MOSFETs and IGBTs are two of the most important components in modern power electronics. They are used to control how electricity flows in devices ranging from small gadgets to large industrial machines. Both works as electronic switches but differ in how they handle voltage, current, and speed. MOSFETs are known for fast switching and efficiency at lower voltages, while IGBTs are preferred for handling higher power and voltage levels.

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What is a MOSFET?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor that controls electrical current using an electric field. It is part of the field-effect transistor (FET) family and is widely used in both analog and digital electronics. The gate of a MOSFET is separated from its conducting channel by a thin layer of oxide, which allows voltage at the gate terminal to control the current between the drain and source terminals. Because the gate is insulated, the MOSFET is also known as an Insulated-Gate Field-Effect Transistor (IGFET).

There are two main categories of field-effect transistors: JFET (Junction Field-Effect Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). Compared with JFETs, MOSFETs offer greater flexibility in circuit design and are more suitable for integrated electronic systems.

The symbols of MOSFETs represent their electrical connections and behavior. Each symbol shows the gate (G), drain (D), source (S), and substrate, with the direction of the arrow indicating current flow for N-channel or P-channel operation. These circuit symbols help engineers identify and interpret MOSFET behavior in schematic diagrams.

Inside a typical MOSFET, there is also a parasitic diode, often called a body diode, that forms naturally between the source and drain. This diode allows reverse current flow when needed and protects the device from voltage spikes or reverse polarity during switching. It is an inherent part of the MOSFET’s semiconductor structure and plays a crucial role in maintaining safe operation.

What is an IGBT?

An IGBT (Insulated Gate Bipolar Transistor) is a semiconductor device that combines the characteristics of a MOSFET and a bipolar junction transistor (BJT). It is designed to be controlled by voltage through its insulated gate, allowing efficient regulation of current flow in electronic circuits. The IGBT is widely used in power electronics where precise and stable current control is required.

The device consists of three terminals: Gate (G), Collector (C), and Emitter (E). The gate terminal controls the conductivity between the collector and emitter, enabling the device to switch electrical power on and off effectively. Because it merges the fast switching capability of a MOSFET and the current handling capacity of a BJT, the IGBT serves as a reliable element in many high-power systems.

In the diagram above, the N-channel and P-channel IGBT symbols illustrate the two common types of IGBTs used in circuit design. The direction of the arrow inside each symbol indicates the current flow and polarity of the device. These standardized symbols help identify IGBTs accurately in schematic diagrams and distinguish them from other semiconductor components.

The circuit symbol for IGBTs is not completely unified across manufacturers, as slight variations may exist. However, it typically combines features of both MOSFET and transistor symbols, emphasizing the IGBT’s hybrid nature. The device type can be confirmed through the model number or datasheet provided in design documentation.

Most IGBTs also include a freewheeling diode (FWD), which is integrated to handle reverse current during switching. This diode, shown in the parasitic diode symbol, connects across the collector and emitter terminals. Its purpose is to provide a safe path for reverse current and maintain circuit protection during voltage transitions. If not specified in the datasheet, the diode is usually assumed to be present as part of the IGBT design.

MOSFET vs IGBT: Structure

While they look similar externally, their internal structures differ in how they handle current and voltage. Both devices have three main terminals-Gate (G), Source/Emitter (S/E), and Drain/Collector (D/C)-and operate through gate-controlled conduction.

MOSFET Structure

A MOSFET is primarily a unipolar device, meaning its operation depends on the movement of only one type of charge carrier (either electrons or holes).

It is built from several semiconductor layers: an n⁺ source, n⁻ drift region, and p-body, all formed on an n⁺ substrate. The gate is insulated from the channel by a thin oxide layer, which acts as a dielectric barrier.

When voltage is applied to the gate, an electric field is generated that allows current to flow between the drain (D) and source (S). This structure provides high-speed switching and is ideal for low to medium power applications, such as power supplies, DC-DC converters, and motor drives.

IGBT Structure

The IGBT combines the best characteristics of both a MOSFET and a bipolar junction transistor (BJT). Structurally, it resembles a MOSFET but with an additional p⁺ substrate layer added to the drain side. This modification creates a bipolar conduction path, enabling higher current and voltage handling capability.

Inside, the IGBT includes an n⁺ buffer, n⁻ drift, p-base, and n⁺ emitter. This hybrid construction gives the IGBT a high input impedance (like a MOSFET) while maintaining a low on-state voltage drop (like a BJT). As a result, it performs efficiently in high-power and high-voltage systems, such as inverters, industrial drives, and renewable-energy converters.

IGBT Equivalent Circuit

The internal equivalent circuit of an IGBT can be viewed as a MOSFET driving a bipolar transistor. The MOSFET section controls the base current of the BJT part, allowing current to flow between the collector (C) and emitter (E).