Popular types of transistors (and when to use them)

Transistors are the backbone of modern electronics. From simple signal amplification to high-power motor control and RF communication, the right transistor choice directly affects performance, efficiency, cost and long-term availability.

While many guides introduce transistors at a high level, this article focuses on popular types of transistors used in real-world designs today, explaining how they work, where they’re used and why engineers still choose them.

What is a transistor?

A transistor is a semiconductor device used to amplify or switch electrical signals. By controlling current or voltage at one terminal, a transistor can regulate a much larger current flowing through another part of the circuit.

Broadly speaking, transistors fall into two fundamental operating families:

• Current-controlled devices (Bipolar Junction Transistors)
• Voltage-controlled devices (Field-Effect Transistors)

Understanding this distinction is key when selecting between transistor types.

Bipolar Junction Transistors (BJT)

A bipolar junction transistor (BJT transistor) uses both electrons and holes as charge carriers, which is where the term bipolar comes from. BJTs are current-controlled devices: a small base current controls a larger collector-emitter current.

Despite being an older technology, bipolar junction transistors are still widely used due to their predictable behaviour, strong gain characteristics and robustness in analog circuits.

NPN Transistors

The NPN transistor is by far the most commonly used BJT configuration.

In an NPN bipolar transistor:

• A positive voltage at the base (relative to the emitter) allows current to flow
• The transistor turns on when base current is applied
• It is generally faster and easier to bias than PNP types

NPN transistors are popular for:

• Signal amplification
• Low-to-medium power switching
• Logic-level interfacing
• Educational and prototyping designs

Common parts like the BC547 transistor or 2N2222 remain popular because they are inexpensive, well-documented and available from multiple manufacturers - an important consideration for procurement and long-term production.

You’ll often see NPN transistors chosen when:

• The circuit references ground
• Simpler drive circuitry is preferred
• High availability and second-source options are required

PNP Transistors

A PNP transistor operates in the opposite polarity to an NPN transistor.

In a PNP configuration:

• Current flows when the base is pulled lower than the emitter
• The transistor turns on when base current is removed (relative to emitter)

PNP transistors are commonly used in:

• High-side switching
• Complementary push-pull amplifier stages
• Certain power and control circuits

Although used less frequently than NPN transistors, PNP transistors are still essential in many designs where supply-side switching is required.

Junction Transistors and Legacy BJTs

The term junction transistor is often used interchangeably with bipolar junction transistor. While many modern designs favour MOSFETs for switching, junction transistors still appear in:

• Analog signal chains
• Audio amplification
• Bias-sensitive circuits
• Temperature-stable designs

For operations and maintenance teams, BJTs are also valued for their predictable failure modes and tolerance of electrical noise.

Field-Effect Transistors (FET)

Unlike BJTs, field-effect transistors are voltage-controlled devices. Applying a voltage at the gate creates an electric field that controls current flow between the source and drain.

This gives FETs several advantages:

• Very high input impedance
• Lower drive current requirements
• Better efficiency in switching applications

Junction Field-Effect Transistor (JFET)

JFETs are an older type of FET where the gate forms a junction with the channel.

They are still used in:

• Low-noise amplifiers
• Sensor interfaces
• Analog front-end circuits

JFETs are valued for their low distortion and predictable behaviour, though they are less common in new designs than MOSFETs.

MOS Field Effect Transistor (MOSFET)

The MOS field effect transistor, more commonly known as a MOSFET transistor, is the most widely used transistor type in modern electronics.

MOSFETs use an insulated gate, meaning virtually no current flows into the gate during operation. This makes them ideal for fast switching and low-power control.

N-Channel and P-Channel MOSFETs

MOSFETs are available in:

• N-Channel configurations (more common, better performance)
• P-Channel configurations (useful for high-side switching)

N-channel MOSFETs generally offer:

• Lower on-resistance
• Faster switching speeds
• Better efficiency at higher currents

P-channel MOSFETs simplify certain designs by eliminating the need for complex gate drive circuitry.

Power MOSFETs

Power attach of MOSFETs designed for higher currents and voltages are known as power MOSFETs.

They are commonly used in:

• Power supplies
• Motor control
• Battery management systems
• DC-DC converters
• Automotive and industrial electronics

From a procurement and operations perspective, MOSFET selection often balances:

• On-resistance (RDS(on))
• Thermal performance
• Package type
• Availability across multiple suppliers

Insulated-Gate Bipolar Transistor (IGBT)

The IGBT combines the gate structure of a MOSFET with the output characteristics of a BJT.

This hybrid approach makes IGBTs suitable for:

• High-voltage applications
• High-current switching
• Industrial motor drives
• Inverters and power conversion systems

IGBTs are not typically used for high-frequency switching, but they excel where efficiency and robustness are more important than speed.

Advanced and specialised transistors

Many articles stop at BJTs and MOSFETs, but modern designs often rely on more specialised devices.

High-Electron Mobility Transistors (HEMT)

HEMTs, including GaN-based devices, are increasingly popular in:

• RF applications
• 5G infrastructure
• High-frequency power electronics

They offer:

• Extremely fast switching
• High power density
• Improved efficiency at high frequencies

Heterojunction Bipolar Transistors (HBT)

HBTs are an evolution of the bipolar junction transistor, optimised for:

• RF amplification
• Microwave applications
• High-speed communication systems

Practical considerations when choosing a transistor

Across all transistor types, engineers and decision-makers typically evaluate:

• Electrical performance (gain, switching speed, voltage ratings)
• Thermal behaviour and packaging
• Drive complexity and control circuitry
• Cost and long-term availability
• Second-source and supply chain resilience

For procurement and operations teams, these factors often matter just as much as raw electrical performance.

Why popular transistors stay popular

Certain transistors remain widely used not because they are cutting-edge, but because they are:

• Well-understood
• Reliable
• Supported by multiple manufacturers
• Easy to source globally

Examples like the BC547 transistor, common NPN transistors and standard MOSFET transistors continue to appear in new designs for exactly these reasons.

Final thoughts

There is no single “best” transistor — only the best transistor for a specific application.

Understanding the strengths and limitations of bipolar junction transistors, MOS field effect transistors, NPN and PNP transistors and modern power devices allows engineers and decision-makers to design more efficient, reliable and scalable systems.

As transistor technology continues to evolve, the fundamentals remain the same: match the device to the job, consider the full lifecycle and design with both performance and availability in mind.