Side-by-side comparison
| Parameter | BJT | MOSFET |
|---|---|---|
| Control mechanism | Current-controlled (base current drives I_C) | Voltage-controlled (V_GS controls I_D) |
| Input impedance | Low (kΩ range, β-limited) | Very high (>10 MΩ, capacitive gate) |
| Switching frequency | Up to ~5 MHz (limited by minority carriers) | Up to 100 MHz+ (majority carrier device) |
| On-state voltage drop | V_CE(sat) ≈ 0.2–0.5 V | I_D × R_DS(on), e.g., 0.05 V at 1 A for IRF540 |
| Drive power requirement | Continuous base current needed | Only charge/discharge gate capacitance |
| Thermal runaway risk | Yes — I_C rises with temperature | Lower risk — R_DS(on) increases with temperature |
| Noise performance | Lower 1/f noise at low frequencies | Higher 1/f noise; better at RF frequencies |
| Typical applications | Audio amplifiers, low-frequency switching, current sources | Power supplies, motor drives, RF amplifiers, digital logic |
| Common devices | BC547, 2N3904, TIP31C | IRF540, IRF9540, BS170, 2N7000 |
| Fabrication complexity | Simpler bipolar process | CMOS process, more complex but scalable |
Key differences
BJTs are current-controlled — a 2N3904 needs a continuous base current of ~1 mA to keep 100 mA flowing in the collector; remove it and the transistor turns off instantly. MOSFETs need zero steady-state gate current — but the gate capacitance (e.g., 800 pF for IRF540) demands a peak charging current at each switching transition. BJTs suffer from minority carrier storage delay, capping useful switching at ~5 MHz; MOSFETs hit 100 MHz routinely in buck converters. BJTs also risk thermal runaway as V_BE drops 2 mV/°C — an issue MOSFET designs sidestep because R_DS(on) has a positive temperature coefficient.
When to use BJT
Use a BJT when you need a low-noise analog amplifier, a precision current source, or a low-frequency switch driven from a high-impedance signal that can spare a few milliamps of base drive. A 2N3906 in a microphone preamplifier delivers lower 1/f noise than an equivalent MOSFET at audio frequencies below 10 kHz.
When to use MOSFET
Use a MOSFET when switching at frequencies above 50 kHz, driving large currents with minimal steady-state drive power, or in any power electronics application. An IRF540N switching at 200 kHz in a buck converter handles 10 A with an R_DS(on) of just 44 mΩ, keeping conduction losses under 5 W.
Recommendation
For any power electronics or high-frequency switching task, choose a MOSFET — the gate needs no steady-state current, switching losses are lower, and thermal runaway is not a concern. Use a BJT when the circuit demands precise transconductance, low 1/f noise, or when the drive source is inherently current-limited.
Exam tip: Examiners in GATE and university finals often ask to compare input characteristics — BJT input is a forward-biased diode (nonlinear), while MOSFET input is essentially a capacitor (linear V-I at gate); know how to sketch both.
Interview tip: Interviewers at semiconductor and power electronics companies expect you to explain why a MOSFET's R_DS(on) increases with temperature and how that actually helps prevent thermal runaway — if you can't, they assume you only know theory.