Silicon Carbide Semiconductors in High Temperature, Power, and Radiation Environments

Silicon carbide (SiC) is a compound of carbon and silicon atoms. It is a very hard and strong material with a very high melting point. Hence, it is used in a variety of niche applications like abrasive machining processes, ceramic plates of a bulletproof vest, and refractories. The material can also be made into an electrical conductor, which finds applications in resistance heating, flame igniters, and electronic power components. In this article, we will discuss SiC technology used in Electronic devices. The chemical and electrical properties of SiC improve the efficiency and thermal conductivity of semiconductor devices. SiC based semiconductors are developed for use in high temperature, high power, and high radiation environments. They find application in electronics for Electric vehicles, Locomotive Traction, and Power Generation from Wind, Fuel Cells and Solar.

Production of Silicon Carbide

SiC is also known as Carborundum. It was originally produced by a high-temperature electrochemical reaction of an equal number of Silicon and carbon atoms. In a SiC compound material each silicon atom has three strong covalent bonds with three carbon atoms and one valance electron having chemical bond with an adjacent carbon atom. Since each carbon is attached to three silicon atoms in the same manner this chemical bonding is called Si-C bilayers (also called Si-C double layers). Si-C bilayers chemical structure is shown in Figure 1.

Figure 1: Si-C bilayers chemical bond

The formation of SiC is represented by the chemical equation as below:

                                                     SiO2 + 3C = SiC + 2CO

In pure semiconductors, devices are operating in a low-temperature environment and the intrinsic carriers are negligible. Dopant impurities are added to increase the electron or holes pairs in semiconductor materials. SiC can be doped n-type by phosphorus or nitrogen and p-type by beryllium, aluminum, boron, or gallium. Metallic conductivity has been achieved by heavy doping with boron, aluminium or nitrogen.

SiC Semiconductor's Properties

Being a wide bandgap semiconductor material, Silicon carbide (SiC) can operate at very high frequencies. SiC is not attacked by any acids or alkalis or molten salts all the way up to 800°C. It also has a very low coefficient of thermal expansion.  SiC has excellent mechanical properties like extreme hardness and low friction reducing mechanical wear-out. It exists in about 250 crystalline forms (called polytypes), out of which 3C-SiC, 4H-SiC, and 6H-SiC are the most common polytypes used in semiconductor devices. Each polytype exhibits unique fundamental electrical and optical properties are given in Table 1.

Table 1: Properties of SiC Polytypes

Why Silicon Carbide Technology?

Traditional Silicon power devices have reached the limit in terms of blocking voltage, operational temperature and switching characteristics. The Industrial, Automotive and Power Generation industry is consistently demanding devices that operate at very high temperatures, lower form factors and very high efficiency. Silicon devices like iGBT’s can operate at high power, high voltage but low frequencies and Mosfets which can operate at high frequencies but cannot carry high power and voltages.

SiC devices have very high blocking voltage, high breakdown electric field, and very low on resistance, and very high thermal conductivity. SiC allows power devices to operate at very high power, higher voltages, higher temperatures, and higher frequencies. SiC devices reduce the amount of energy lost in a system, improves performance, reliability, and cuts operating costs. By using SiC devices, systems can be built smaller with high efficiency and system flexibility.

Commercially Available SiC Devices

• SiC-based LED: SiC-based P-N junction light - emitting diode (LEDs) was first discovered in 1907, with yellow, green and orange emission. Yellow LEDs are made up of 3C-SiC, whereas blue LEDs are made up of 6H-SiC.
• SiC-based sensors: The wide bandgap of SiC is more efficient at absorbing UV rays (short-wavelength light) and dark current. The low dark current of SiC diodes are used in X-ray, heavy ion, neutron detection in nuclear reactor monitoring and enhanced scientific studies of high-energy particle collisions and cosmic radiation. UV-sensitive photodiodes are used as flame sensors in turbine-engine combustion monitoring and control.
• SiC-based Power Diode and Rectifiers:SiC-based rectifiers have features like high current density, high voltage, high power density, high temperature, high-frequency operation, and high switching speed. The 4H-SiC power Schottky diodes are used in Bipolar and Hybrid Power Rectifiers.
• SiC High-Power Switching Transistors: SiC MOSFETs features the industry’s highest junction-temperature rating of 200°C. There is only a very small variation of the on-state resistance at high temperatures. A SiC MOSFET is suitable for use in hard and resonant switching topologies like Low Conduction Loss (LLC) and Zero Voltage Switching (ZVS) converters.
• SiC-based RF device: The high power density of wide bandgap transistors has  proven to be quite useful in realizing solid-state transmitter applications. SiC-based high-frequency RF MESFETs and static induction transistors (SITs) are used in RF devices. SiC mixer diodes also show excellent promise for reducing undesired intermodulation interference in RF receivers.

Applications

• Industrial production: SiC-based MOSFETs and IGBTs are used in industrial applications like motor drive controllers, since they have features like high Switching frequency (40kHz) that allows the 20kVoltage range (in high voltage DC applications),  and operate at high temperatures of  200°C.
• Electric vehicles: In Electric vehicles, SiC electronic devices are used in battery interface, Main Inverter, DC/DC converter, Auxiliary Inverter/ Converters and Onboard Charger. SiC-based devices improve the system efficiency from 62.6% to 79.6 and energy consumption is reduced from 124.4 to 83.8 Wh/km as compared to Si-based Devices.
• Battery charging and energy storage applications: SiC-based Schottky diode and JFETs are used in Battery charging and power storage applications.
• Aerospace, Jet-aircraft, Automotive: SiC based sensors are used in Aircraft, Automotive engine sensors and Jet engine ignition systems. These sensors have reduced maintenance, pollution, weight along with higher fuel efficiency, and increased operational reliability.