Silicon carbide power device characteristics, applications and challenges: an overview

Muhamad Faizal Yaakub, Mohd Amran Mohd Radzi, Faridah Hanim Mohd Noh, Maaspaliza Azri 1 Fakulti Teknologi Kejuruteraan Elektrik dan Elektronik, Universiti Teknikal Malaysia Melaka, Malaysia 1,2 Fakulti Kejuruteraan, Universiti Putra Malaysia, Malaysia 3 Fakulti Teknologi Kejuruteraan, Universiti Tun Hussein Onn Malaysia, Malaysia 1,4 Centre for Robotics & Industrial Automation, Universiti Teknikal Malaysia Melaka, Malaysia 1,2 Advanced Lightning, Power and Energy Research Centre, Universiti Putra Malaysia, Malaysia


INTRODUCTION
In recent decades, power electronics have gained high interest due to new materials invented for the new power devices. Ever since the silicon (Si) based device was created, the innovation in power device materials has been evolved to meet the application requirement and performance needs in various fields [1]. Up till now, the advancement is predominantly driven by the Si power devices such as an insulated-gate bipolar transistor (IGBT) for high voltage, high power, and low-frequency application, and power metaloxide-field-effect transistor (MOSFET) which are particularly targeted for low voltage, low power, and highfrequency application. While MOSFET which is based on the trench gate structure dominates the global market for applications below 600 V, its super junction version (SJ-MOSFET) and IGBT based on field stop and injection enhancement concept have secured the market shares for application in the range from 600 V to 6.5 kV. Even with these advancements, the Si power device has reached its performance limitation. To date, the most high-end Si IGBT breakdown voltage capability is only 6.5 kV with limited switching performance. Furthermore, there is no Si device could operate above 200°C [2].
Owning these restrictions, power converter efficiency is reduced severely which leads to the needs of a complex cooling system and expensive passive components. As a result, an upgraded material power 2195 device is expected. The exploration of the solution to the silicon limitation leads to the investigation of a wide bandgap (WBG) semiconductor materials. The advancement in WBG semiconductor material has made it possible to improve the efficiency of electric energy transformation. Having a trade-off between process/manufacturing maturity, theoretical characteristics, and commercial marketability, Silicon Carbide (SiC) and Gallium Nitride (GaN) are becoming perfect semiconductor material candidates for the next generation of power devices. Despite offering decent high-frequency and high-voltage performance, GaN comes with inferior thermal conductivity and a lack of good-quality bulk substrates required for vertical devices. Consequently, it makes SiC become a better option for power device material [3], [4]. Nevertheless, GaN-based power devices are still being used and play a major role in other specific applications. Therefore, due to the interesting features of the SiC power device, this paper presents an overview of the SiC power device and its contribution to the improvement in the state-of-the-art selected applications. SiC properties and their characteristics are presented in section 2. Adaptation in applications and their contribution to enhancing the overall system performance is discussed and summarized in section 3. In addition, the system design challenges of SiC are summarized in section 4.

THE CHARACTERISTICS
Recent trend discloses that Silicon Carbide (SiC) based power device has established its popularity among power electronics practitioners in modern applications. Since Infineon introduced SiC Schottky diode back in 2001, the SiC-based power device has kept its momentum in technology development and market growth as projected by [3] depicted in Figure 1. This is due to the fact that the properties and characteristics are well suited to solve innumerable power electronics-related problems and performance demands. From the application perspectives, the most advantageous features of SiC, compared to Si are the breakdown field, its higher conductivity, and its wide bandgaps as tabulated in Table 1.
In principle, the SiC scored a breakdown field ten times higher compared to what Si has. Therefore, the thinner drift layer and higher doping concentration can be used on the SiC power devices at the same blocking voltage. As depicted in Figure 2, SiC power devices such as Unipolar Schottky diodes and MOSFET would have lower specific on-resistance compared to Si counterpart by having thinner blocking layer and higher doping concentration [5]. As a WBG material, it offers other incredible material properties that are attractive to power device design and development. WBG semiconductor material such as SiC, in particular, has advantages of operating in higher temperatures and greater radiation hardening. Theoretically, the thermal energy of the electron valance band is directly proportional to the temperature. As such, it has enough energy to propagate to the conduction band at a certain temperature. But, this uncontrolled conduction event must be avoided. For Si-based material, this could occur around 150°C [6], [7]. In contrast, bandgap energy is higher in WBG semiconductor material, as such, more thermal energy is required by the valence electron to travel to the conduction band. Remarkably, for SiC this intrinsic temperature is about 900° C. The aforementioned argument is also valid for radiation hardening. Radiation energy likewise can stimulate an electron like the thermal energy and make it travel to the conduction band. Consequently, devices developed based on WBG material such as SiC can endure more in extreme heat and radiation without sacrificing its delightful characteristic [11].  In principle, as the drift velocity of the semiconductor increases, the switching frequency capability is also increased. The drift velocities of SiC semiconductor material are more than twice the velocity of Si as shown in Table 1. Higher drift velocity permits the minority carriers to be removed faster from the depletion region at the moment of turn-off transient. As such the SiC's switching speed is faster due to the higher electron saturated drift velocity. Additionally, the increased switching frequency is also contributed by the lower on-resistance at the same breakdown voltage of the SiC semiconductor material. The junction capacitance of the SiC devices e.g. MOSFET is lower than the Si devices, taking into account trade-offs between thinner drift regions and smaller dice size [12]. Furthermore, numbers of Coss and Qg of SiC material are lower compared to in the Si, which makes SiC device able to switch at much higher dv/dt. Figure  3 depicts high-frequency switching produced by SiC MOSFET compared to Si IGBT, which offers low switching loss and thus enhancing the converter power density and efficiency [13].

SiC APPLICATIONS 3.1. Electric vehicle/hybrid electric vehicle
At the beginning of 2013, the electric vehicle (EV) market shares cover 0.02% of the total passenger car segment [14]. However, it is projected that there will be over 600 million cars will be on the road due to strong policies worldwide, and by 2030, it is estimated that passenger EV will secure 50% of market shares [15], [16]. 32% reduction in size and 40% of reduction in weight and loss of the EV electronics circuit is expected as per an announcement made by The EV Everywhere Challenge in 2013. This will be only possible with the deployment of SiC devices in the circuit design [17]. Numerous research and development activities have been conducted globally. For instance, in Japan, Denso Corporation has developed 100A SiC-MOSFET with the SiC-Schottky Barrier Diode (SiC-SBD) inverter module back in 2007. Later in 2008, NISSAN has introduced to the market an inverter with SiC diode for fuel cell vehicle (FCV). ROHM in collaboration with Honda also developed a 1200V 230A high power inverter module. SiC-SBD and SiC-MOSFET have been adopted by them in the design with an integrated three-phase inverter and one phase converter module. As a result, a 25% reduction of switching loss can be offered compared to the Si version [18]. A year after that, Mitsubishi Electric has introduced 11kW inverter. The inverter module comes with SiC-MOSFET and SiC-SBD on-board and produces 70% and 75% loss and volume reduction, respectively [19]. The race of research and development of SiC power devices for EV application continues as demand increases, not only by introducing new devices but also in improving the quality and reliability of existing technology. This is true when Toyota Corporation has put SiC power technology into trial on its hybrid Camry and fuel cell bus in 2015 [20]. Figure 4 shows the SiC power control unit developed by Toyota. Table 2 summarizes the research and development of SiC based power device and module for EV application.

Motor Drives
The importance of motor drive, especially in the industrial domain sparks an interest in improving and enhancing its capability to address the issues raised in various applications. Generally, things like efficiency, cost, and footprint/sizing always become a major concern for SiC-based motor drives. Medium voltage (MV) application motor drives are said to be more suitable to fully take advantage of SiC devices compared to low voltage type, in particular, taking into account the incorporation of high-speed medium voltage motors [31]. SiC power device offers a distinctive prospect to achieve a highly efficient motor drive with simpler topology. For example, SiC-based MV motor drives improve its efficiency by having a lesser loss of power semiconductors. Due to that, the aim of having simpler inverter topology is also achievable. Principally, higher power density is triggered by the high switching frequency, which allows MV direct drive variable speed controlled motor to abandon the usage of the gearbox. Therefore, causing a decent reduction in system size. A research work done by [32] is an example of how the SiC power device has made the aforementioned argument feasible. The 20kW MV motor drive is designed by adopting a two-level topology. It incorporates a 10 kV/10 A SiC MOSFET with 6 kV DC-link and 3.3 kV line-to-line AC output which leads to the simplification of the drive topology. In-term of performance, efficiency, and power density are remarkably promising, estimated approaching 97% at 20 kHz switching frequency and 4.11 W/inch3, respectively. In 2019, Infineon Technologies has introduced in the market its EVAL-M5-E1B1245N evaluation board which is optimized for motor drive application. Table 3 tabulates a summary of other selected SiC power device contribution in a power drive application.
SiC devices have been deployed, investigated, and analyzed in different inverters by numerous researchers. For instance, a group of researchers in the United State [33], [34] have been working on a solar inverter that based on SiC Schottky diode which demonstrated the improvement of inverter's efficiency above 96% and reduced reverse recovery losses.

Solar inverter
With the remarkable performance of the SiC power device, it is the best candidate in the applications where efficiency, power density, cost, and speed are the ultimate concern. One of them is the solar inverter. The capability to reach higher efficiency in the energy conversion process is directly proportional to the cost reduction i.e. reduced solar panel area, simpler circuitry/topology, and even lower installation fee [7], [44]. Furthermore, in the past few years, a group of GE's researchers and engineers have been working on a single-stage megawatt level solar inverter [45]. The inverter integrates an advanced SiC MOSFET module with an innovative system engineering design. As a result, at 900V dc input, the California 2199 energy commission (CEC) efficiency of the inverter is reaching close to 99%. Figure 6 and Figure 7 illustrate the efficiency of GE SiC megawatt solar inverter and GE's LV5+ SiC-based solar inverter, respectively. Table 4 summarizes several selected researches works on SiC devices in the emerging solar inverter applications for the past few years.

CHALLENGES
Although comes to the market with the appealing characteristic and performance, there are still many challenges raised from the device and application point of views which require close attention from the developer as well as end-users [3].
The gate driver design is likely to be more challenging for SiC devices due to the occurrence of the positive counterfeit voltage gate [56]. This is due to the higher dv/dt of the SiC devices which imposes higher common mode current to be injected to the gate loop. Consideration to have an advanced gate driver with dv/dt and/or di/dt control is a must. Since SiC devices also hold a faster current rise during fault events, therefore a faster response protection requirement is indispensable. Nevertheless, timing mismatch caused by parallel or/and series also must be taken into consideration in the system design.
Some high voltage and high power applications that adopt SiC devices may face severe electromagnetic interference (EMI) noise because of the high dv/dt formed by the huge parasitic capacitance and fast switching speed. An appropriate EMI filter is necessary for motor drive applications to prevent the occurrence of voltage doubling effect caused by the high dv/dt. Similarly, extreme EMI conduction caused by high dv/dt will appear on the grid side in the grid-connected application. Therefore, the EMI filter is indispensable and advanced technology of EMI suppression method is also crucial to be investigated [57], [58].
Long term reliability of the SiC devices across all applications must vividly be demonstrated. This is vital for applications that are sensitive and extremely critical such as aviation, space program, and military [59]. The thermal ripple coming from the SiC-based converter is possibly larger than the Si converter due to its capability in providing a higher current rating. Consequently, a more rigid specification for package materials is compulsory as the SiC device operates at higher temperature conditions. Then again, due to SiC devices' capability operates in high-temperature conditions, peripheral components associated with it become crucial, especially the capacitors because connectors linking high-temperature devices to the capacitor may further raise the capacitor's temperature [60].
The full potential of the SiC device somehow cannot be achieved due to the packaging limitation [61]. As such it must be justified that the manufacturing for both front end and back-end processes must be improved to address the challenge. Since 2012, most major semiconductor and their silicon foundry companies have moved towards 150mm wafer fabrication which allows more dies to be planted. As such, lower cost per each device is achievable

CONCLUSION
Silicon Carbide devices including MOSFET, diode, and other unipolar/bipolar switches are well accepted in the high-performance power electronics application. This due to its capability in yielding a remarkable benefit to at least in three following aspects; Noticeable reduction in power loss which leads to simplification of cooling requirement and poses higher system efficiency. Overall system improvement stimulated by high switching frequency and wide bandwidth control capability. Simplification and optimization of system circuitry and topology especially for converters.
However, SiC advancement characteristic brings several challenges to the device's applications, including gate driver design, electromagnetic interference, reliability, and manufacturing/process. Some of the arguments presented in this paper are yet to be solved and active research is ongoing. No doubt that the continuous improvement of SiC technologies is crucial for it to be implemented in far-reaching applications

ACKNOWLEDGMENTS
The author would like to acknowledge the Universiti Teknikal Malaysia Melaka and Universiti Putra Malaysia for their financial and technical support.