Types of transistors explained: BJT, MOSFET & IGBT

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 the types used in real-world designs today — how they work, where they're used, and why engineers still choose them.

What is a bipolar junction transistor (BJT)?

A bipolar junction 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, BJTs are still widely used for their predictable behaviour, strong gain characteristics and robustness in analogue circuits — and, for maintenance teams, their predictable failure modes and tolerance of electrical noise.

NPN transistors

The NPN transistor is by far the most commonly used BJT configuration. A positive voltage at the base relative to the emitter allows current to flow, making NPN devices straightforward to drive directly from the logic outputs of microcontrollers and PLCs in low-side switching arrangements. They are the default choice for signal amplification, low-to-medium power switching, logic-level interfacing and educational or prototyping designs.

Common parts like the BC547 and 2N2222 remain popular because they are inexpensive, well documented and available from multiple manufacturers — an important consideration for procurement and long-term production. Rapid's transistor range covers low, medium and high power NPN types in both through-hole and surface mount packages.

PNP transistors

A PNP transistor operates in the opposite polarity: current flows when the base is pulled lower than the emitter. PNP devices are used less frequently than NPN — electrons move faster than holes, so NPN types switch faster — but they remain essential wherever supply-side (high-side) switching is required, and in complementary push-pull amplifier stages.

What is a field-effect transistor (FET)?

Unlike BJTs, field-effect transistors are voltage-controlled. Applying a voltage at the gate creates an electric field that controls current flow between source and drain. This gives FETs very high input impedance, minimal drive current requirements and better efficiency in switching applications.

JFETs

JFETs are an older FET type where the gate forms a junction with the channel. They are valued for low distortion and predictable behaviour and still appear in low-noise amplifiers, sensor interfaces and analogue front-end circuits, though they are less common in new designs than MOSFETs.

MOSFETs

The MOSFET is the most widely used transistor type in modern electronics. Its insulated gate means virtually no current flows into the gate during operation, making it ideal for fast switching and low-power control. N-channel devices are the more common choice, offering lower on-resistance, faster switching and better efficiency at higher currents; P-channel devices simplify high-side switching by eliminating complex gate drive circuitry.

MOSFETs designed for higher currents and voltages are known as power MOSFETs, and they dominate power supplies, motor control, battery management, DC-DC converters and automotive and industrial electronics. Selection typically balances on-resistance (R_DS(on)), thermal performance, package type and availability across multiple suppliers. For surface mount designs, the SOT-23 package has become the industry-standard small-signal format — see Rapid's SOT-23 general purpose transistors.

What is an IGBT and when is it used?

The insulated-gate bipolar transistor combines the easy-to-drive gate structure of a MOSFET with the high-current, low-saturation-voltage output of a BJT. That hybrid makes IGBTs the standard for high-voltage applications — industrial motor drives, inverters and power conversion systems — where efficiency and robustness matter more than raw switching speed. They are not typically used for high-frequency switching; that territory belongs to MOSFETs and, increasingly, GaN devices.

What about specialised transistor types?

Many articles stop at BJTs and MOSFETs, but modern designs often rely on more specialised devices. High-electron mobility transistors (HEMTs), including GaN-based devices, are increasingly popular in RF applications, 5G infrastructure and high-frequency power electronics, offering extremely fast switching and high power density. Heterojunction bipolar transistors (HBTs) are an evolution of the BJT optimised for RF amplification, microwave applications and high-speed communication systems. Where a design needs several matched devices in one package, discrete semiconductors such as transistor arrays keep board area and part count down.

How do you choose the right transistor?

Across all types, the evaluation comes down to five things: electrical performance (gain, switching speed, voltage ratings), thermal behaviour and packaging, drive complexity, cost, and long-term availability with second-source options. For procurement and operations teams, those last two often matter as much as raw electrical performance — certain transistors stay popular not because they are cutting-edge but because they are well understood, reliable, supported by multiple manufacturers and easy to source globally. There is no single best transistor, only the best transistor for a specific application: match the device to the job, consider the full lifecycle, and design with both performance and availability in mind.

Choosing between transistor types

What is the most commonly used transistor?

The MOSFET is the most widely used transistor type in modern electronics, thanks to its insulated gate, minimal drive current and switching efficiency. Among bipolar types, general-purpose NPN parts such as the BC547 and 2N2222 remain the most common in analogue, educational and prototyping circuits.

Should you use a BJT or a MOSFET?

Use a MOSFET for efficient switching, especially at higher currents or from low-power control signals. Use a BJT where linear amplification, low cost at small scale, or predictable analogue behaviour matters. As a rule: MOSFETs switch, BJTs amplify — though both can do either job.

Why are NPN transistors more common than PNP?

Electrons — the majority carriers in NPN devices — move faster than the holes used in PNP devices, giving NPN transistors faster switching and better high-frequency performance. NPN types also suit low-side switching, where the emitter connects to ground, making them easier to drive directly from microcontroller and PLC logic outputs.

What is the difference between an IGBT and a MOSFET?

A MOSFET switches faster and suits high-frequency, lower-voltage applications; an IGBT handles higher voltages and currents with lower conduction losses, making it standard above roughly 600V in motor drives and inverters. IGBTs trade switching speed for robustness and efficiency at high power.

Can Rapid supply transistors for production and education?

Yes. Rapid stocks NPN and PNP bipolar transistors, MOSFETs, Darlingtons, SOT-23 surface mount devices and transistor arrays across low, medium and high power ratings.