BJT vs MOSFET — Complete Explanation

1. Introduction

Bipolar Junction Transistors (BJTs) and Metal–Oxide–Semiconductor Field-Effect Transistors (MOSFETs) are the two most widely used semiconductor devices in analog and digital electronics. Both are used for switching and amplification, but they operate on fundamentally different physical principles, which makes each suitable for specific applications.

2. Basic Operating Principle

2.1 BJT (Bipolar Junction Transistor)

A BJT is a current-controlled device. It has three terminals: Base (B), Collector (C), and Emitter (E). A small current flowing into the base controls a much larger current flowing from collector to emitter.

The name “bipolar” comes from the fact that both electrons and holes participate in current conduction.

2.2 MOSFET (Metal–Oxide–Semiconductor FET)

A MOSFET is a voltage-controlled device. It has three main terminals: Gate (G), Drain (D), and Source (S). Applying a voltage to the gate creates an electric field that controls the current between drain and source.

The gate is insulated from the channel by a thin oxide layer, so ideally no current flows into the gate.

3. Control Mechanism

BJT

• Controlled by base current
• Requires continuous base current to remain ON
• Collector current ≈ β × Base current

MOSFET

• Controlled by gate-to-source voltage (V_GS)
• Practically zero steady-state gate current
• Acts like a voltage-controlled resistor or switch

4. Input Impedance

BJT

BJTs have low to moderate input impedance because the base-emitter junction behaves like a diode. This means the driving circuit must supply base current.

MOSFET

MOSFETs have very high input impedance because the gate is insulated. This allows them to be driven easily by logic circuits and microcontrollers.

5. Switching Behavior

BJT

BJTs are generally slower in switching applications due to charge storage in the base region, especially when driven into saturation.

MOSFET

MOSFETs switch much faster because they are majority-carrier devices. This makes them ideal for high-frequency and high-speed switching applications.

6. Power Handling and Efficiency

BJT

• Lower conduction losses at low currents
• Requires base drive power
• Less efficient in high-power switching

MOSFET

• Very low ON resistance (R_DS(on)) in modern devices
• Higher efficiency in switching power supplies
• Gate drive consumes almost no DC power

7. Thermal Stability

BJT

BJTs are prone to thermal runaway. As temperature increases, collector current increases, which further increases temperature unless proper biasing and thermal compensation are used.

MOSFET

MOSFETs are inherently more thermally stable. As temperature rises, R_DS(on) increases, which naturally limits current.

8. Analog Performance

BJT

BJTs offer excellent linearity and predictable gain, making them very popular in low-noise amplifiers, audio stages, and precision analog circuits.

MOSFET

MOSFETs are less linear in small-signal operation but excel in power and switching roles. Specialized MOSFETs are used in analog ICs where high input impedance is required.

9. Typical Applications

BJT Applications

• Audio amplifiers
• Low-noise preamplifiers
• Analog signal processing
• Current mirrors

MOSFET Applications

• Switching power supplies
• Motor drivers
• Microcontroller outputs
• Digital logic (CMOS)

10. Summary Table

11. Conclusion

BJTs and MOSFETs are not competitors but complementary devices. BJTs excel in analog performance and predictable gain, while MOSFETs dominate modern power electronics and digital systems due to their high efficiency and voltage-driven control. Choosing between them depends entirely on the application requirements.