The transistor family of power devices, including the Bipolar Junction Transistor (BJT), the Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and the Insulated Gate Bipolar Transistor (IGBT), offers a different set of switching characteristics compared to thyristors. These devices are generally controlled by a continuous signal at their base or gate terminal, offering different control paradigms compared to the triggered latching behavior of thyristors, making them suitable for a wider range of applications, including high-frequency power conversion systems.

4.1.1 Bipolar Junction Transistor (BJT)

The Bipolar Junction Transistor (BJT) is a three-terminal device whose operation depends on the contact between two types of semiconductors. It is termed “bipolar” because its operation involves two types of charge carriers: holes and electrons.

Basic Structure

A BJT is constructed from two P-N junctions connected back-to-back, sharing a thin central region. This creates a three-layer structure with three terminals: the emitter, the base, and the collector.

Key Electrical Characteristics

The operation of a BJT is governed by the relationship between the currents flowing through its terminals. A small current at the base terminal controls a much larger current flow between the collector and the emitter.

• Current Relationship: The emitter current is the sum of the base and collector currents: IE = IB + IC
• Transport Factor (α): This is the ratio of the collector current to the emitter current: α = IC / IE
• Current Gain (β): This is the ratio of the collector current to the base current and defines the amplification capability of the device: β = IC / IB = α / (1 – α)

Primary Functions

In electronic circuits, a BJT is a versatile component that can be configured to function as a switch, an amplifier, or an oscillator.

4.1.2 Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is a type of transistor primarily used for switching electronic signals. It is a four-terminal device—Source (S), Drain (D), Gate (G), and Body (B)—though it is commonly used as a three-terminal device by connecting the Body to the Source.

Defining Structural Feature

A MOSFET’s operation is defined by its unique gate structure. The gate terminal is electrically isolated from the main current-carrying channel by a very thin layer of silicon dioxide. This insulating layer results in an extremely high DC input impedance, meaning the device is voltage-controlled and requires negligible steady-state gate current to maintain its state.

Forms of Operation

MOSFETs exist in two main operational forms, defined by their default state at zero gate voltage:

• Depletion State: The device is normally ON when the gate-source voltage is zero. A specific gate-source voltage must be applied to switch the device OFF.
• Enhancement State: The device is normally OFF when the gate-source voltage is zero. A gate-source voltage must be applied to create a conducting channel and switch the device ON.

Key Performance Characteristics

A key advantage of the MOSFET is its control efficiency. It requires a very low current (often less than one milli-ampere) to be switched on, yet it can control a high current load, with some devices capable of delivering more than 50 Amperes to a load.

4.1.3 Insulated Gate Bipolar Transistor (IGBT)

The Insulated Gate Bipolar Transistor (IGBT) is a three-terminal semiconductor device engineered as a hybrid to leverage the best attributes of both MOSFETs and BJTs. It combines the isolated gate structure of a MOSFET for simple voltage control with the high current and low saturation voltage capabilities of a BJT.

Operational Principles

The IGBT is a unidirectional device, meaning current can only flow in the forward direction from the collector to the emitter. A key operational feature is that it requires only a small voltage applied to its gate to maintain conduction, making it relatively easy to drive.

Primary Advantages and Characteristics

The IGBT is characterized by several key advantages that make it suitable for demanding applications:

• Fast switching speeds
• High operational efficiency
• A low ON-state resistance (RON) compared to a MOSFET, which reduces conduction losses.

Typical Applications

The unique combination of high current handling and fast switching makes IGBTs ideal for medium to ultra-high power applications. They are commonly found in electric cars, variable frequency drives (VFDs) for motor control, and traction motors.

The individual analysis of these devices reveals a range of characteristics. To facilitate effective system design, a direct comparison is necessary to guide component selection.