MOSFET vs BJT Transistor — A Fun, Field‑Ready, No‑Nonsense Guide

MOSFET: field‑effect in a nutshell

A MOSFET is a voltage‑controlled resistor whose channel forms when the gate electric field attracts carriers. Conduction is roughly Ohmic in the linear region, transitioning into saturation where current becomes weakly dependent on V_DS. In switching we care about charging and discharging the gate’s effective capacitance (C_iss, C_gs, C_gd)—that’s your Qg budget per cycle.

BJT: charge control with gain

A BJT is a current‑controlled device. Injecting base charge allows a much larger collector current—amplification—until saturation. The price: continuous base drive and storage delay if you over‑saturate. In linear stages BJTs deliver smooth transconductance and low noise; in switching they demand smart base resistance and flyback management.

For MOSFETs

• RDS(on) at the actual V_GS and temperature. Double the temp, double the pain.
• Qg (total gate charge) and Miller plateau voltage determine drive power and speed.
• Body diode reverse recovery (Q_rr) dominates dead‑time loss in synchronous converters.
• SOA curves for linear use; avalanche energy (E_AS) for inductive abuse.

For BJTs

• V_CE(sat) vs I_C at realistic β and temperature.
• h_FE bins: design base current with margin (I_B ≈ I_C/10 is a safe start).
• Storage time and f_T (gain‑bandwidth) for speed estimates.
• Secondary breakdown SOA—the classic BJT gotcha in linear and pulse regions.

Driving a MOSFET

A controller must shove and yank charge across the gate, fast. That’s why we use dedicated gate drivers (source/sink amps), bootstraps for high‑side, and dead‑time control. The metric is P_drive ≈ Q_g·V_drive·f.

// STM32 pseudo‑code: center‑aligned PWM, complementary MOSFET drive with deadtime
TIM1->BDTR = (1<

Driving a BJT

You budget base current. For switching, a rough rule is I_B ≈ I_C/10, verified by V_CE(sat) at the target current. Add a base‑emitter resistor (22–100 kΩ) to bleed charge and speed turn‑off; a Schottky clamp across base‑collector prevents deep saturation in fast logic stages.

// Rule of thumb: Ib = Ic/10
float I_c = 0.2; // 200 mA relay
float I_b = I_c / 10.0; // 20 mA base drive target

Power converters (buck/boost)

MOSFETs all day. Synchronous rectification slashes loss; look at RDS(on), Qg, Qrr.

Relay/solenoid drivers

Either works. For <200 mA, a jellybean BJT is simple; higher current → logic‑level MOSFET with low RDS(on).

Analog amplifiers

BJTs deliver lovely transconductance and predictable V_BE tempco. MOSFETs excel in source followers and high‑Z inputs.

RF/small‑signal

BJTs and JFETs historically; MOSFETs dominate in CMOS RFICs. Board‑level? Choose by noise figure and f_T.

Motor drives

<100 V → MOSFETs; >600 V → IGBTs/SiC. BJTs are rare here now.

Low‑drop ORing / ideal diode

MOSFET with controller beats a BJT diode drop by far.

A) MOSFET low‑side switch (logic level)

MCU GPIO → 33Ω → Gate
Source → GND
Drain → Load− ; Load+ → +VIN
Schottky across inductive load; TVS to chassis if cabled

Notes: Check V_GS(th) vs guaranteed RDS(on) at 2.5/4.5 V. Add a 100 kΩ gate‑to‑source resistor to keep it off at boot.

B) BJT relay driver (low‑side)

GPIO → Rb → Base
Emitter → GND
Collector → Relay− ; Relay+ → +12V
Diode across relay; Base‑emitter 100 kΩ bleed
Rb ≈ (Vgpio−0.8)/Ib ; Ib ≈ Ic/10

C) Simple linear LED with BJT

+VIN → Rset → LED → Collector
Emitter → GND ; Base ← Vref via Rb
Choose Rset for LED current; thermal on BJT!

D) Synchronous MOSFET ideal diode

MOSFET + controller IC; sense Vdrop, drive gate
RDS(on) ≪ 50 mΩ; watch reverse body‑diode conduction

Switching observations

• MOSFET V_GS plateau indicates charging C_gd; slow there → high loss.

• BJT saturation stores charge; too deep → slow turn‑off. Schottky clamps help.

What trips engineers up

• Choosing MOSFET by RDS(on) alone without checking Qg.

• Under‑driving the gate from a micro pin directly.

• Forgetting base power in BJTs; it heats too.

• Ignoring SOA during startup/short‑circuit tests.