What is a BJT transistor?

A bipolar junction transistor (BJT) is a three-terminal, current-controlled transistor built from three semiconductor layers that form two PN junctions. A small current into the base controls a much larger current between the collector and emitter. Because it conducts using both electrons and holes, it is called bipolar. The BJT transistor is a workhorse of analog electronic circuits, from audio amplifiers to current mirrors.

For a broader introduction to the underlying device family, see what a transistor is. For the full working principle, see how transistors work.

How does a BJT work?

A small base current biases the two junctions so that a much larger collector-emitter current can flow, with the ratio set by the current gain (β).

In practice, a BJT operates in one of three regions depending on that bias.

A bipolar transistor operates in three primary regions:

  • Cutoff region: transistor is OFF and no collector current flows
  • Active region: transistor operates linearly and can amplify signals
  • Saturation region: transistor is fully ON and behaves like a closed switch

In amplifier circuits, the transistor is usually biased in the active region, enabling signal amplification. A common configuration is the common emitter amplifier, widely used in analog circuits.

Types of BJT

BJTs come in two complementary polarities — the two main types you will encounter:

  • NPN: the default and most common type; turns on when the base is driven positive relative to the emitter.
  • PNP: the mirror image; turns on when the base is pulled below the emitter, and is used in high-side switches and complementary output stages.

The two are often combined in push-pull and complementary amplifier stages.

For more on how each conduct, see how transistors work.

What is a MOSFET?

A MOSFET (metal-oxide-semiconductor field effect transistor) is a three-terminal, voltage-controlled transistor from the field effect transistor family. Instead of a base current, a voltage on an insulated gate controls the current between drain and source.

What makes it different is the underlying MOS structure: a gate electrode separated from the silicon by a thin oxide. That insulated metal-oxide-semiconductor field arrangement sets the MOSFET apart from the junction FET (JFET), which controls current with a reverse-biased PN junction instead of an insulated gate. As a building block, the MOS transistor is the foundation of modern integrated circuits, microprocessors, and CMOS digital logic.

For the full picture, see how MOSFETs work and what a Field Effect Transistor (FET) is.

The gate terminal and gate capacitance

A MOSFET (metal-oxide-semiconductor field effect transistor) is a three-terminal, voltage-controlled transistor from the field effect transistor family. Instead of a base current, a voltage on an insulated gate controls the current between drain and source.

What makes it different is the underlying MOS structure: a gate electrode separated from the silicon by a thin oxide. That insulated metal-oxide-semiconductor field arrangement sets the MOSFET apart from the junction FET (JFET), which controls current with a reverse-biased PN junction instead of an insulated gate. As a building block, the MOS transistor is the foundation of modern integrated circuits, microprocessors, and CMOS digital logic.

For the full picture, see how MOSFETs work and what a Field Effect Transistor (FET) is.

The body diode

Every power MOSFET also contains a parasitic body diode between source and drain, a byproduct of its internal structure. This diode conducts whenever the drain is pulled below the source, regardless of the gate voltage. In many designs this is a nuisance to account for, but in others it is essential: in motor control and H-bridge circuits, the body diode provides a path for inductive freewheeling current the moment the MOSFET switches off, protecting the device from voltage spikes.

BJTs have no equivalent built-in diode, so this is one of the practical reasons MOSFETs dominate in switching applications with inductive loads.

How does a MOSFET work?

A voltage applied to the gate sets up an electric field across a thin insulating oxide layer. Once that gate-source voltage rises above the threshold, it forms a conducting channel between the drain and source; below the threshold, the channel disappears and the device is off.

Because the gate is insulated, it draws virtually no steady current — so a MOSFET needs almost no drive power and switches very fast.

For the detailed structure and operation, see how MOSFETs work.

To see how this switching behavior plays out in a real circuit, see how a MOSFET works as a switch.

Difference between MOSFET and BJT

The core difference is how each device is controlled: a BJT is current-controlled (a base current), while a MOSFET is voltage-controlled (a gate voltage). That single distinction drives most of the practical differences below.

 

For more detail on how MOSFET voltage ratings are specified and how to apply them when comparing devices, see MOSFET voltage ratings: how to read and apply breakdown voltage specs.

The thermal behavior row matters more than it might look: as a BJT heats up, its collector current tends to rise at a given base drive, which can push it to heat up further — a positive-feedback effect known as thermal runaway that becomes a real risk when multiple BJTs are paralleled to share current. A MOSFET behaves the opposite way: as it heats up, its RDS(on) rises, which reduces current through it and cools it back down. This self-balancing behavior is a key reason MOSFETs can be paralleled safely for higher current handling, while paralleling BJTs requires extra care (e.g., emitter ballast resistors).

These differences explain where each device dominates: BJTs in precision analog stages, MOSFETs across power electronics and digital circuits, where their fast, low-drive-power switching is a clear advantage.

When to use BJT vs MOSFET

So, when to use BJT vs MOSFET? The advantages of each device flow directly from how it is controlled. Use a BJT when simplicity and cost matter more than switching efficiency — a BJT driven by a resistor from a microcontroller GPIO is often the quickest solution for low-current switching, and BJTs suit precision linear amplifiers and current mirrors. Choose a MOSFET when efficiency, speed, and low drive power matter most, as in power electronics, motor control, and digital circuits.

Ultimately, the BJT versus MOSFET choice — the heart of any MOSFET vs BJT transistor decision — comes down to whether you value a cheap, simple current-controlled part or an efficient, fast voltage-controlled one.

For an example of how this plays out in a modern power MOSFET product family, see OptiMOS™: Infineon's power MOSFET family architecture explained.

Frequently asked questions about MOSFETs and BJTs

Which is better, MOSFET or transistor?

Neither is universally "better" — a MOSFET is also a type of transistor, so the real question is MOSFET vs BJT. MOSFETs tend to win on switching speed, efficiency, and drive simplicity in power electronics and digital logic, while BJTs remain a cheap, straightforward choice for low-current switching and precision linear amplification. The right pick depends on the application; see when to use BJT vs MOSFET above.

For high-power applications — typically above a few hundred volts and tens of amps, such as motor drives, inverters, and traction systems — the IGBT (insulated-gate bipolar transistor) is often the actual choice, not the BJT. An IGBT combines a MOSFET-style insulated gate for easy voltage control with a bipolar output stage that handles high voltage and current efficiently.

In practice, the real-world decision at high power levels is frequently MOSFET vs. IGBT rather than BJT vs. MOSFET, with BJTs mostly reserved for lower-power or legacy designs.

Yes — a MOSFET is one specific type of transistor, part of the field effect transistor (FET) family. What sets it apart from other transistors, such as the BJT, is that it is voltage-controlled through an insulated gate rather than current-controlled through a base.

For the broader category it belongs to, see what a transistor is article, and what a Field Effect Transistor (FET) is article.

A MOSFET is often used as a switch, but that's only one of its uses. Because its channel turns fully on or off based on gate voltage, it makes an efficient switch with very low drive power and fast switching speed — which is why it dominates power switching and digital logic. It can also be operated in its linear region as an amplifier.

For a closer look at the switching use case, see how MOSFETs work as a switch.

The two different types are NPN and PNP, which differ in polarity. NPN is the default choice for most circuits, while PNP is used where the load is switched on the high side or in complementary output stages.

Match the device to the circuit: it must handle the maximum collector-emitter voltage and collector current with margin, provide enough current gain (β) for your base drive, and switch fast enough for the application — all within its power-dissipation and package limits.

For most switching, an enhancement-mode N-channel MOSFET with a low RDS(on) gives the best efficiency, with a P-channel device or a gate driver for high-side switching.

For how the device operates, see how MOSFETs work.

Voltage margin also matters — see MOSFET voltage ratings: how to read and apply breakdown voltage specs for guidance on reading breakdown voltage specs, and OptiMOS™: Infineon's power MOSFET family architecture explained for an example product family built for this.

Yes — Infineon provides online product finders and parametric search to filter its transistor and MOSFET portfolio by voltage, current, gain, RDS(on), and package, along with SPICE models and evaluation boards to validate a design.

A BJT is driven by current into the base, usually through a series base resistor from a control signal such as a microcontroller GPIO. To use it as a switch, you supply enough base current to push it fully into saturation; to use it as an amplifier, you bias it in the active region.

The base current must be at least the collector (load) current divided by the current gain β. For reliable switching, designers add margin — driving the base harder than the bare minimum to keep the transistor firmly in saturation and minimise its on-state voltage drop.