Side-by-side comparison
| Parameter | Unipolar | Bipolar Transistor |
|---|---|---|
| Current Carriers | Both electrons and holes (minority + majority) | Majority carriers only (electrons in N-channel FET, holes in P-channel) |
| Control Mechanism | Current-controlled — base current IB controls collector current IC | Voltage-controlled — gate-source voltage Vgs controls drain current Id |
| Input Impedance | Low to moderate — 1 kΩ to 100 kΩ typical | Very high — 10⁸ to 10¹⁵ Ω (MOSFET gate is oxide-isolated) |
| Input Bias Current | 10 nA to 10 µA (BJT op-amp input) | 1 pA to 10 nA (FET op-amp: TL071 = 30 pA) |
| Transconductance (gm) | High — gm = IC/VT ≈ 40 mA/V at IC=1 mA | Lower — JFET gm ≈ 2–10 mA/V; MOSFET varies with W/L |
| Noise Performance | Lower noise at low frequencies; BJT 1/f noise corner ~10 kHz | Higher 1/f noise in MOSFETs; JFET better than MOSFET for audio |
| Temperature Sensitivity | Vbe decreases ~2 mV/°C; sensitive to thermal runaway | Positive TC of Rds_on prevents thermal runaway in power MOSFETs |
| Speed | GHz range (SiGe HBT for RF); limited by minority carrier storage | Higher switching speed in digital; MOSFET dominates in CMOS logic |
| Common Devices | BC547, 2N3904, TIP31C, 2N2222 | 2N3819 (JFET), IRFZ44N (MOSFET), CD4007 (CMOS) |
| Power Applications | Darlington pair TIP122 for motor drive | IRFZ44N, GaN EPC2001C for SMPS above 100 kHz |
Key differences
BJTs are minority carrier devices — the transistor action depends on minority carrier injection across the forward-biased emitter-base junction, which causes charge storage and limits switching speed. The base must be driven with current (IB = IC/β; for 2N3904 with β=200, driving 100 mA load needs 0.5 mA base current). FETs control drain current with gate voltage through an electric field, with no DC gate current in MOSFETs (gate-oxide insulation). This makes MOSFETs dominant in CMOS digital ICs (billions of transistors on a chip would be impossible with BJT base current). BJT transconductance (gm = IC/VT = 40 mA/V at 1 mA) exceeds typical MOSFET gm, making BJTs preferred in precision analog amplifier input stages.
When to use Unipolar
Use BJT (2N3904, BC547, TIP31C) when you need high transconductance at low quiescent current — audio amplifier input stages, transimpedance amplifiers for photodiodes, and motor driver stages where base current drive is available from a microcontroller.
When to use Bipolar Transistor
Use MOSFET or JFET (IRFZ44N, 2N3819) when input impedance must be in the megaohm or gigaohm range, when driving power loads at high switching frequency, or in CMOS digital logic — essentially every IC produced after 1980 uses MOSFET as the switching element.
Recommendation
Choose BJT for precision analog amplification where high gm and low noise at audio frequencies matter. Choose MOSFET for power switching, digital logic, and any application needing high input impedance. In modern power electronics, MOSFETs and IGBTs have almost completely displaced BJTs above 1 A.
Exam tip: Examiners ask the fundamental difference in control mechanism — write "BJT is current-controlled, FET is voltage-controlled" and support this by comparing base current IB for BJT vs gate current IG ≈ 0 for MOSFET.
Interview tip: Interviewers at analog design companies ask why op-amps like TL071 use JFET input stages — state the input bias current difference (30 pA vs 80 nA for LM741) and link it to high-impedance sensor interfacing applications.