Transistors Explained: BJT vs MOSFET for Switching and Amplification

Transistors Explained: BJT vs MOSFET for Switching and Amplification

Transistors are the “muscles” of electronics. They turn things on/off, drive motors and LEDs, amplify signals, and protect circuits. But picking the wrong type—especially confusing BJT and MOSFET—leads to common problems like overheating, weak drive, slow switching, or a circuit that doesn’t fully turn on.

This guide explains BJTs vs MOSFETs, when to use each, and the key specs you must check.

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What does a transistor do?

A transistor is basically a controllable valve for electricity. It can work in two main modes:
1. Switching: OFF or ON (power control, digital outputs, drivers)
2. Amplifying: small signal in → larger signal out (audio, sensors, analog)

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BJT vs MOSFET (the real difference)

BJT (Bipolar Junction Transistor)
• Controlled by current into the base
• Has a fairly fixed voltage drop when on (VCE(sat))
• Often simpler for low-cost, low-current switching and analog amplification

MOSFET (Metal-Oxide Semiconductor FET)
• Controlled by voltage on the gate
• When on, it behaves like a low resistance (RDS(on))
• Usually best for power switching (efficient, low heat)

Simple memory trick:
• BJT = current-driven
• MOSFET = voltage-driven

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When to use a BJT

Use a BJT when:
• You need a simple switch for small loads
• You want predictable behavior in analog amplification
• Your circuit naturally provides base current easily

Common BJT applications:
• Switching small relays (with proper flyback diode)
• Driving small LEDs (low current)
• Audio amplifiers and analog gain stages
• Signal conditioning circuits

BJT switching reality (important)

A BJT needs base current to turn on strongly. If you don’t provide enough base current, it runs hot.

Rule of thumb for “hard switching”:
• Base current is often designed as roughly collector current / 10
(Exact depends on transistor and conditions, but this keeps it safe.)

That’s why BJTs become less convenient as load current rises.

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When to use a MOSFET

Use a MOSFET when:
• You need efficient switching at medium/high current
• You want low heat at high current
• You are switching frequently (PWM, power converters, motor control)

Common MOSFET applications:
• DC motor drivers
• LED strips and lighting
• Battery power paths (power switching)
• DC-DC converters
• Load switches for high current rails

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The 3 most common MOSFET types you’ll see

1) N-channel MOSFET (most common)
• Best efficiency and performance
• Often used as low-side switch (between load and ground)

2) P-channel MOSFET
• Often used as high-side switch (between supply and load)
• Convenient but usually higher resistance than N-channel

3) Logic-level MOSFET (very important)

This means it turns on well at low gate voltage (like 3.3V or 5V).

If your MCU is 3.3V, you must use a MOSFET that fully turns on at 3.3V gate drive. Many “power MOSFETs” don’t.

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Key specs you must check (BJT)

1) Vce (maximum voltage)

Must be higher than your supply + spikes.

2) Ic (collector current rating)

Must exceed your load current with margin.

3) Vce(sat) (on-state voltage drop)

Lower Vce(sat) = less heat.

Heat ≈ Vce(sat) × current

4) Gain (hFE / beta)

Gain varies a lot. Don’t depend on “typical” gain for switching—design base current with margin.

5) Base resistor

You almost always need a base resistor to control base current.

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Key specs you must check (MOSFET)

1) Vds (maximum drain-source voltage)

Choose above your rail + spikes.

2) Id (drain current)

But this number alone is not enough—thermal matters more.

3) Rds(on) (on resistance)

This is the #1 spec for heat in a switch.

Heat ≈ I² × Rds(on)

Example:
If current is 5A and Rds(on) is 20mΩ (0.02Ω)
Heat = 5² × 0.02 = 25 × 0.02 = 0.5W (manageable with good copper)

If Rds(on) is 80mΩ
Heat = 25 × 0.08 = 2W (hot, may fail without heatsinking)

4) Vgs(th) is NOT the “turn-on voltage”

This is one of the biggest mistakes people make.

Vgs(th) is where the MOSFET just begins to conduct a tiny current, not where it’s fully on.

Always check Rds(on) at your gate voltage:
• If MCU is 3.3V → look for Rds(on) specified at Vgs = 2.5V or 3.3V
• If MCU is 5V → look for Rds(on) at 4.5V

5) Gate charge (Qg)

Qg affects switching speed and driver requirements.
Higher Qg = harder for a small MCU pin to switch quickly (more switching loss, slower edges).

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Switching use-case: low-side vs high-side

Low-side switching (easy and common)
• N-MOSFET between load and ground
• Gate driven by MCU
• Very efficient and simple

Downside: load is not directly tied to ground when off (sometimes matters for sensors)

High-side switching
• Switch is between supply and load
• Often uses P-MOSFET (simple) or N-MOSFET with a driver (best performance)

Use high-side when:
• You must keep the load grounded
• You’re switching power rails to modules

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Common mistakes (and why circuits fail)
1. Using a MOSFET that doesn’t fully turn on at 3.3V
Result: high resistance → heat → weak output → failure
2. Choosing MOSFET by Id rating only
Result: thermal limit exceeded even though “Id looks high”
3. Using Vgs(th) as the decision factor
Result: MOSFET barely turns on
4. No flyback diode on inductive loads (relay, motor)
Result: voltage spike kills transistor
5. Driving big MOSFET gates directly from MCU at high PWM speed
Result: switching loss, EMI, unstable behavior (needs gate resistor/driver)

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Quick selection cheats (good defaults)

For small loads (under ~200mA)
• BJT can work fine and be cheaper

For medium/high loads (0.5A and up)
• MOSFET is usually better

If using MCU 3.3V output
• pick a logic-level MOSFET with Rds(on) specified at 2.5V/3.3V

For inductive loads (relay/motor)
• always add a flyback diode
• consider a TVS diode for harsh environments

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FAQ

BJT or MOSFET for LED strips?

MOSFET (less heat, better efficiency). Use N-channel low-side switching in most cases.

Why is my MOSFET hot even at low voltage?

Because heat depends on I² × Rds(on). If the MOSFET isn’t fully enhanced (wrong gate voltage), Rds(on) can be high and it will heat up fast.

Can I replace a BJT with a MOSFET directly?

Sometimes, but you must check gate drive voltage, polarity, and whether the circuit expects current-driven behavior.