Silicon carbide (SiC) is a widely used old-fashioned industrial material that has been mass-produced since 1893 and is still in use today. However, silicon carbide is difficult to find in nature, and the mineral moissanite in meteorites contains silicon carbide. Due to the high hardness of silicon carbide, the main use of silicon carbide is as an abrasive, it is also used in automobile brake discs, as an additive for automobile lubricants and as a substitute for jewelry diamonds, etc. In recent decades, however, it has been used as an electronic material, initially in light-emitting diodes (LEDs) and more recently in power electronic devices, including Schottky barrier diodes (SBDs), junction field effect transistors (JFETs). ) and MOSFET transistors. SiC MOSFETs have received particular attention due to their potential to replace existing silicon superjunction (SJ) transistors and integrated gate bipolar transistor (IGBT) technologies.

The semiconductor device potential of silicon carbide has been known for many years. In 1962 Lloyde Wallace was granted a patent (US3254280A) for a silicon carbide unipolar transistor device. It is essentially a junction field effect transistor. Figure 1 shows a diagram of the Lloyde 1962 patent. An N-type channel region is formed in the P-type SiC body. Source/drain contacts are formed to the N-type channel. The gate structure is located between the source and drain, and the corresponding gate electrode is located on the bottom side of the SiC substrate. Currently, UnitedSiC is producing SiC-based JFETs, although they are based on vertical designs for improved performance, with the source and gate on the top of the SiC die and the drain on the back.

Figure 1 from US3254280A (Silicon Carbide Unipolar Transistor)

The benefits of using SiC for power electronics were first described in 1989 by B. Jayant Baliga of North Carolina State University (NCSU). Baliga invented the IGB while at GE. He is now a Distinguished University Professor at NCSU. He derived a figure of merit called BHFFOM, which suggests that power losses can be reduced by using semiconductors with greater mobility and higher critical breakdown fields, such as SiC or even diamond. A series of patents related to power semiconductor applications of silicon carbide appeared during this time.

One of the main inventors at the time was John Palmour, who co-founded Cree in 1987 at Research Triangle Park, North Carolina. Now he is the CTO of Power and RF Technologies. Cree has been one of the main drivers of SiC power device technology. While still a graduate student at NCSU, he filed an important patent in 1987 that led to the invention of the SiC-based MOSFET transistor.

This seminal patent (US4875083A) concerns the formation of MOS capacitor structures on SiC substrates.

Here is an excerpt from the introduction of this patent published in 1987, describing the development of SiC at that time:

Silicon carbide has been a candidate material for semiconductor devices. Silicon carbide has long been recognized as having unique properties that give it superior properties to semiconductor devices formed from other commonly used semiconductor materials such as silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP). Silicon carbide has a wide band gap, high melting point, low dielectric constant, high breakdown field strength, high thermal conductivity and high saturation electron drift velocity. These properties make it possible for devices made from silicon carbide to operate at higher temperatures, at closer distances, at higher power levels, and in situations where some other devices made of other semiconductor materials simply cannot.
Despite these known properties, high-quality commercial devices made of silicon carbide have yet to emerge. Silicon carbide is a very difficult material to crystallize in over 150 different polytypes. Therefore, the fabrication of large single crystals of single polytype or specific polytype silicon carbide films required for electronic devices on semiconductor materials remains an elusive goal.
Recently, however, many advances in the field have made it possible for the first time to produce commercial-quality electronic devices on silicon carbide.
However, recent advances in this area have made the production of commercial high-quality silicon carbide electronic devices possible for the first time.

Figures 1 and 2 of this patent show the structure of the MOS capacitor, as shown in Figure 2 below. The capacitor is composed of a circular ohmic contact to a doped silicon carbide substrate with a central circular metal contact over a layer of oxide. The capacitance varies with the applied voltage due to carrier losses in the underlying SiC. The MOS capacitor structure is the key to forming a MOSFET transistor.

Figure 2: From US4875083A (metal insulator-semiconductor capacitor formed on silicon carbide)

Strangely, a device patent describing a simple planar MOSFET transistor on a SiC substrate does not appear to exist. In all likelihood, the concept would have been considered obvious at the time and not required to be patented. There are also patents describing methods of fabricating MOSFET transistors on silicon carbide substrates, describing variations on the basic structure of a simple MOSFET. For example, Yoshihisa Fujii, Akira Suzuki and Katsuki Furukawa filed US5170231A in 1990, describing a SiC MOSFET with asymmetric source/drain conductivities. Shortly thereafter, in 1992, John Palmour filed a groundbreaking patent (US5506421A) describing the structure of a vertical trench gate SiC MOSFET. The application was granted in 1996 and is now over 20 years old, so the patent has expired and the concept described is now in the public domain. However, there are many patents related to SiC MOSFETs that are still valid after this patent. For example, a search revealed that Cree has more than 700 active patents related to SiC MOSFET technology.

The structure of the vertical channel SiC MOSFET shown in US5506421A is shown in Figure 3 below. The patent claims that vertical power MOSFETs with low on-resistance and high temperature range are formed on the C-plane of a silicon carbide substrate, similar to N-type. An N-drift layer is formed over the substrate, followed by a P-channel layer. The trench gate penetrates the P-channel layer and forms an N+ source region. Metal source and drain electrodes are located on the top and bottom of the die, respectively. This trench architecture is sometimes referred to as UMOS (U-shaped gate) to distinguish it from planar DMOS (drift MOS) designs.

Figure 3 US5506421A (Power MOSFET in SiC)

By 2011 , Cree introduced the first SiC power MOSFET on the market, the CMF20120D device. The CMF20120D is a vertical N-channel enhancement mode SiC MOSFET. Figure 4 shows a cross-sectional SEM micrograph of the planar transistor gate in the CMF20120D device. The N+ source and P-type body implants are depicted in this SEM micrograph.

Figure 4 Cree CMF20120D SiC planar MOSFET cross section

The SiC power MOSFET market has expanded significantly since 2010 and now exceeds $200 million per year. As SiC replaces silicon technology in multiple markets such as automotive, photovoltaics, and railways, many new players have entered the market and are expected to achieve double-digit compound annual growth rates. Typically, SiC power MOSFETs operate at 1200 or 1700 V and are designed to replace IGBT technology. 650 V SiC MOSFET devices were recently released, which may be aimed at competing with silicon superjunction and GaN-based technologies.

It appears that Cree continues to focus on planar SiC MOSFET technology. However, others, including Infineon and Rohm, are using trench or UMOS technology. In contrast, STMicroelectronics is also focusing on planar SiC MOSFET technology. Figure 5 shows a cross-sectional SEM micrograph of the trench gate found on a ROHM SCT3022AL 650 V SiCN channel MOSFET.

A comparison of Figure 5 with claim 1 of US5506421A shows that the ROHM SCT3022AL uses many of the features in the John Palmour fine trench SiC MOSFET patent. For example, SEM images show the presence of trenches, insulating layers and gate electrodes.

Figure 5 Rohm SCT3022AL 650 V SiC MOSFET cross section

Silicon carbide is a disruptive technology that is starting to gain traction as it replaces silicon-based technologies in a variety of key power electronics markets. . Innovative work by key inventors has made this possible since the mid-1980s. It is predicted that the SiC power electronics market will exceed $1 billion by 2025, and possibly sooner. The war is getting bigger