to the transistor or when there is no battery connected between its terminals. It is then referred to as the transistor’s unbiased state. We’ve already talked about how a PN junction diode works in the absence of bias. A transistor is made up of two PN junctions, as we already know.

As a result, under no biased conditions, electrons in the emitter region begin to move towards the base region due to temperature variations. However, a depletion region forms at the transistor’s emitter-base junction after a certain amount of time has passed. Only about 5% of electrons combine with holes in this region after reaching the base region, while the rest drift across the collector region. Similarly, a depletion region forms at the transistor’s base-collector junction after a period of time.

It’s worth noting that the thickness or thinness of the depletion region is determined by the material’s doping concentration. To put it another way, in the case of a lightly doped region, the width of the depletion region will be greater than in the case of a highly doped region. This is why the depletion width at the collector-base junction is wider than at the emitter-base junction. These two depletion regions serve as a potential stumbling block to any further majority carrier flow.

The following diagram depicts the biased condition of an NPN transistor:

The width of the depletion region, also called PN Junction, narrows as a result of the forward applied voltage at the emitter-base junction. Similarly, the width of the collector-base junction is widened by the reverse applied voltage. This is why, in comparison to the collector-base junction in the previous figure, the emitter-base junction has a thin depletion region.

Electrons begin to inject into the emitter region as a result of the forward applied voltage VBE. The electrons in this region have sufficient energy to overcome the emitter-base junction’s barrier potential and reach the base region.

The charge carrier movement in an NPN transistor is depicted in the diagram below:

Because the base region is very thin and doped lightly. As a result, only a few electrons combine with the holes once they reach their destination. Because of the strong electrostatic field, electrons begin to drift at the collector region due to the very thin base region and the reverse voltage at the collector-base junction. As a result, these electrons are now collected at the transistor’s collector terminal. The electrons begin to move towards the collector as recombined holes and electrons become separated from one another. A very small base current also flows through the device as a result of this movement. This is why the emitter current is equal to the sum of the base and collector currents. IE = IB + IC

Transistor Current

The following equation shows the relation between emitter current, base current and collector current in a biased transistor.

I_E = I_B + I_C

Where:

I_E = Emitter Current
I_B = Base Current
I_C = Collector Current

It shows the the emitter current is equal to the sum of base current and collector current. The value of base to emitter current is 2 – 5% while the collector current is almost 95 – 98%. That’s why both the base and collector current is equal to the emitter current.

The transistor gain equation can be written as follow as well by using Kirchhoff’s current law.

I_E + (- I_B) + (-I_C) = 0

I_E – I_B – I_C = 0

I_B = I_E – I_C

I_C = I_E – I_B

I_E = I_B + I_C

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Transistor Gain, Current Gain, Voltage Gain & Power Gain (α, β and γ)

Transistor Gain

Transistor Gain is known as the ration between circuit output and input.

Transistor Gain = Output / Input

Current Gain:

The ratio between collector current and emitter current is known as current gain of a transistor represendt by the greek symbol alpha “α” or h_FE

Current Gain; α_DC = A_i = -I_C / I_E = I_OUT / I_IN

α_DC = I_C / I_E = Collector Current / Emitter Current

The DC written with alpha “α” symbol is for DC values. The more the value of α, the better of a transistor will be as it shows the quality of the transistor.

α_DC can be written as simple “α” and it is known as forward current transfer ratio or amplification factor which is also represented by h_FB. In h_FB, the “F” represents the “Forward” and “B” represents the “Common Base” where the the alpha (α) is generally derived from transistor’s common base circuits.

The AC alpha (α_AC) of a transistor:

α_AC = ΔI_C / ΔI_E = Change in Collector Current / Change in Emitter Current

α_AC is also known as the short circuit gain of a transistor which can be represented by h_fb.

Read More : Analog to Digital Converters (ADCs)

Good to know:

h_FB = DC alpha α_DC
h_fb = AC alpha α_AC

Also, the ratio between DC collector current and DC base current is known as current gain which is represented by the Greek symbol Beta “β“.

β_DC = -I_C / -I_B = I_C / I_B

I_C = βI_B

It is also known as common emitter DC forward transfer ratio represented by H_FE.

We use AC beta “β_AC” when analyzing a transistor for AC operations

β_AC = ΔI_C / ΔI_B

β_AC can be also represented by h_fe.

Lastly, the ratio between emitter current and base current in common collector is also known as current gain and represented by the Greek symbol Gamma “γ”

γ = I_E/ I_B

putting the value of I_E in the above equation from “I_E = I_C + I_B“

γ = β +1

Voltage Gain

The ratio between input and output voltage is known as the voltage gain of a transistor.

Voltage Gain = A_V = α I_E R_CB / I_E R_EB

Voltage Gain = A_V = Voltage across R_CB / Voltage across R_EB

A_V = α x (R_CB / R_EB)

A_V = V_OUT / V_IN

Power Gain

Transistor Power gain can be calculated by the following equation.

Power Gain = A_P = P_OUT / P/_IN

A_P = α² x A_R

Where:

A_P = Power Gain
α = Current Gain
A_R = Resistance Gain

The overall expressions for relation between alpha, beta and gamma (α β & γ) are given below:

α = β / ( β + 1 )
β = α / (1-α)
γ = β +1

Characteristics Curve & Operation Regions of NPN Transistor

There are mainly four regions of operations of a BJT transistor namely:

Active Region
Cutoff Region
Saturation Region
Breakdown region

Active Region: It is known as the normal operation of a transistor. Or the region between saturation and breakdown region is known active region.

Cutoff Region: A region where the value of base current I_B becomes zero and make the first (or lower) curve is known the cutoff region of a transistor. In this region, both the emitter-base diode and collector-base diodes become and operate in reverse bias.

Saturation Region: In this region, both the emitter-base diode and collector-base diodes become and operate in forward bias. It is the initial slop (or almost perpendicular) area near to the origin (in the curves) when the initial voltage increases from zero to 1 and so on.

Breakdown Region:

When the collector voltage increases too much crossing the rated value, it leads to breakdown the collector diode. For this reason, a transistor should not be operated in breakdown region as it will damage and destroy the transistor circuit.

The graph between the collector current and the collector-emitter voltage with varying base current is called the output characteristics curve of bipolar transistor. The figure below show the output characteristics curves of a NPN transistor.

The transistor is ON when there is a small current through the base terminal and small positive voltage relative to the emitter terminal. Otherwise the transistor is OFF. This is reflected in the graph as well. The collector current depends on the collector voltage only until the collector voltage reaches 1V level.

There is also a straight line joining points A and B. This straight line is called ‘Dynamic Load Line’. This line connects the points where V_CE = 0 and Ic = 0. The straight line and the region around it is the active region of the transistor.

Collector-Characteristics-Curve-Operation-Regions-of-a-Transistor-Common-Emitter-CE-NPN-Transistor

Given the base current and the collector voltage, the common emitter configuration characteristics are used to calculate the collector current.

Common Emitter Configuration of NPN Transistor

There are three possible configuration for the transistor, and one in the discussion is the common emitter configuration. The circuits that use common emitter configuration are generally used in voltage amplifiers. As the name suggests, we take one of the three terminals, in this case the emitter as common. This common terminal will act as both input and output to the transistor.

The voltage amplification which is achieved using the common emitter configuration can be done only in one step. So these circuits are also called single stage common emitter amplifier circuit. The input terminals as we discussed earlier is the base terminal, collector as output terminal.

The emitter remains as common terminal. The amplification process starts with biasing the base-emitter junction forward. This means that there is a greater positive potential at the base terminal than at emitter terminal. This process will enable us to control the current flow in the transistor.

As the output required has to have amplification, we use the common emitter amplifier which has a very high gain even though the output is inverted. Due to the dependencies of the diode characteristics on the ambient conditions, the gain is very much influenced by the surrounding temperature and the bias current.

This is the most commonly used configuration of the NPN transistor because it has a very low output impedance and it also provides a high input impedance. This configuration also has a very high power and voltage gain.

The typical value of the current gain for this configuration is around 50. This configuration is generally used where there is a low frequency amplification to be done. The radio frequency circuits also use this configuration. The common emitter amplifier configuration is shown below.

Common-Emitter-CE-Amplifier-by-NPN-Transistor

NPN Transistor in Amplification Circuits

Like in the push pull transistor circuit. Although all the configurations of the push-pull amplifier can technically be called push-pull amplifier, only the Class B amplifier is the actual push-pull amplifier. In contrast to Class A amplifier, Class B amplifier has two transistors for the push-pull electrical action of which one is NPN and the other is PNP.

Each transistor will work for one half of the cycle of the input producing the necessary output. This improves the efficiency of the Class B amplifier many times higher than the Class A amplifier. The conduction angle for this amplifier is 180 degrees, because each transistor works for one half only.

NPN Transistor as a Switch & Inverter

in digital logic circuit, a BJT transistor works in saturation and cut-off region as bias voltage is not provided to the base of the transistor in logic circuits. This way, the low or high output voltage are gain which can be used for switching purposes in those digital logic circuits.

The following circuit shows an inverter NPN circuit used for switching purpose.

It clearly shows that there is no input biasing voltage at the base but a square wave form as input provided through a resistor connected in series to the base of the the NPN transistor which working as an inverter.

The circuit shows that both the V_CC and input high level value si +5V where the voltage between collector and emitter V_CE is output voltage.

When the input voltage is high i.e. +5V:

The base – emitter junction is forward bias.
Current flows to the base through the series resistor R_B.
The value of R_B and R_C provide I_B current which situates the circuit i.e transistor operates in situation region.

In other words, when the input to the inverter is high “+5V”, the transistor saturated and its output is low “≈0V”. When the input to the inverter is low, the transistor is cut-off and its output us high. In short,

In saturate region, it is “ON”
In cut-off region, it is “OFF”.

As shown by the circuit input and output i.e when the input is low, the output is high, that’s why it is also known as BJT inverter circuit.

The above explanation of ON and OFF operation of transistor inverter circuit is similar to the close and open switch connected between collector and emitter. That’s the reason a transistor inverter circuit is also known as a transistor switch.

When the transistor is in saturation mode, the voltage between collector and emitter is zero same like the voltage across the closed or ON switch and the amount of current is maximum.

Similarly, In cut-off region (when transistor is Open of OFF), the value of flowing current from collector to the emitter becomes zero same like an open or OFF switch and the voltage across the switch is maximum.

The ON-OFF operation of the circuit depends on the values of input voltages i.e. when

Input Voltage is High = +5V = Switch is ON
Input Voltage is Low = 0V = Switch is OFF

The following fig shows the switching operation in cutoff and saturation regions of BJT (NPN transistor).

Applications of NPN diode:

Transistors with NPN Diodes (NPN) are used in a variety,

High-frequency applications make use of these.
Switching applications are where NPN transistors are most commonly used.
This component is used in amplifying circuits.
To amplify weak signals, it’s used in Darlington pair circuits.
NPN transistors are used in applications where a current sink is required.
Some classic amplifier circuits, such as ‘push-pull’ amplifier circuits, make use of this component.
In temperature sensors, for example.
Applications with extremely high frequency.
In logarithmic converters, this variable is used.
Because signal amplification is done with NPN transistors. In amplifying circuits, it is used in this way.
Logarithmic converters are another area where it is used.
The switching characteristic of the NPN transistor is one of its most significant advantages. As a result, it’s commonly used in switching applications.

NPN transistor terms that are important to know:

Region of the emitter: It is the largest section of the structure, which is larger than the base region but smaller than the collector region. It has a lot of doping in it. It is used to transfer majority carriers into the base region, which are electrons. It is a forward-biased region, which means it is always provided with the base region forward biased.

Region of the base: The base region is located in the middle of the structure. In comparison to the transistor’s emitter and collector regions, it has a small region. It is lightly doped to ensure that there is minimal recombination and a high current at the collector.

Region of the collector: It is the structure’s rightmost section, and its function is summed up in its name: it collects the carriers transferred by the base region. When compared to the base region, this region receives reverse biassing.

Read More : Transistor vs. Thyristor (SCR)

Advantages of NPN Transistors

The NPN has several advantages over other electronic components:

High Efficiency: Handles high current and voltage efficiently.
Cost-Effective: Widely available and affordable for most electronic projects.
Fast Switching: Suitable for high-frequency circuits.
Simple Design: Easier to use in circuits with straightforward connections.
Versatile Applications: Used in analog, digital, and power circuits.

These advantages make NPN a cornerstone of modern electronics.

Disadvantages of NPN

Despite their advantages, NPN also have some limitations:

Base Current Requirement: Requires a small base current to initiate conduction.
Limited Power Handling: High power can cause overheating.
Sensitivity to Temperature: NPN may not perform well in extremely high temperatures.
Complex Configurations: Requires careful biasing in some circuits.

Frequently Asked Questions on NPN Transistor

1. What are the three terminals of the transistor?

Emitter, base and collector.

2. What are the majority charge carriers in NPN transistors?

Electrons.

3. How does current flow in NPN transistor?

In an NPN transistor, the current flows from the emitter to the collector. The base terminal controls the flow of current through it.

4. Define emitter current.

Sum of the base current along with the collector constitutes emitter current.

5. What are the operative modes of a transistor?

Cut-off mode
Saturation mode
Active mode