Understanding NPN Transistors: Characteristics, How They Work, And More

Bipolar Junction Transistors (BJT) come in two forms, which are NPN and PNP. Some refer to N as negative and P as positive, but in real essence, they mean N-type and P-type. The most commonly used type of the two is NPN transistors because they have several benefits, such as faster switching speeds, better conductivity, better frequency response, etc.

To understand why NPN transistors have these benefits and are more popular in building electronics, let’s look at their properties, characteristics, construction, and how they work.

What Is an NPN Transistor?

An NPN transistor is a 3-layer BJT that consists of two N-type semiconductors sandwiching a P-type semiconductor, hence the name NPN. Because N-type semiconductors have electron charge carriers, the two layers provide more electrons than holes (for P-type), so this electronic component has electrons as the majority charge carriers and holes as the minority.

The inner structure of an NPN transistor

NPN Transistor Symbol and Terminals

NPN transistors are equivalent to two diodes connected with forward-biased PN junctions facing outward on either side. These components have three terminals, and the third one (base) connects to the inner P-type semiconductor. The other two terminals are the collector and emitter.

Considering the symbol, NPN transistors have the arrow pointing to the emitter terminal from the middle. In comparison, PNP transistor symbols have the arrow pointing to the middle from the emitter because the polarities are reversed.

An NPN transistor symbol with its equivalent 2-diode representation

NPN Transistor Construction

The semiconductor materials used to make NPN transistors are usually silicon or germanium, and each layer in this component is doped differently to create the required electrical switching and amplification properties.

Base regions are usually lightly doped, while emitters are heavily doped. The doping level of collectors is somewhere between these two. Depletion regions form where these semiconductors meet and these are influenced by the doping levels.

Because the heavily doped emitter has more electrons, the depletion region pushes more toward the base, making it thin. But the relatively lightly doped collector pushes less towards the base, making the joint have a thicker depletion region.

How an NPN Transistor Works

NPN transistors operate in three modes/regions depending on the biasing of the PN junctions.

• Active Mode: In active regions, the base-emitter junction is forward-biased, while the base-collector junction is reverse-biased. With this connection, NPN transistors operate as amplifiers, where the output current is proportional to the input current.

• Cutoff Mode: NPN transistors operate as switches in the off state in this cutoff region. It occurs when the base-collector and base-emitter junctions are reverse-biased, cutting current flow from the collector to the emitter terminal.

• Saturation Mode: To turn on the NPN transistor switch, you saturate its junctions by forward biasing both base-emitter and base-collector junctions. This connection allows maximum current to flow from the collector to the emitter.

Active Region

To achieve an active region, let’s consider this circuit.

An NPN circuit with two voltage sources (Vbe and Vcb)

In this configuration, the base-emitter junction is forward-biased, while the base-collector junction is reverse-biased. The depletion region at the base-emitter PN junction narrows further due to the applied forward-biasing voltage.

Therefore, electrons are injected into the emitter from the voltage source and overcome the potential of the depletion barrier to cross over to the base. But the base semiconductor is lightly doped, so it has few holes. Therefore, only a few electrons cross over and reach their destination.

This leaves an excessive supply of electrons, which creates a strong electrostatic field with the collector region due to the positive voltage applied by the battery. As a result, these electrons flow into the collector and combine with holes from the voltage source.

These holes then flow across the depletion region into the base and out through the emitter. Since we consider holes (+) as the conventional current, the output from the emitter becomes:

IE = IB + IC, which results in amplification.

This circuit is the Common Emitter (CE) configuration and is the most commonly used BJT amplifier topology. Essentially, the movement of electrons or negative charge carriers across the base makes up the transistor action.

Cutoff and Saturation Regions

To create an on/off switch using an NPN transistor, connect a voltage source to the collector (with the load in between) and ground the emitter. Next, connect the controlling voltage to the base and emitter terminals, then adjust the polarity of the voltage source connecting to the base.

An NPN switch circuit

If the input at the base is set to positive, current (holes) flows through R2, forward biasing both base-collector and base-emitter PN junctions. Remember, there is a load between V1 and Q1 so the base-collector junction won’t be reverse-biased. Therefore, electrons will flow from the emitter to the collector, while conventional current will go in the opposite direction.

Once you switch the polarity of V2, both PN junctions are reverse-biased, which increases the width of their depletion regions and current flow resistance. This turns off the device, making it operate as an insulator.

In all three scenarios, the base controls the transistor using current, which explains why NPN transistors and BJTs, in general, are current-controlled devices.

Common Emitter (CE) NPN Transistor Properties

Input Resistance

A CE NPN transistor’s input resistance to the signal flow is low because its input circuit is forward-biased (emitter-base). This resistance is defined as the small change ratio in the emitter-base voltage to the change in base current (with the emitter-collector voltage kept constant).

Output Resistance

A CE transistor’s output resistance refers to the ratio of emitter-collector voltage change to the change in the collector current when the base current is kept constant. It is usually high.

Current Gain

Current gain is the ratio of the change of the transistor’s collector current to the change in base current. It usually varies from 20–500.

Voltage Gain

This property is defined as the ratio of the change of the output voltage to the change in input voltage.

Power Gain

Power gain is the ratio of the output to input signal power.

CE NPN Transistor Characteristics

Common Emitter NPN transistors have a medium voltage and current gain due to their moderate input and output impedance levels. But their power gain is significant, and we’ll look at their input and output characteristics to understand their behavior.

Input Characteristics

Input characteristics of an NPN transistor

These characteristics simply refer to the different curves between the base current and voltage across the base and emitter terminals for a specific CE voltage. This graph compares the curves of 1V, 10V, and 20V CE voltages.

Output Characteristics

Output characteristics of an NPN transistor

These curves show the link between the collector current and the voltage applied across the collector-emitter terminals with varying base currents. As you can see, these transistors turn on after a low base current is applied at a limited CE voltage.

But the collector current is mostly impacted by the collector voltage at low levels of around 1V, and this effect diminishes past this voltage.

Applications of NPN Transistors

• Signal amplification

• Digital switching circuits

• Oscillators

• Amplitude modulation

• Signal processors

• Logarithmic converters

NPN vs. PNP Transistors

Advantages of NPN Transistors

• Compact size

• Fast switching speed

• Low noise

• High power gain

• Low cost

Disadvantages of NPN Transistors

• Low breakdown voltage

• Cannot handle high temperatures

• Unsuitable for high-power applications

• Generates a lot of heat

Emerging Technologies in NPN Transistor Construction

High-power NPN transistors and BJTs, in general, have emerged to provide better thermal performance while handling high current levels across their PN junctions. Additionally, with the increasing advancements and adoption of AI, transistor manufacturers are turning toward AI algorithms to help develop dependable and highly efficient NPN transistors to run various devices.

Key Drivers That Are Increasing Demand for NPN Transistors

• Increasing demand for power electronics

• Higher appetite for consumer electronics and IoT devices

• Expansion of the car industry, especially EVs

• High demand for energy-efficient solutions

• Advancements in BJT construction technology (high-voltage and high-current PNP transistors) that make them suitable for more applications

Restraints in NPN Transistor Usage/Adoption

• Increasing competition from other transistor types, such as FETs and IGBTs

• Compatibility issues in some circuits

• Limited operating temperature range

• Regular NPN transistors are affordable, but the advanced types can be costly

Final Words

In conclusion, NPN transistors have a simple construction, so the secret ingredient to their performance lies in their chemistry, more specifically the doping level in each semiconductor region.

As you can see in the output characteristics curve, these doped materials enable the device to operate in 3 modes by tweaking various inputs, the key one being the base current. And the active region is particularly important when building amplification circuits configured in the CE topology.

So whether you are looking for single or array NPN transistors for amplification, high-frequency RF circuits, or any other application, we can ship them to you at pocket-friendly costs. We also have pre-biased BJTs in stock (single and array), which operate more efficiently and produce a more stable, distortion-free output signal. These are better for switches, amplifiers, and medical electronics