What are Silicon Carbide Semiconductors

Silicon Carbide is a compound semiconductor used in many semiconductor devices where it provides higher breakdown voltages and a number of other advantages.

Semiconductors Includes:
What is a semiconductor Holes & electrons Semiconductor materials Compound semiconductors Silicon carbide, SiC Gallium nitride, GaN

Silicon carbide, SiC is use for semiconductor devices where this compound semiconductor enables it to provide some useful advantages for a number of types of electronic circuit design.

Silicon carbide is probably best known as carborundum which it is a particularly hard material. However it can also be used as a semiconductor material.

Silicon carbide is a compound semiconductor and even though it has silicon as one of its main constituents, it nevertheless has some very different properties to silicon on its own.

This enables it to be used in semiconductor devices which have some unique properties which can be used in a number of electronic circuit designs where other semiconductor device would not be used.

Silicon carbide, SiC properties

Silicon carbide possesses a number of properties that make it an ideal choice for use within semiconductors. Although the material is exceedingly hard, it can still be processes chemically for use in semiconductor devices or electronic components.

Main Physical Properties of Silicon Carbide
Properties	Units	Silicon Carbide
Density	g/cm³	3.15 - 3.20
Hardness	MPa	2300 - 2900
Fracture Toughness	MPa.m^1/2	14 - 120
Thermal Expansion Coefficient	10^-6/°K	7.9 - 11

In view of its physical properties and in particular its hardness, silicon carbide is often used for very hard drills and a number of other abrasive situations. Its properties in this arena have been known for very many years.

Applications for silicon carbide, SiC devices

The hardness of silicon carbide gives it many used within cutting and drilling areas. However it has some significant advantages for use within electronic components and more specifically semiconductor devices.

The two main areas where silicon carbide can be used as a semiconductor:

Silicon carbide, SiC Schottky diodes: SiC Schottky diodes are used in many power circuits where they provide a higher switching performance, efficiency, power density and lower systems costs when compared to silicon diodes. These diodes provide zero reverse recovery, low forward voltage drop, current stability, high surge voltage capability and positive temperature co-efficient.
SiC MOSFETs : The other main area where silicon carbide is used is for MOSFETs. Silicon carbide MOSFETs came to the market after sSiC diodes as a result of fabrication process difficulties, but now they are available they are able to use the improved performance allowed by silicon carbide in many areas. They tend to be used within electronic circuit designs for power switching. Here they are able to outperform both silicon power MOSFETs and also IGBTs which are probably the main competitors.

These are the main uses for silicon carbide within semiconductor devices or electronic components. However as the material lends itself to semiconductor technology in a variety of ways, it is likely other devices might use the technology in the coming years.

Silicon carbide semiconductor basics

Silicon carbide is what is referred to as a compound semiconductor as the material has two elements. Unusually for compound semiconductors both elements are within the same group, group IV, of the periodic table of elements.

The main highlight electrical properties of silicon carbide that are important for their use in semiconductor devices are summarised in the table.

Main Electrical Properties of Silicon Carbide Used in Semiconductor Devices
Property	Si	SiC
Energy Gap: E_G(eV)	1.12	3.26
Electron Mobility: µ_n(cm²/V_S)	1400	900
Hole Mobility: µ_p(cm²)	600	100
Breakdown Field: E_B(V/cm)X10⁶	0.3	3
Thermal Conductivity(W m^-1 °K^-1)	1.5	320 - 350
Saturation Drift Velocity: v_s(cm/s)X10⁷	1	2.7
Relative Dielectric Constant: e_S	11.8	9.7

One of the key features of silicon carbide, SiC as a semiconductor is that it features a breakdown electric field strength which is about ten times that of silicon on its own.

This significantly higher breakdown level makes it possible to fabricate devices that can operate at very high voltages - some are in the region above 600 volts and this can extend to even a few thousand of volts. A figure that would not be possible for silicon devices.

This is achieved through a thinner drift layer in the device and at a higher impurity concentration.

Because most of the resistance within high voltage devices arises in the drift layer, the use of silicon carbide, SiC makes it possible to achieve much higher voltages while also achieving extremely low "ON" resistance levels for a given unit area.

Theoretically, the drift layer resistance per area can be reduced by a factor of around 300 when compared to silicon at the same withstand voltage. This gives a significant improvement in performance for SiC MOSFETs devices.

Silicon carbide manufacturing

Silicon carbide is formed when silicon and carbon react together.

One of the simplest processes for this is known as the Acheson Process. This uses a graphite electric resistance furnace which is maintained at a temperature between 1,600°C and 2,500°C.

To produce bulk silicon carbide crystals for semiconductor manufacture a process called the Lely method is often used. In this process the silicon carbide powder is loaded into a graphite crucible, which is purged with Argon gas and heated to approximately 2,500°C. Then the silicon carbide near the outer walls of the crucible sublimes and is deposited on a graphite rod near the center of the crucible, which is at a lower temperature.

Silicon cabide exists in around 250 crystalline forms, but the form most often used because it is more suitable is known as the 4H form which has a hexagonal structure.

The various different forms of silicon carbide have different characteristics - thermal conductivity, bandgap and the like.

Doping silicon carbide

In order for silicon carbide to act as a semiconductor, it requires a small amount of impurities or dopants to be added. In its pure form silicon carbide, like other semiconductors acts like an insulator because no free electrons are available.

Different elements are added to make the SiC crystal either P-type or N-type.

P type silicon carbide: P-type silicon carbide can be created by adding aluminium, boron, or gallium.
N-type silicon carbide: N-type silicon carbide can be created by adding nitrogen or phosphorus. N-type silicon carbide is the most widely used because it uses electrons as the majority carrier and therefore operates in a much swifter fashion as holes have a much lower mobility.

One of the advantages of silicon carbide within the fabrication process is that it is possible to control the P-type and N-type regions within a device over wide ranges. This has proved to be another advantage of using silicon carbide over silicon for power devices.

Silicon carbide is a semiconductor material that is being used increasingly in a number of areas. Silicon carbide electronic components are particularly useful where high voltages need switching, rectifying, etc. Both SiC Schottky diodes and SiC MOSFETs are available and being used increasingly in a variety of power electronic circuit designs.