What is a MOSFET?

Field-effect transistors (FETs) are mainly categorized into two types: Junction FETs (JFETs) and Metal-Oxide-Semiconductor FETs (MOSFETs).

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. Due to the insulating layer between the gate and the channel, it’s also called an insulated-gate field-effect transistor.
MOSFETs are further divided into four types:

• N-channel Depletion Mode
• N-channel Enhancement Mode
• P-channel Depletion Mode
• P-channel Enhancement Mode

▲ MOSFET types and circuit symbols

Some MOSFETs include an internal diode, known as a body diode, also referred to as a parasitic diode or freewheeling diode.

There are two main functions of the parasitic diode:

1. Overvoltage Protection: In the event of excessive VDD voltage, the diode may break down in reverse and conduct current to ground before the MOSFET is damaged, thereby protecting the MOSFET.
2. Reverse Polarity & Flyback Protection: When the drain and source are reversed or when reverse inductive voltage occurs, the diode provides a current path to prevent MOSFET damage.

MOSFETs offer high input impedance, fast switching speed, excellent thermal stability, and voltage-controlled current characteristics. They are commonly used as amplifiers or electronic switches in various circuits.

What is an IGBT?

IGBT (Insulated Gate Bipolar Transistor) is a composite semiconductor device that combines a bipolar junction transistor (BJT) and a MOSFET.

As a new-generation power semiconductor device, IGBT offers the following characteristics:

• High input impedance
• Low power loss with voltage-controlled drive
• Simple control circuitry
• High voltage withstand capability
• High current handling capacity

The IGBT symbol is not yet standardized. On schematics, it may be represented by the symbol of a BJT or MOSFET. To distinguish it, check the part number labeled in the circuit diagram.

Note that IGBTs may also have a body diode, which is not parasitic but intentionally designed to protect the IGBT’s relatively weak reverse voltage characteristics. This diode is referred to as a Free Wheeling Diode (FWD).

To determine whether an IGBT has a built-in diode, a multimeter can be used to measure the resistance between the collector and emitter. If the resistance is infinite, the device does not include a body diode.

IGBTs are well-suited for applications such as AC motor drives, inverters, switch-mode power supplies, lighting circuits, and traction systems.

Structural Characteristics of MOSFETs and IGBTs

The internal structures of MOSFETs and IGBTs differ:

An IGBT is essentially a MOSFET with an additional P-layer at the drain side.
Its ideal equivalent circuit shows that an IGBT is a combination of a MOSFET and a BJT.

While MOSFETs suffer from high on-resistance at high voltage, IGBTs overcome this issue by maintaining low on-state voltage drop even under high voltage conditions.

However, for devices with similar power ratings, IGBTs generally have slower switching speeds due to tail current during turn-off, which increases dead time and limits the switching frequency.

Choosing Between MOSFETs and IGBTs

Selecting between a MOSFET and an IGBT is a common design decision. The choice depends on system voltage, current, switching power, and other factors. Here are some general guidelines:

• For low-voltage, high-frequency applications, choose a MOSFET.
• For high-voltage, high-current, low-to-medium frequency applications, choose an IGBT.

The diagram (not shown here) illustrates the application boundaries of each device. The shaded areas indicate where both devices can be used, and the areas marked with “?” represent technical limitations of current semiconductor processes.

Summary

MOSFETs excel in high-frequency performance (up to hundreds of kHz or even MHz), but suffer from higher conduction losses in high-voltage/high-current applications due to their resistive nature.

• IGBTs are superior in handling high voltages and large currents with lower conduction loss, but their slower switching speed limits their use in high-frequency applications.

Typical Applications:

• MOSFETs: Switch-mode power supplies, electronic ballasts, high-frequency induction heating, RF inverters, telecom power systems.
• IGBTs: Inverters, welding machines, motor drives, electroplating/electrolysis power supplies, ultrasonic heating systems.