MOSFET vs BJT: two kinds of transistor, compared
MOSFETs and BJTs are both transistors: three-terminal devices in which a small signal controls a larger current. A BJT (bipolar junction transistor) has a base, a collector and an emitter, and a small current into the base controls a much larger collector current; it comes as NPN or PNP. A MOSFET (metal-oxide-semiconductor field-effect transistor) has a gate, a source and a drain, and the gate is insulated from the channel by a thin oxide, so the gate voltage, not a gate current, decides whether a conducting channel forms; it comes as NMOS or PMOS. When people ask about a MOSFET vs a transistor, they usually mean a MOSFET vs a BJT: a MOSFET is a transistor too. BJTs are current-controlled and MOSFETs voltage-controlled; that one difference explains most of where each is used.
What each one is good at
MOSFETs dominate digital logic: because the insulated gate draws essentially no steady current, a CMOS gate built from NMOS and PMOS pairs uses almost no power when it is not switching, and MOSFETs shrink well, which is how billions fit on one chip. They are also common as power switches. BJTs remain widely used in analog circuits such as amplifiers and in many discrete designs, and the classic TTL logic family, such as the 74LS series, was built from them; our TTL vs CMOS page compares the two logic families.
See the MOSFET at work in digiwleea
digiwleea is built on the MOSFET. In the browser you wire NMOS and PMOS transistors as switches into gates, then adders, memory and an 8-bit CPU, in a 40+ level course with an AI tutor (Wleea). The MOSFET simulator shows the channel forming as the gate voltage rises, the MOSFET lesson explains the switch, and NMOS vs PMOS compares the two polarities. digiwleea has no BJT model; it simulates logic at the switch level, not analog transistor behaviour.
Side by side
| BJT | MOSFET | |
|---|---|---|
| Terminals | Base, collector, emitter | Gate, source, drain (and body) |
| Controlled by | Base current | Gate voltage (insulated gate) |
| Types | NPN, PNP | NMOS, PMOS |
| Steady input current | Needs base current | Essentially none |
| Typical uses | Analog amplifiers, classic TTL logic | CMOS digital logic, power switching |
No sign-up is needed to start: open the lab and place your first transistor, or read the free theory course. The account (and the AI tutor) are optional.
More comparisons: Digital logic concepts compared
See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.
Open the digiwleea lab →Frequently asked
Is a MOSFET a transistor?
Yes. A MOSFET is a field-effect transistor. When people compare a MOSFET with a transistor they usually mean a bipolar junction transistor (BJT).
What is the main difference between a MOSFET and a BJT?
A BJT is controlled by a current into its base; a MOSFET is controlled by the voltage on its insulated gate, which draws essentially no steady current.
Why are computer chips made of MOSFETs?
Because CMOS logic built from NMOS and PMOS MOSFETs draws almost no power when it is not switching, and MOSFETs can be made extremely small, so billions fit on one chip.
What are the terminals of a BJT and a MOSFET?
A BJT has a base, a collector and an emitter. A MOSFET has a gate, a source and a drain, plus a body (substrate) connection that is often tied to the source.