MOSFET vs BJT: Which to Choose for Switching?
When you want to electronically switch a motor, LED strip, relay or any load on and off, you face two basic options: MOSFET and BJT. Both are transistors and both can switch, but their operating principles and strengths differ. In this article we compare which is the right choice when, specifically for switching applications.
The core difference: current control vs. voltage control
The most critical distinction is the control method. A BJT is current-controlled: it conducts as long as you feed a continuous base current, which means the control circuit constantly spends current (and therefore power). A MOSFET is voltage-controlled: you apply a voltage to its gate and, in the steady state, it draws almost no current. That's why driving a MOSFET is generally more efficient, especially for loads that stay on continuously.
Comparison table
| Feature | MOSFET | BJT |
|---|---|---|
| Control | Voltage (gate) | Current (base) |
| Drive power | ~0 in steady state | Requires continuous base current |
| Conduction loss | Low RDS(on), advantageous at high current | Fixed VCE(sat) drop |
| Switching speed | Very fast (high frequency) | Slower |
| High current | Very suitable | Limited |
| Cost (low power) | Slightly higher | Very cheap |
| Static (ESD) sensitivity | High (sensitive gate) | Low |
When a MOSFET?
If you're switching a high-current load (motor, heater, powerful LED), operating at high frequency (speed/brightness control with PWM, a switch-mode power supply), or want to minimize the control circuit's power use, the MOSFET is the right choice. Thanks to a low RDS(on) it heats up little at high current, and driving the gate doesn't require continuous power. When driving with a microcontroller, you often need to choose a logic-level MOSFET or use a gate driver.
When a BJT?
For low-power, simple switching jobs (driving a small relay, a single LED or a low-current signal), a BJT is still practical and very cheap. At loads of a few milliamps, the MOSFET's advantages aren't pronounced; a base resistor and a cheap NPN transistor do the job cleanly. The BJT is also preferred in analog amplifier applications and low-voltage circuits.
The drive difference: base resistor vs. gate drive
The approach differs when driving the two types. With a BJT, you put a resistor on the base to limit the base current and provide enough current to push the transistor into saturation. With a MOSFET, what matters is providing the voltage to fully turn on the gate and charging/discharging the gate capacitance quickly; at high frequency this may require a gate driver circuit. A wrongly driven MOSFET can stay half-on and overheat.
Power loss and heat: the critical difference
The real consequence of the switching-element choice shows up in heat. When a MOSFET is conducting, its loss is roughly the current squared times RDS(on); a MOSFET with a low RDS(on) heats up little even at high current. In a BJT, there's a roughly constant VCE(sat) voltage drop in saturation, and the loss is the product of current and that voltage; as current grows, this loss increases linearly. That's why, for loads above a few amps, the MOSFET runs both more efficiently and cooler. Do your math and plan a heatsink if needed.
Protection on an inductive load
If you're switching an inductive load such as a motor, relay coil or solenoid, the high-voltage spike the coil produces when current is cut can kill the transistor. The solution is the same for both MOSFET and BJT: place a flyback (freewheeling) diode in reverse parallel with the load. In high-speed switching, a snubber circuit also softens voltage spikes. This protection is the most commonly overlooked but most expensive detail in switching circuits.
Common mistakes
- Driving a standard MOSFET with a 5 V microcontroller: a non-logic-level MOSFET won't fully turn on with a 5 V gate; choose a logic-level type.
- Not saturating the BJT: insufficient base current leaves the transistor semi-conducting, it heats up and loss increases.
- Leaving the gate floating: the MOSFET gate is high-impedance; without a pull-down/pull-up resistor it can turn on unintentionally from noise.
- Ignoring ESD: the MOSFET gate is sensitive to static; take precautions during assembly.
Frequently asked questions
If both do the same job, why does it matter?
At low power the difference is small; but when high current, high frequency or efficiency matters, the MOSFET clearly comes out ahead.
What is a logic-level MOSFET?
It's a MOSFET whose gate can be fully turned on with low logic voltages like 3.3 V–5 V; it's suitable for driving directly with a microcontroller.
How do I choose the right MOSFET?
VDS, ID, RDS(on) and the gate threshold voltage are the basic criteria.
Conclusion
The general rule for switching: MOSFET when you need high current, high frequency and efficiency; BJT for low-power, simple, cost-focused jobs. To narrow the choice by parameters, you can search with the right MPN, review candidates side by side with the comparison tool, and find a suitable transistor through search.