How Does a MOSFET Work: Basic Understanding, Types, and Applications

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a voltage-controlled device that has revolutionized modern electronics. As a fundamental building block in both analog and digital circuits, understanding MOSFETs is crucial for any electronics engineer or hobbyist.

Basic Structure and Operation

Physical Structure

Operating Principles

The MOSFET operates by controlling the channel between source and drain through an electric field created by the gate voltage. This creates three main operating regions:

1. Cut-off (OFF state)
2. Linear (ON state)
3. Saturation (Current limiting)

Types of MOSFETs

Enhancement vs Depletion

N-Channel vs P-Channel

Operating Regions

Key Operating Points

Region Characteristics

Key Parameters

Electrical Parameters

Thermal Parameter

Applications

Power Applications

Signal Applications

Selection Guide

Application-Based Selection

Package Selection

Common Issues

Failure Modes

Frequently Asked Questions

Q1: What's the difference between BJTs and MOSFETs?

Key differences include:

• MOSFETs are voltage-controlled, BJTs are current-controlled
• MOSFETs have higher input impedance
• MOSFETs generally switch faster
• BJTs have better linearity
• MOSFETs have lower power dissipation in switching applications

Q2: How do I protect a MOSFET from damage?

Protection methods include:

• Gate resistor for switching speed control
• Gate-source zener for overvoltage protection
• Snubber circuits for dV/dt protection
• Proper thermal management
• ESD protection during handling

Q3: Why do MOSFETs get hot?

MOSFETs generate heat due to:

• I2R losses in RDS(on)
• Switching losses at high frequencies
• Body diode conduction losses
• Gate charge losses
• Operation in linear region

Q4: How do I calculate MOSFET power dissipation?

Power dissipation calculations include:

• Conduction loss = ID2 × RDS(on)
• Switching loss = VDS × ID × f × (tr + tf)/2
• Gate drive loss = QG × VGS × f
• Total power = Conduction + Switching + Gate losses

Q5: What parameters are most important for MOSFET selection?

Critical parameters include:

• Maximum voltage rating (VDSS)
• Current rating (ID)
• On-resistance (RDS(on))
• Gate threshold voltage (VGS(th))
• Switching speed (Qg, tr, tf)