BJT vs MOSFET: Differences Explained

A bipolar junction transistor (BJT) is controlled by a base current, while a MOSFET is controlled by the voltage on an insulated gate. That one difference explains most of the rest: MOSFETs draw almost no steady input current and are easy to integrate by the billions, which is why they dominate digital chips, while BJTs offer high transconductance and low noise, which keeps them useful in analog and RF circuits.

AspectBJTMOSFET
ControlCurrent-controlled: base current sets collector currentVoltage-controlled: gate voltage sets drain current
Input currentA steady base current is needed (collector current divided by the gain)Almost no steady gate current, but the gate capacitance must be charged and discharged at each switching event
CarriersBoth electrons and holes take part (bipolar)Current is carried by one type of carrier in the channel (unipolar)
TransconductanceHigh for a given current (gm = IC / VT, about 38 mS at 1 mA)Lower for the same current (gm = 2 ID / (VGS - Vth))
Switching speedSlowed by minority-carrier storage when driven into saturationFast, because there is no minority-carrier storage
On-stateFixed saturation voltage VCE(sat) of about 0.1 to 0.3 VBehaves like a resistor RDS(on); the loss is I squared times R
Temperature behaviorCurrent rises with temperature, which can cause thermal runaway and makes paralleling harderOn-resistance rises with temperature, so current spreads out and paralleling is easy
NoiseLower low-frequency (1/f) noiseHigher 1/f noise
IntegrationLarger and draws bias current; used in analog, RF and high-speed circuits (SiGe HBT, BiCMOS)Small, nearly zero static power in CMOS; the basis of microprocessors and memories
Typical usesAnalog amplifiers, RF circuits, bandgap references, simple discrete switchesDigital logic, memory, power switching, most modern ICs

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

BJT
When you need high gain per unit current, low noise or a precise voltage reference: analog front ends, audio and RF amplifiers, bandgap references.
MOSFET
When you need a switch or a very large number of devices on one chip: digital logic, memory, power converters, motor drives.
IGBT
The IGBT combines a MOS gate with a bipolar output stage. It is used where voltage and current are high, such as motor drives and inverters.

FAQ

What is the main difference between a BJT and a MOSFET?

A BJT is controlled by a base current, while a MOSFET is controlled by the voltage on an insulated gate. A MOSFET draws almost no steady gate current, which makes it easy to integrate in huge numbers.

Why are MOSFETs used in digital chips instead of BJTs?

MOSFETs are small and, in CMOS circuits, draw almost no static power. BJTs need bias current and take more area, so billions of them on one chip would overheat.

When is a BJT better than a MOSFET?

A BJT has higher transconductance for the same current and lower low-frequency noise, so it remains useful in analog amplifiers, RF circuits and bandgap voltage references.

What is an IGBT?

An IGBT combines a MOS gate with a bipolar output stage. It is used for high voltage and high current, such as motor drives and inverters, where it has lower conduction loss than a MOSFET.

Check your understanding

Which correctly describes the difference in how bipolar transistors and MOSFETs are controlled?

  1. AA bipolar transistor is controlled by base current, while a MOSFET is controlled by gate voltage (almost no DC gate current flows)
  2. BA bipolar transistor is controlled by light and a MOSFET by temperature
  3. CBoth are controlled by a gate voltage, and no DC current flows into the control terminal
Show answer and explanation

Answer: A. A bipolar transistor is controlled by base current, while a MOSFET is controlled by gate voltage (almost no DC gate current flows)

A bipolar transistor is current-controlled: base current controls collector current. A MOSFET is voltage-controlled with an insulated gate, so steady-state gate current is nearly zero, though current does flow to charge and discharge the gate capacitance at each switching event. Drive power is thus tiny at low speed, while fast or large devices need capable gate drivers. The IGBT combines both, driving a bipolar structure through a MOS gate.

What is the typical collector-emitter voltage of a silicon bipolar transistor in the saturation region?

  1. AAbout 0.1–0.3 V
  2. BAbout 5 V
  3. CAbout 20 V
Show answer and explanation

Answer: A. About 0.1–0.3 V

In saturation both junctions are forward-biased and their forward voltages largely cancel, so VCE(sat) is small, about 0.1–0.3 V. This is the on-state drop of the switch, and the loss is VCE(sat) × IC. At high current the collector's resistance adds to it. Compared with the drop across a MOSFET's on-resistance, MOSFETs often win at low voltage and high current.

What is the approximate transconductance gm of a bipolar transistor operating at a collector current of 1 mA? (Room temperature, thermal voltage VT ≈ 26 mV)

  1. AAbout 0.38 mS
  2. BAbout 38 mS
  3. CAbout 3.8 S
Show answer and explanation

Answer: B. About 38 mS

A bipolar transistor's transconductance is gm = IC/VT, so 1 mA ÷ 26 mV ≈ 38 mS. It depends only on current, not on device size or process variation. A MOSFET's gm = 2ID/(VGS − Vth), and since VGS − Vth is usually over 0.1 V, a bipolar gives 2–4 times more gm at the same current. This high gm is one reason bipolars are used in low-noise, high-gain analog circuits.

  1. Why is the static (steady-state) power consumption of a CMOS inverter small?
  2. Power MOSFETs are easy to use in parallel. Which statement about the temperature dependence of on-resistance explains why?
  3. Which correctly describes the typical division of use between silicon MOSFETs and IGBTs?

Practice Bipolar transistors questions

Key terms explained

Power MOSFET
A MOSFET built to handle large currents and voltages. It switches quickly and is widely used in power supply circuits and the like.
HBT (heterojunction bipolar transistor)
A bipolar transistor using materials with different band gaps for the emitter and base. Types include SiGe and GaAs-based devices, used in high-frequency amplifiers.

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