Differences Between MOSFET, BJT and IGBT.

MOSFET, BJT, and IGBT: Principles of Operation

Transistors are among the most important components in modern electronics. They can be used to amplify electrical signals, switch currents, regulate power, and control other circuits. Although MOSFETs, BJTs, and IGBTs all perform similar basic functions, their internal structures and methods of control are quite different.

The most important distinction is this:

• A BJT is primarily a current-controlled device.
• A MOSFET is primarily a voltage-controlled device.
• An IGBT is a voltage-controlled device that uses a bipolar conduction structure.

Understanding this difference makes it much easier to decide which type of transistor is appropriate for a particular circuit.

1. The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

A MOSFET is a field-effect transistor (FET). Unlike a bipolar transistor, it does not normally require a continuous input current to control the main current. A typical power MOSFET has three terminals:

• Gate (G)
• Drain (D)
• Source (S)

Internal physical structure of a MOSFET.

The Gate is electrically insulated from the semiconductor by a very thin insulating layer, traditionally silicon dioxide. This insulation is the reason why the MOSFET has an extremely high input resistance.

Power MOSFET package and schematic symbol.

How the MOSFET works

Consider an N-channel enhancement MOSFET, which is one of the most common types. When there is no suitable voltage between Gate and Source, the transistor is normally off. There is no conducting channel connecting the Drain and Source.

If a positive voltage is applied to the Gate relative to the Source, an electric field is created through the insulating layer. When the Gate-to-Source voltage, V_GS, exceeds a certain value called the threshold voltage (V_TH), this electric field creates a conducting channel in the semiconductor. The channel allows current to flow between Drain and Source.

Gate voltage → electric field → conducting channel → Drain current

This is the fundamental operating principle of a MOSFET.

Why almost no Gate current is required

The Gate is insulated from the semiconductor. Therefore, under steady-state conditions, essentially no DC current flows into the Gate. A small amount of current may flow temporarily while the Gate capacitance is being charged or discharged. This is extremely important in practical circuits. For example, a microcontroller can control a power MOSFET because the microcontroller does not have to continuously supply the transistor's Drain current. It only has to charge and discharge the MOSFET's Gate.

MOSFET as a switch

The MOSFET is particularly useful as an electronic switch. When the Gate voltage is sufficiently high, the MOSFET becomes strongly conductive. Its Drain-to-Source resistance, known as R_DS(on), can become very small.

• Gate LOW → MOSFET OFF
• Gate HIGH → MOSFET ON

When the MOSFET is fully turned on, the voltage across it can be relatively small, and therefore its power dissipation can also be relatively low. This is one of the reasons MOSFETs are widely used in:

• DC-DC converters
• Switching power supplies
• Battery-powered equipment
• Motor controllers
• Inverters
• Audio amplifiers
• Electronic load switches

MOSFET as an amplifier

A MOSFET does not have to operate only as a switch. If the Gate voltage is varied continuously, the Drain current can also be varied. This makes the MOSFET useful as an amplifier.

In an amplifier circuit, the transistor is usually biased so that it operates in an appropriate region between complete cutoff and strong conduction. A small change in Gate voltage can then produce a corresponding change in Drain current.

• small voltage change at the Gate → change in Drain current → amplified output signal

This principle is widely used in radio-frequency amplifiers, audio amplifiers, oscillators, and many other circuits.

2. The Bipolar Junction Transistor (BJT)

The Bipolar Junction Transistor, or BJT, is a different type of transistor. The three terminals are:

• Base (B)
• Collector (C)
• Emitter (E)

The two most common types are NPN and PNP. For beginners, the NPN transistor is usually the easiest type to study first.

Simplified structure of the npn transistor.

BJT (TO-92 package) and schematic symbol.

How a BJT works

A BJT is controlled primarily by the current flowing into or out of its Base. In an NPN transistor, a small Base current allows a much larger Collector current to flow. In simplified form:

Base current → control of Collector current

The relationship is commonly described by the transistor's current gain: I_C ≈ β × I_B. where: I_C is the Collector current; I_B is the Base current; β (beta) is the transistor's DC current gain.

For example, if a transistor has a current gain of 100, a Base current of 1 mA can, under appropriate operating conditions, control a Collector current of approximately 100 mA. This does not mean that every BJT will always obey this equation exactly. The actual current depends on the operating region, voltage, temperature, and characteristics of the particular transistor.

The Base-Emitter junction

The Base-Emitter junction of a silicon BJT behaves somewhat like a semiconductor diode. For a typical silicon NPN transistor, the Base must be approximately 0.6–0.7 V more positive than the Emitter for significant conduction under ordinary conditions. This voltage should not be regarded as an exact universal value. The actual Base-Emitter voltage varies with Collector current, temperature, and transistor type. Once the Base-Emitter junction is forward biased, charge carriers are injected into the Base region. Because the Base is deliberately made very thin, many of these carriers pass through it and are collected by the Collector. This allows a relatively small Base current to control a much larger Collector current.

BJT as an amplifier

The BJT is particularly well suited to analog amplification. Suppose a small signal is applied to the Base. The Base signal produces a corresponding change in Collector current. When this current flows through a suitable load resistor, the change in current produces a much larger voltage variation at the output.

• small Base signal → larger Collector current variation → amplified voltage signal

This principle is fundamental to transistor amplifiers. BJTs have historically been widely used in:

• Audio amplifiers
• Radio-frequency amplifiers
• Oscillators
• Preamplifiers
• Signal-processing circuits
• General-purpose switching circuits

BJT as a switch

A BJT can also be used as an electronic switch. There are two important states:

• Cutoff: The Base current is essentially zero. The transistor is off, and the Collector current is very small.
• Saturation: The Base is supplied with sufficient current to drive the transistor strongly on. The Collector-Emitter voltage becomes relatively low, allowing current to flow through the load.

A simple switching circuit can therefore be controlled by applying or removing Base current. The disadvantage compared with a MOSFET is that the control circuit must supply Base current while the BJT is conducting.

3. The Insulated-Gate Bipolar Transistor (IGBT)

The Insulated-Gate Bipolar Transistor (IGBT) combines important characteristics of the MOSFET and the BJT. It has three terminals:

• Gate (G)
• Collector (C)
• Emitter (E)

The Gate is insulated, just as it is in a MOSFET. The main current path, however, uses a bipolar semiconductor structure. This combination gives the IGBT its distinctive characteristics.

Internal physical structure of an IGBT.

Schematic symbol of an IGBT.

How the IGBT works

The Gate of an IGBT controls the device through an electric field. When the Gate-to-Emitter voltage is below the required threshold, the device is essentially off. When a sufficient positive voltage is applied between Gate and Emitter, a conducting channel is created near the Gate. This channel controls the operation of the bipolar portion of the device. The bipolar structure then allows a large current to flow between Collector and Emitter. In simplified form:

Gate voltage → MOSFET-like control → bipolar conduction → large Collector current

This allows the IGBT to combine: the high input impedance of a MOSFET with the high-current capability of a bipolar transistor.

Why use an IGBT instead of a MOSFET?

The major advantage of the IGBT appears in applications involving relatively high voltage and high power. Power MOSFETs are excellent switching devices, particularly at low and moderate voltages and at high switching frequencies. However, as the required voltage rating increases, a MOSFET may have to use a thicker and more lightly doped semiconductor region. This can increase its conduction resistance.

The bipolar conduction mechanism of an IGBT can provide relatively low conduction losses at high voltage and high current. For this reason, IGBTs have traditionally been widely used in:

• Motor drives
• Industrial inverters
• Uninterruptible power supplies (UPS)
• Induction-heating equipment
• Welding equipment
• Electric-power converters
• High-power switching systems

The principal disadvantage of the IGBT

The same bipolar mechanism that gives the IGBT excellent high-power characteristics also makes it slower than many MOSFETs. When an IGBT is switched off, stored charge inside the semiconductor must be removed. This produces a characteristic current tail during turn-off.

As a result, an IGBT generally has greater switching losses than a suitable MOSFET operating at the same switching frequency. Therefore, the choice between a MOSFET and an IGBT is not simply a question of which device is "better."

The correct question is: Which device is better suited to the required voltage, current, switching frequency, and power level?

MOSFET vs. BJT vs. IGBT Comparison

Characteristic	MOSFET	BJT	IGBT
Control principle	Voltage-controlled	Current-controlled	Voltage-controlled
Control terminal	Gate	Base	Gate
Main terminals	Drain–Source	Collector–Emitter	Collector–Emitter
Input impedance	Very high	Relatively low	Very high
Continuous control current	Almost none	Required	Almost none
Switching speed	Very high	Moderate	Moderate
High-frequency operation	Excellent	Good	More limited
High-voltage operation	Good	Good	Excellent
High-current operation	Excellent	Good	Excellent
Typical strength	Fast switching	Analog amplification	High-power switching

An Important Practical Point for Radio Amateurs

For a beginner, it is useful to remember the three devices in a very simple way:

• BJT: "Control current with current." A small Base current controls a larger Collector current.
• MOSFET: "Control current with voltage." A voltage applied to the Gate creates an electric field that controls the Drain current.
• IGBT: "Use a MOSFET-like Gate to control a bipolar high-power device." The Gate requires very little steady-state current, while the internal bipolar structure allows the device to handle substantial voltage and current.

What Happens Inside a Switching Circuit?

Consider a microcontroller controlling a load such as a lamp, relay, or motor.

• With a BJT, the microcontroller supplies Base current. That Base current controls the Collector current.
• With a MOSFET, the microcontroller applies a voltage to the Gate. The Gate voltage controls the Drain-Source channel. Almost no steady-state Gate current is required.
• With an IGBT, the microcontroller also controls the device with Gate voltage, but the internal bipolar structure makes the device particularly suitable for higher-power applications.

The basic concept can therefore be summarized as:

• BJT: Base current → Collector current
• MOSFET: Gate voltage → Drain current
• IGBT: Gate voltage → bipolar Collector current

A Note About the Diode Often Shown With MOSFETs and IGBTs

In power-transistor circuit symbols, a diode is often shown connected between the main terminals. With a power MOSFET, this is the device's intrinsic body diode, which is an inherent consequence of the semiconductor structure. With an IGBT power device, a diode is also commonly used in parallel with the IGBT in practical switching circuits, especially in inverter and motor-drive applications. Depending on the exact device and symbol convention, the diode may be shown as a separate component rather than being treated as an intrinsic transistor terminal.

This diode is important when the transistor is used with inductive loads, such as motors, transformers, and coils. Inductors resist sudden changes in current, so a suitable path must be provided for the current when the transistor switches off.

The Main Idea to Remember

A transistor is not simply a device that "lets electricity through." It is a controllable semiconductor device in which a relatively small control signal determines the behavior of a much larger current. The three devices achieve this in different ways:

• BJT: current controls current.
• MOSFET: voltage controls current through an electric field.
• IGBT: voltage controls a bipolar power-conduction structure.

Once this distinction is understood, the symbols and practical applications of these three transistor families become much easier to understand.

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