BASIC KNOWLEDGE - SEMICONDUCTORS What you need to know about power semiconductors

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Power semiconductors are efficient devices that can withstand high voltage and current with lesser losses. From acting as a switch in power electronics to controlling the speed of a fan, power semiconductors are reliable devices with high power ratings. Read this article to get everything about power semiconductors explained!

What is a power semiconductor?

Power Semiconductors perform the modified electronic functions of regular semiconductors with a high power rating. Unlike simple semiconductor devices, power semiconductors are capable of withstanding high voltage and current with lesser leakage, voltage drop, and other power losses. The current and voltage we deal with in power semiconductors start in the order of kilos and mega.

Basic uses of power semiconductors:

• 1. Switching to turn ON/OFF electricity

• 2. A component in converters and inverters

• 3. Used in power amplifiers to amplify a signal

The main use of power semiconductors is for switching and converting purposes in power control systems. Most importantly, power semiconductors are part of systems that enable power generation and long-distance transmission and distribution of electricity.

As components in power management subsystems, power semiconductors are typically used as switching devices and rectifiers (to convert electrical signals), as well as to change the voltage or frequency of an electrical current.

E-BOOK

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Si, SiC, and GaN, explained

The semiconductor material silicon (Si) is still used for many high-voltage and high current applications today, though it has been joined by silicon carbide (SiC) and gallium nitride (GaN) in recent years. The latter two materials have a wider bandgap, which significantly reduces power loss and increases efficiency. Of these semiconductor materials, GaN offers the best performance.

Types of power semiconductor devices

Power semiconductor devices categorize into three types:

• 1. Power Diodes

• 2. Thyristors

• 3. Power Transistors

Another classification of power semiconductors is based on their operation.

Just as the name suggests, uncontrollable power semiconductors cannot be controlled by altering input or any of the terminals. In contrast, fully controllable devices are easily controlled through input voltage or current.

Power Diodes

A power diode is an uncontrollable power semiconductor device capable of rectifying very strong electrical signals. It can handle hundreds of amperes and thousands of kilovolts. A typical PN junction diode is upgraded into a power diode by inducing an additional N- lightly doped intrinsic semiconductor layer. The lightly doped layer between the P and N layers of the diode is termed the drift layer. Adding a drift layer enables a power diode to withstand high voltage. Power diodes are used as rectifiers, voltage multipliers, clipper circuits, etc.

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Thyristors

A Thyristor is a semi-controllable four-layer and three-terminal power semiconductor switch. It consists of an alternating PNPN layer and three terminals - anode, cathode, and control. Most Thyristors can easily switch ON but lack the ability to turn OFF. Thyristors are often used to control electric power and act as protection circuits in home appliances, electrical tools, and outdoor equipment.

Silicon Controlled Rectifier

An SCR (Silicon Controlled Rectifier) converts an AC signal to a DC signal. It is a three-terminal device that follows a two-transistor analogy. There are three terminals in an SCR - anode, cathode, and gate. The anode and cathode are heavily doped, the gate is moderately doped, and the drift layer is lightly doped. SCR consists of a four-layer PNPN configuration that forms three PN junctions. It is an uncontrollable power semiconductor device that does not turn off until the main current is interrupted, increasing its turn-off time. SCR has one of the lowest ON-state resistance and high conductivity modulation compared to other devices. But it has the least switching frequency. SCR is used in switching circuits and motor drives. Another type of SCR - LASCR (Light-activated Silicon Controlled Rectifier) - is used in HVDC transmission.

Gate Turn Off

A GTO (Gate Turn Off) Thyristor is a three-terminal and four-layer device that allows the gate to switch off the device. The negative pulse at the gate terminal is used to turn it off. There are three PN junctions formed in the structure of GTO that lead to increased conductivity of the device. GTO has comparatively faster rise and fall times, lower size, and more efficiency than SCR. GTOs are used in AC/DC motor drives, robotics, etc.

Mos-controlled Thyristor

An MCT (Mos-controlled Thyristor) is a fully controllable thyristor consisting of two MOSFETs. An n-channel MOSFET and p-channel MOSFET are connected in an MCT Thyristor. Each MOSFET is responsible for turning on and off the state of the device.

Reverse Conducting Thyristor

An RCT (Reverse Conducting Thyristor) is a device that has a fabricated anti-parallel diode on the same IC. It is a semi-controllable unipolar device that cannot block reverse voltage during operation. RCT is used in inverters and high-power choppers.

Integrated Gate Commutated Transistor

An IGCT (Integrated Gate Commutated Transistor) is a thyristor that turns off like a transistor. It is a fully controllable Thyristor that acts like a GTO but with a faster turn-off time and negligible conduction losses. IGCT is used in high-power semiconductor devices such as frequency inverters, drivers, and compensators.

Triode for Alternating Current

A TRIAC (Triode for Alternating Current) combines two SCR in an antiparallel configuration with gate terminals connected. It consists of two main terminals and a single gate terminal that is a combination of both SCRs. There is no meaning of cathode or anode because bidirectional current flows through it. TRIACs are used to control AC signals in a variety of electronics such as fans and lights.

Diode for Alternating Current

A DIAC (Diode for Alternating Current) is a bidirectional thyristor diode that is similar to a TRIAC but with the absence of a gate terminal. Two SCRs are connected in an antiparallel configuration with two main terminals. There is no gate terminal, leading DIAC to operate as an uncontrolled switch. DIACs are mostly used to trigger TRIACs in power electronics.

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Power Transistors

Unijunction Transistor

A UJT (Unijunction Transistor) is a semiconductor device that has a p-type emitter terminal connected to an n-type bar of two bases B1 and B2. It is sometimes referred to as a Double-base device. For simplification, the UJT circuit is represented by a PN diode connected to two resistances. Just as the name suggests, a UJT device forms a single PN junction. UJT exhibits a negative resistance and is used as a relaxation oscillator to trigger SCR Thyristor.

Power BJT

A Power BJT is a four-layer three-terminal semiconductor device that has high current ratings. It has three terminals: base, emitter, and collector. A power BJT serves a variety of applications like power amplifiers, relays, and power control systems.

Power MOSFET

Power metal-oxide-silicon transistors are fully controllable power semiconductor switches designed to handle large amounts of power. Power MOSFET is a three-terminal voltage-controlled majority carrier device that has a vertical channel configuration to increase the current rating. The source and drain are placed on the opposite side of the silicon to increase the power rating of the device. Power MOSFETs offer low gate drive power, rapid switching speed, and wide bandwidth, in addition to being easy to operate and repair. They are the most commonly used type of power transistors that perform well at high frequencies. Power MOSFET dominates 53 % of the transistor market, making it one of the most popular power semiconductor devices.

Insulated-gate Bipolar Transistor

Insulated-gate bipolar transistor (IGBT) is a fully controllable power semiconductor switch used for low-to-medium frequency applications. IGBT is an integration of power MOSFET and BJT to offer high efficiency. The device forms a PNPN-configuration semiconductor device that enables voltage control. The gate terminal is insulated with a metal oxide coating while the emitter and collector form conducting regions inside the device. There is an additional buffer layer and injection region makes an IGBT more efficient than most devices. Unlike power MOSFET, a bipolar current flows inside an IGBT device. IGBTs have high power ratings, and low on-state voltage and are typically used as discrete devices in power electronics units such as consumer electronics, air conditioners, and electric cars.

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Applications of power semiconductors

There are three types of power switches either used discretely or as part of power integrated circuits (PICs): metal-oxide-silicon transistors (MOSFETs), insulated-gate bipolar transistors (IGBTs), and bipolar junction transistors (BJTs). The main industries that benefit from power semiconductors are power transmission and distribution, automotive and transport, renewable energy, and consumer electronics. Power semiconductors play a key role in the sustainable and efficient use of energy and can be used to transport energy over long distances with minimal losses.

Reliability and failure of power semiconductors

Power semiconductors can fail or be damaged if the operating voltage or current is too high. Overvoltage can puncture the insulating gate oxide layer on a power MOSFET or IGBT. To ensure reliable functioning, experts recommend operating them at 20 % below their operating range.

Overheating is another major reason why power semiconductors can fail. Electronic components naturally heat up due to ON-state resistance, in example. the resistance between the drain and source during the transistor operating mode. The higher the ON-state resistance, the greater the power loss and the heat. It is essential to consider thermal management in any power electronics system. Currently, research focuses on reducing ON-state resistance, performing proper insulation within the layer, and maintaining high-performance components.

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