**2. High-Temperature Components**

The electrical system, from power generation, power conversion, and power transmission to all kinds of power equipment, runs in a wide temperature range. However, these systems cannot be developed without the improvement of advanced material, power electronic devices, gate drives, and passive components [20]. This section outlines the development profiles of the high-temperature components.

#### *2.1. The Properties of SiC Material*

In 1824, a Swedish scientist called J. J. Berzelius discovered the existence of SiC material, and subsequent research revealed that this material has good performance. However, SiC material had not been well-developed due to the outstanding achievement and rapid development of Si technology at that time. Until the 1990s, Si-based devices could not meet the high requirement of power electronics, such as high frequency, high voltage, high temperature, and high power density. This has once again ignited the interest of researchers in SiC material.

Since the covalent bond between carbon and silicon is stronger than that between silicon atoms, SiC material have higher breakdown electric field strength, carrier saturation drift rate, thermal conductivity, and thermal stability compared to Si material. SiC material have a variety of di fferent crystal structures (polytypes), and more than 250 have been identified to date. Although there are many types of polytypes, only three crystalline structures exist—cubic, hexagonal, and rhombohedral. The physical properties of the current available semiconductor materials are listed as Table 2. Despite the same atomic composition in all SiC polytypes, the electrical properties di ffer. For instance, the band-gap for SiC ranges from 2.2 eV for 3C-SiC to 3.2 eV for 4H-SiC. Since 4H-SiC has higher electron mobility than 6H-SiC, it is a preferable option for SiC-based devices. Due to that the thermal conductivity of SiC, which is three times that of Si, and it is expected to withstand higher operating temperature for devices equipped with SiC material.


**Table 2.** The physical properties of the available semiconductor materials under room temperature (25 ◦C).

However, high-purity SiC powder, which can be used to grow SiC boules, is only available from a limited number of suppliers, and is relatively expensive [25]. At present, the United States is the global leader in the production of SiC substrates and wafers, followed by Europe and Japan. The quality of SiC substrate is critical for the manufacturing of high-quality chips, and the SiC substrate constitutes a major portion of the chip cost. However, the cost of epi-growth and chips can also be reduced by the use of larger-area substrates, so manufacturers that are able to successfully fabricate 6-inch diameter SiC substrates with acceptable quality. From the Yole's report, the market size of SiC N-type wafers will increase to US\$110 million by 2020 with a 21% compound annual growth rate (CAGR). With a fast growing rate of CAGR, the production of SiC-based devices will be dramatically increased.

#### *2.2. SiC-based Power Electronic Devices*

#### (1) Development of SiC devices

As early as 2001, Infineon produced the first commercial SiC Schottky barrier diode (SiC SBD) with characteristics of high blocking voltage, better thermal stability, and hardly any reverse recovery time. This paved the way for the development of SiC power devices in the field of power electronics. Since then, more discrete devices and power modules have gradually come out [26]. Figure 1 shows the milestones of the development process of commercialized SiC semiconductor devices. Until 2014, GeneSiC and Micross components have sold SiC bipolar junction transistor (BJT) with junction temperature up to 210 ◦C. At the research and development level, the operating junction temperature of SiC-SBD can reach up to 300 ◦C, and the performance of SiC positive-negative (P-N) diode under the temperature of 600 ◦C has also been verified.

**Figure 1.** The milestones of the development process of SiC power electronic devices.

As the most market-oriented SiC devices at this stage, SiC MOSFETs have a fast switching speed and low on-resistance. In 1987, Palmour et al. from NCSU in USA developed the world's first high-temperature depletion layer N-channel MOSFET. Subsequently, Brown et al. from GE integrated a simulated operational amplifier (OPA) using depletion MOSFET, and it can work at 300 ◦C. In 2011, Purdue University reported a SiC CMOS digital integrated circuit with the maximum temperature of 350 ◦C, but as the temperature continues to increase, the gate leakage current will increase rapidly. Studies have shown that the long-term reliability of the gate oxide structure of SiC MOS is not good, especially at high temperature, this issue is exacerbated. At present, commercial SiC MOSFETs can operate up to 200 ◦C [27].

Unlike SiC MOSFETs, SiC BJTs, with high reliability, are very suitable for high-temperature conditions. However, the disadvantage is that a continuous and stable driving current is required to cause a large loss, and the current gain decreases as the temperature increases, and then the driving loss is further increased. The commercialized 1,200 V SiC BJT produced by GeneSiC can withstand temperature up to 210 ◦C, which is the highest level in the market. Actually, SiC junction field-effect transistor (JFETs) has developed since the 1990s, and the first commercial SiC JFETs came out around 2006. In general, a lateral channel structure or a vertical trench structure is employed in a SiC JFETs. It shows that Infineon uses lateral channels, while Semi South mainly uses vertical channels. SiC JFETs produced by Semi South and packaged by Micross are resistant to temperature up to 200 ◦C. SiC JFETs is currently being studied by NASA Glenn Research Center, Rutgers University and Caesar Western Reserve University, and it is reported that SiC JFETs can operate reliably for 521 h at 460 ◦C. In 2016, NASS reported that SiC JFETs can operate for 25 h at 727 ◦C. Figure 2 compares the highest tolerated temperature for commercial power electronic devices at the current stage [28]. The advanced semiconductor materials are becoming the new choice for high-temperature power electronics.

**Figure 2.** The maximum operating temperature of SiC power electronic devices.

Theoretically, SiC devices, with wide band-gap, can allow a very high voltage and high operating temperature. However, the thermal capability of all materials has not reached the same technological maturity. The maximum operating junction temperature for most commercial SiC devices is only up to 210 ◦C. Tennessee University has developed the 1.2 kV/100 A SiC JFET power module operating at 200 ◦C. In [29], a 1.2 kV/60 A SiC MOSFET phase-leg power module with the optimized internal layout is presented for an operating frequency of 100 kHz and junction temperature of 200 ◦C. In [30], a SiC power module with a junction temperature of 250 ◦C is presented for military hybrid electric vehicle applications, which is designed as half or full bridge structure. Some discrete devices and ICs are demonstrated laboratory level to operate above 500 ◦C for a short while. Reference [31] shows the characteristics of MOSFET fabricated on β-SiC thin films, which can operate at the temperature of 650 ◦C. The research on material and fabrication of SiC devices is still ongoing to develop the high-temperature commercial SiC devices and modules.

#### (2) Fabrication of SiC devices

For an example of processing of SiC MOSFETs, wafer sizes and material quality for SiC have improved over time. The main di fference between the processing of Si and SiC wafers is the temperature range, shown in Reference [32] for details. Since the strong bonds between silicon and carbon need more energy for the growth of material, post-annealing of damaged material after ion implantation, bond breaking during thermal oxidation or contact alloying. A simplified SiC MOSFET process flow in Figure 3 starts with ion implantation, field oxide formation, and polycrystalline silicon gate stack alignment. For SiC MOSFETs, the channel mobility improves slightly at higher temperature, and there is a maximum voltage rating that strictly limits the current drive and on-resistance. For ICs operating at high temperature, the condition will approach that of accelerated lifetime testing. Still, threshold voltages have to be designed with some safety margin at the highest temperature expected, where it is reduced by about 1 V with the rise of 100 ◦C.

At present, packaging technology is a big issue for design and fabrication of SiC power electronic devices or modules for high-temperature applications. Most packaging solutions are developed for mild ambient, and permissible junction temperature of SiC devices is far below the theoretical value. Packaging material and technology are critical factors for the further increase of the operating temperature. The future high-temperature converters require higher power density, which requires that the heat dissipation conditions would be as simple as possible, and the packaging technology should also adapt to the high-temperature situation.

Ohmic contact and Schottky interface are critical factors that may limit the devices in the application of high temperature. High operating temperature could result in a di ffusion process in the contact layer and a reaction between the contact components, which may result in changes in contact properties during high temperature operation, as well as degradation of the devices. In general, low contact resistivity should be maintained to decrease the voltage drop, but to ge<sup>t</sup> the low resistivity of ohmic contact is di fficult for SiC devices due to the di fficulty in doping and, in the case of p-type materials, due to the high electron a ffinity and high width of the band-gap. In addition, with the increase of temperature, Schottky barriers between metal and semiconductor become larger, which makes ohmic contact on SiC more difficult. By contrast, with Si and GaAs devices, the operating temperature is limited by the electronic properties of the semiconductor material; but the maximum operating temperature of SiC devices is limited by the stability of the contacts. Some device parameters such as response time and output power, depend strongly on the ohmic contact resistivity and its stability at high operating temperature. Therefore, the thermal stability of ohmic contacts and Schottky interfaces at high operating temperature is considered the critical factor for determining their power application.

**Figure 3.** Simplified non-self-aligned SiC metal oxide MOSFET (**a**) Ion implantation of p-type dopants (arrows) into source and drain (dashed boxes) is performed with an ion implant hard mask; (**b**) during ion implantation annealing, the surface has to be protected with a carbon cap, which prevents Si out-diffusion and surface roughening even at 1,800 ◦C; (**c**) the field oxide (FOX) and polycrystalline silicon gate (poly) stack is aligned with global alignment marks (not shown), and some overlap must be allowed; (**d**) the metal must be patterned before annealing at ~800–900 ◦C to form the metal silicide (otherwise the reaction will occur with SiO2).

Device applications are presented as discrete devices and power modules, and wire-bonded package, shown in Figure 4, is commonly used in high-temperature converters. The substrate, base plate, die attach and heat sink are included, and the following will analyze the physical structure and composition material of SiC power module.

**Figure 4.** The package of SiC power module.
