Knowledge popularization: What is silicon carbide

2024-01-30 321

The development of semiconductor materials has been decades, at present, the first generation of semiconductor silicon Si and germanium Ge have reached the application bottleneck, with the deepening of research, the shortcomings and application boundaries of the first generation of semiconductor materials are gradually revealed. The third generation semiconductor represented by gallium nitride and silicon carbide provides more possibilities for the performance improvement of electronic devices with its excellent electrical characteristics.

Understanding silicon carbide

silicon carbide (English: silicon carbide, carborundum), chemical formula SiC, commonly known as emery, gem name diamond pulp, natural mineral for moissanite, for silicon and carbon phase bonding of ceramic compounds, silicon carbide in nature in the form of moissanite this rare mineral. Silicon carbide is currently widely used in the third generation of semiconductor materials, the natural formation is very rare, mainly by artificial synthesis, silicon carbide high-purity powder is the raw material of the growth of silicon carbide single crystal by PVT method, at a high temperature of more than 2000℃, the carbon powder and silicon powder through high temperature decomposition into atoms, through temperature control deposition on the silicon carbide seed crystal to form silicon carbide crystal.

Material property

SiC is a wide band gap semiconductor composed of silicon (Si) and carbon (C). The energy gap is around 3.26 eV. Its binding force is very strong and it is very stable in thermal, chemical and mechanical aspects. SiC has a variety of polytypes (polycrystalline types), and their physical properties are different. Here is a detailed look at the detailed characteristics of silicon carbide.

Physical characteristics

Silicon carbide (SiC) is a hard dark green odorless powder with a molar mass of 40.097g·mol⁻¹, a density of 3.16 g/cm3 (hexagonal silicon carbide), and a melting point of 2830 °C.

Chemical characteristic

Silicon carbide (SiC) is insoluble in water and ethanol, and has high chemical inertness. Compared with crystalline silicon, it has higher thermal conductivity, electric field breakdown strength and maximum current density, and the coefficient of thermal expansion is also very low. In addition, there are various polycrystalline types of silicon carbide, the more common types are (β)3C-SiC, 4H-SiC, (α)6H-SiC these three crystal types, the following is its structure diagram.

(β)3C-SiC

4H-SiC

(α)6H-SiC

Silicon carbide exists in about 250 crystalline forms. Alpha-silicon carbide (alpha-sic) is the most common of these polytypes, forming at temperatures greater than 1700°C and having a hexagonal crystal structure similar to wurtzite. β-silicon carbide (β-SiC), with a diamond-like sphalerite crystal structure, is formed at temperatures below 1700°C. Because of its higher specific surface area than α-silicon carbide, β-silicon carbide can be used as a negative carrier for heterogeneous catalysts.

Pure silicon carbide is colorless, and industrial silicon carbide is brown to black due to impurities such as iron. The iridescent luster of the crystal is due to the passivation layer of silica produced on its surface.

Research and development background

Silicon carbide was first used as an abrasive because of its hard texture. In the application process, scientists found that silicon carbide has a very good performance when applied to electronic devices, and then scientists began to develop large-scale artificial silicon carbide technology. The development trajectory of silicon carbide is similar to that of gallium nitride, but from the perspective of application history, silicon carbide is the predecessor of gallium nitride.

R&d history

In 1810 Berzelius reported the synthesis method of reducing potassium fluosilicate with potassium metal.

In 1849, Charles Mansuete Despretz reported a synthetic method for burying electrified carbon rods in sand grains

In 1881 Robert Sydney Marsden reported a synthesis method of dissolving silica with molten silver in a graphite crucible.

In 1882, Albert Colson synthesized silica by heating it in an ethylene atmosphere and in 1881, Paul Schutzenberger reported the synthesis of silica and silica mixtures by heating them in a graphite crucible.

However, the real production of silicon carbide in large quantities was first achieved in 1890 by Edward Goodrich Acheson. Acheson discovered this method of synthesizing silicon carbide while attempting to make synthetic diamonds by heating a mixture of clay (aluminum silicate) and coke powder in an iron pan. He mistmisted the resulting blue crystals of emery for a corundum like substance made of carbon and aluminum.

In 1893, Henry Moissan discovered a rare natural silicon carbide ore while studying meteorite samples from the Dayabro Canyon in Arizona, and named it Moissanite. Moissan also synthesized silicon carbide by several methods, including melting elemental carbon with molten elemental silicon, melting a mixture of SIC and silica, and reducing silica with elemental carbon in an electric furnace. Moissan, however, credited Acheson with the discovery of silicon carbide in 1903.

Acheson applied for patent protection on February 28, 1893 for a method of synthesizing silicon carbide powder.

In 1907, Henry Joseph Round, an employee of the Marconi Company and assistant to Marconi, obtained the world's first light-emitting diode by applying a voltage to a silicon carbide crystal and observing yellow, green, and orange light emitted on the cathode. These experimental results were later repeated in 1923 by the Soviet scientist Oleg Loshev.

Scope of application

Silicon carbide devices have wide application scenarios. Due to its high thermal conductivity, high breakdown electric field strength and high current density, semiconductor devices based on silicon carbide materials can be used in automotive, charging equipment, portable power supplies, communication equipment, robot arms, aircraft and other industrial fields. The scope of its application is also constantly popularized and deepened, and it is a very broad application prospect and very valuable material.

Silicon carbide power devices have achieved mature applications in wind power generation, industrial power supply, aerospace and other fields. With the rapid development of new energy vehicles, photovoltaic power generation, rail transit, smart grid and other industries, the use of power devices has increased significantly.

According to IC Insights' 2019 Optoelectronics, Sensors, and Discrete Devices Market Analysis and Forecast Report, global power device sales grew by 14% in 2018 to reach $16.3 billion. The IHSMarkit data show that in 2018, the silicon carbide power device market size of about 390 million US dollars, driven by the huge demand for new energy vehicles, as well as power equipment and other fields, it is expected that by 2027, the market size of silicon carbide power devices will exceed 10 billion US dollars. The market demand for silicon carbide substrate will also increase significantly.

Fast charge field

We focus on the application of silicon carbide in the field of fast charging, like gallium nitride, silicon carbide is mainly used as a power electronic component in charging products. The difference is that silicon carbide is mainly used in high-power, especially more than 100W fast charging products, while fast charging products for gallium nitride applications contain almost all power ranges.

Working principle

Silicon carbide is the third generation semiconductor that has been widely concerned at present, and has been mass-produced in the early stage in the application of fast switching, high temperature and high voltage. The first available component was the Schottky diode, followed by junction Fets and high-speed switching power MOSFETs. Bipolar transistors and thyristors are currently being developed.

The most relevant to the charger is the diode and the MOS tube. Silicon carbide is mainly used to form these components, specifically to form the substrate of the above devices to replace silicon. How does this work?

We take the MOS tube as an example, in the component, silicon carbide as the substrate, due to the incorporation of trivalent boron ions, resulting in positive charges on the substrate, so we call this semiconductor P-type semiconductor.

Then we assemble this silicon carbide substrate into a MOS tube with the gate, source and drain, and give it high and low levels to achieve the inverter from direct current to alternating current. Why the MOS tube with silicon carbide as the substrate and the IGBT (the integration of the MOS tube and the transistor) are stronger, this is because the component needs to be constantly on and off the current, and the high voltage will continue to act on the substrate.

The voltage acting on the substrate is very high when it is disconnected, the band gap width of general silicon (a measure of the amount of withstand voltage) is 1.12ev, and silicon carbide is 3.26ev, which is three times that of silicon, which means. At the same voltage, the design area of the silicon substrate only needs to be one-third of that of silicon. In addition, silicon carbide also has better thermal conductivity, higher operating temperature and higher operating frequency (switching frequency).

We have sorted out the performance comparison table of silicon and silicon carbide, and have a look at it together.

In general, in addition to the advantages of band gap width, the silicon carbide substrate can be higher strength and stronger pressure resistance. In addition to this:

1) The higher tolerance temperature means that the high temperature environment of silicon carbide can be tolerated is wider and the application range is larger.

2) The switching frequency is several times higher, so that it, like gallium nitride, can greatly reduce the volume of other components, especially transformers, thereby reducing the volume of charging products.

3) Better thermal conductivity means that the energy conversion rate is higher when the component is working, the loss is small, and the device is more resistant to damage. It can also improve the component density of integrated circuits and reduce the volume of charging products.

In general, the components and advantages of silicon carbide used in electronic products are basically similar to gallium nitride, which are used in power devices such as MOS tubes, IGBTs and other switching tubes. Due to its excellent band gap width, higher heat resistance and better thermal conductivity, it can save device size and BOM cost. And reduce the size of the product specifications, improve the user experience.

However, the application of silicon carbide to tubular components has a short board, that is, such components are more prone to short circuit failure.The specific principle is that silicon carbide often has a smaller chip area due to better thermal conductivity, which will make its conductivity higher, thereby increasing the risk of short circuit. Secondly, in short circuit condition, the silicon carbide MOS tube needs higher forward gate bias, which further aggravates the degradation of gate oxide layer during short circuit.

Other applications

Abrasive and cutting tools

Because of its durability and low cost, emery is used as a common abrasive in modern gem processing. The hardness of emery makes it used in grinding processes such as grinding wheel cutting, honing, water knife cutting and sandblasting in the manufacturing industry. The silicon carbide particles are laminated onto the paper to make sandpaper and the grip of the skateboard.

In 1982, the super-strong composite material composed of alumina and silicon carbide whisker came out, and after the development of the following three years, this composite material came out of the laboratory and became a commodity. In 1985, Advanced Composites and Greenleaf introduced a new commercial cutting tool made from a reinforced composite of alumina and silicon carbide whiskers.

Structural material

In the 1980s and 1990s, several high-temperature gas turbine research projects in Europe, Japan and the United States studied silicon carbide, with the goal of replacing nickel superalloys with silicon carbide for turbine blades or nozzle blades. However, none of these projects achieved mass production, mainly due to the low impact resistance and fracture toughness of silicon carbide materials.

Unlike other ceramic materials such as alumina and boron carbide, silicon carbide can be used to make composite armor (such as Chobham armor) and ceramic plates in bulletproof vests.

astronomy

Silicon carbide has low coefficient of thermal expansion, high hardness, rigidity and thermal conductivity, so that it can be used as a mirror material for astronomical telescopes. Polycrystalline silicon carbide discs with diameters of 3.5 meters and 2.7 meters, fabricated by chemical vapor deposition, have been installed on several large astronomical telescopes, including the Herschel Space Observatory and the Stratoinfrared Observatory, respectively.

Catalyst support

The oxidation resistance of silicon carbide itself and the large specific surface area of cubic β-SiC make it a suitable carrier for heterogeneous catalysts. β-SiC synthesized from rice husk carbonization has been used as heterogeneous catalyst carrier to catalyze the oxidation of hydrocarbons such as n-butane oxidation to maleic anhydride.

Graphene growth

By heating to high temperatures, epitaxial graphene can be obtained on the surface of silicon carbide. This method of obtaining graphene is considered promising for large-scale synthesis of graphene with practical applications.

Material application bottleneck

The main problem in the commercialization of SIC components is how to remove defects, including edge dislocations, spiral dislocations (hollow and closed), triangular defects and base dislocations. Therefore, although there are many studies trying to improve the characteristics, the earliest SiC material components, their reverse voltage blocking ability is not good. In addition to crystal quality, interface problems between SiC and silica also affect the development of SiC MOSFETs and IGBTs. Nitriding has greatly improved the interface problem, but the mechanism is still unclear.

In 2008, the first commercial JFET, rated at 1200V, was followed by the first commercial MOSFETs, rated at 1200V, in 2011. In addition TO the common TO-247 and TO-220 packages for SiC switches and SiC Schottky diodes, many manufacturers have also begun TO place SiC bare crystals in power modules.

SiC SBD diodes have been used in PFC power factor correction circuits and IGBT power modules. Conferences such as the International Conference on Integrated Power Electronics Systems (CIPS) also regularly report on the technical trends of SiC power components.

Some of Japan's newly built high-power cross-transfer railway vehicles have replaced IGBTs with silicon carbide for traction converters (such as the Shinkansen ALFA-X, EMU3000 and E235 series), helping to further reduce vehicle power consumption.

Sum up

The development of technology is inseparable from the innovation of materials, and the transformation from silicon substrate to silicon carbide substrate is not only the replacement of semiconductor materials, but also the technical breakthrough of integrated circuits. Today, the application potential of third-generation semiconductors has not yet been fully released, and the application prospect is very promising in the industry. But like any new thing, silicon carbide is still technologically immature in many places. For example, the degree of crystallization of silicon carbide needs to be improved, and the problem of asymmetric gate drive circuits of silicon carbide components also needs to be solved.

But anyway, with the deepening of the field of material application, I believe that these problems of silicon carbide will be solved sooner or later, after all, practice is the best teacher.