세종대학교

Power Semiconductor Technology for High-Efficiency Power Conversion

Rim You-seung,

Department of Semiconductor Systems Engineering

1. Introduction

The term "power conversion" may seem unfamiliar, but we rely on power conversion technology in all aspects of our daily lives. The electricity we refer to as energy is generated at power plants and supplied to every household. We know that the 220V alternating current (AC) voltage we use is supplied to our houses, but how is it delivered from a power plant?

Fig. 1. Voltage supplied to houses and the role of power semiconductors [1]

The generated electricity needs to undergo a process called voltage step-up to be converted to very high voltages for delivery to urban areas. To achieve significant voltage changes, it is easily done using alternating current (AC), which makes it feasible to cover long distances despite considering losses.  Currently, electricity is transmitted through voltage step-up in the range from 154kV to 765kV, and then through a distribution process, 220V AC is supplied to houses. Up to this point, we can easily understand the fact that "220V AC comes to houses."

The importance of power conversion lies in the fact that almost all electronic devices we use operate on direct current (DC). DC is a type of energy where the voltage does not change over time. It is used by laptops, TVs, refrigerators, washing machines, and almost all electronic devices, each of which requires different voltages. This means 220V AC needs to be converted into various DC voltages such as 5V, 10V, and 15V. Here, the commonly encountered adapter comes to mind. In short, the adapter converts AC to DC and adjusts the voltage to the required level. This is where the rectification function of semiconductors comes into play. Using the feature of allowing current to flow in only one direction, it filters the real-time changing (positive and negative voltages) AC in one direction and converts it into DC. Conversely, when converting DC to AC, it employs the pulse width modulation (PWM) technology of creating an AC form. Semiconductors are also used in this process. Figure 2 illustrates the characteristics of various types of power conversions: converting AC to DC, changing AC frequency, altering DC voltage, and converting DC to AC in order. Each is used in different application environments.

Fig. 2. Types of power conversion [1]

Based on the preceding discussion to provide a brief understanding of power conversion, the main section will describe why high energy gap semiconductor materials are needed and why high efficiency is required.

2. Meaning of Withstanding High Voltage

Semiconductors have an inherent property known as bandgap. When voltage is applied to a semiconductor, breakdown phenomenon occurs at a certain voltage threshold, which leads to a sharp increase in current, making it uncontrollable and leading to degradation. This breakdown phenomenon highly depends on the size of the semiconductor's band gap. For example, silicon (Si) has a band gap of 1.1 eV. After the conversion into breakdown electric field, it has 0.3 MV/cm. In other words, since the breakdown voltage threshold of Si is fixed, increasing the breakdown voltage requires enhancing the semiconductor’s resistance. A widely used method to increase the resistance is to reduce the impurity concentration in a semiconductor. The method makes it possible to control current flow by adjusting a semiconductor’s impurity concentration. More simply, an additional layer with low impurity concentration can be added to create a high-resistance region in the direction of current flow. The wider this high-resistance region, the more difficult it becomes for current flows and the more the breakdown voltage increases. 

Additionally, semiconductors can be created as n-type or p-type, depending on the abundance of electrons or holes, respectively.  A semiconductor with more electrons is called n-type, and a semiconductor with more holes called p-type. When these two types of semiconductors form a junction, we obtain a diode which becomes a semiconductor device capable of rectification. By using such opposite polarity layers, it is possible to increase breakdown voltage. These techniques enable semiconductor device design suitable for various applications by increasing breakdown voltage. In summary, techniques to enhance breakdown voltage include inserting layers with different impurity concentrations of the same polarity and creating a junction with layers of opposite polarities to form a depletion layer. These techniques make it possible to increase a semiconductor’s breakdown voltage and design appropriate devices in application fields. 

3. Necessity of High Voltage Implementation, Limitations of Si, and the Industry

Having previously explored the types of voltage, their conversion, and applications, we now turn our attention to voltage from a different perspective. Voltage, also known as electric potential difference, refers to the electric potential that a charge has within an electric field. Simply put, it can be seen as the difference in potential energy between two points. Higher voltage means that charges can move with greater force, implying a higher quantity of charge flow per unit time. When discussing voltage, we cannot ignore electric power (P), which represents the ability of electric energy to perform work and can be expressed as the amount of the work done with the electrical energy transmitted per unit time or converted. Power (P) is defined as the voltage (V) multiplied by the current (I). Consider two scenarios, A and B, which have the same amount of power. If A has a higher voltage than B, it will have a correspondingly lower current. What does this imply? Higher current means more electrons flowing at the same time. This triggers vibration and collisions, gradually generating heat and leading to heat loss. Therefore, for the same amount of power, higher voltage with lower current is advantageous in terms of heat loss and design.

Increasing voltage, however, is not always the correct approach. Higher voltage requires increasing the breakdown voltage of semiconductors and making an electrical open state. The increase in the thickness of the resistance layer in order to increase the breakdown voltage ends up raising the overall resistance of the semiconductor. In the condition where current flows through the increased resistance, the high resistance leads to more losses. This increased resistance leads to more losses.  As such, since breakdown voltage and resistance are in a trade-off relationship, various attempts to overcome this have been made. Among them, we focus on overcoming material limitations.

Fig. 3. (Left) Band gap and breakdown electric field characteristics of different materials. (Right) On-resistance characteristics according to breakdown voltage design

The left graph in Figure 3 shows the band gap values of various materials and their corresponding breakdown electric field values. Silicon (Si) has a band gap of 1.1 eV, and each material in the figure has different band gap values. Among these, 4H-SiC (3.3 eV) and GaN (3.4 eV) have been studied and commercialized as substitutes for Si. Their bandgaps are more than three times greater than that of Si, which means they have superior breakdown voltage characteristics with advantageous resistance design. Let’s turn to the right graph in Figure 3, which shows the correlation between on-resistance and designed breakdown voltage for a semiconductor device. Let’s assume that a semiconductor was designed to withstand 1,000V. If Si was used, the on-resistance would be over 100 mΩ⦁cm² in consideration of the aforementioned resistance layer’s thickness and resistance. In contrast, if 4H-SiC was used, the on-resistance could be reduced to about 0.5 mΩ⦁cm², nearly 200 times lower. Thanks to such an advantage, attention has been paid to the development of power semiconductors based on wide band gap materials. Research is also actively exploring ultra-wide bandgap materials such as Ga2O3, AlN, and diamond. But do we need such high voltage in our daily lives? We already experience high voltage in our life. A prime example is the electric vehicle. 

Fig. 4. (Left) Electric vehicle battery charging and power conversion by each usage. (Right) Vehicle charging time by voltage

The left diagram in Figure 4 illustrates the power conversion system of an electric vehicle. It provides a comprehensive view of how various power conversion methods are applied to the entire system. Electric vehicles currently use 400V and 800V battery systems, and the battery capability has increased from 60kWh to 80kWh or more. For example, the DC coming from the batteries used in electric vehicles with the 800V system must be converted into lower voltages (12V or 24V) for use in electronic devices such as audio systems and dashboards, and into AC power for motor drive applications. The advantages of high voltage come into play during charging. As shown in the right diagram, high voltage charging significantly reduces charging times. Tesla’s Supercharger system, which is used as a sort of standard in the U.S, supports 480V charging. Hyundai’s Ioniq 5 supports 800V charging, reducing charging time 1.6 times more than Tesla’s. The charging time for electric vehicles is crucial to all users, and shorter charging time significantly contributes to spreading widely electric vehicles. In other words, the high-voltage based system design can produce efficiency and convenience.

One might wonder if it is okay to use semiconductors capable of withstanding 800V in the 800V system. Generally, in power conversion requiring switching, severe waveform changes inevitably occur during the short time of switching and charging/discharging. Accordingly, surge voltage occurs. Although the duration is brief (nanoseconds), it can cause a voltage much higher than the design voltage to be applied to semiconductors, potentially leading to device failure. Therefore, it is common to design with a voltage margin of around 10-20% or more. Typically, for the 800V system, semiconductor devices with a 1200V withstand voltage are used. In other words, high-voltage design is already widely applied in everyday life, and there are a diversity of products, as shown in the figure below.

Ironically, despite the extensive use of these power semiconductors in everything from household appliances to commercial equipment, the domestic self-sufficiency rate achieved through localized technology is less than 10%. Countries leading in the power semiconductor field include Germany, the United States, Japan, and France. There are no key domestic players in the field. Unlike the memory semiconductor industry, which focuses on mass production of a few types, the power semiconductor industry is characterized by small-scale and multi-variety production of a small variety of products. As each company must match different power requirements, the role of field engineers is extremely important. Companies are required to have the capability of not only providing customized products for clients, but also proposing design solutions. For this reason, domestic companies have yet to enter in the field. From their standpoint, overcoming the difficulties in adapting to this ecosystem, which has been built up over decades, remains a significant task. Nevertheless, power semiconductors are emerging as a promising and highly valued industry beyond the memory sector, drawing attention for their potential. Power semiconductors are not only the technology needed in the era of increasingly smart and intelligent electronic devices, but the muscles and power source of all electronic devices. Therefore, the research and development of power semiconductors and the enhancement of domestic self-sufficiency in power semiconductors has become another key challenge and goal for domestic researchers. 

Fig. 5. Global power semiconductor companies (2023 YOLE Report) 

4 Technology of Gallium Oxide with Ultra-Wide Band Gap

Having looked into the advantages of wide band gap materials for overcoming the limitations of silicon (Si), we will discuss actual research, development, and commercialization cases. Silicon carbide has a wide band gap of 3.3 eV and high thermal conductivity (more than three times that of Si). High currents in high-power operations generate a lot of heat in devices. To dissipate this heat rapidly, materials should have excellent thermal dissipation properties, and be supported by appropriate thermal design. In this regard, SiC is considered an optimal material. The most successful commercial replacement of Si with SiC can be seen in the inverter of Tesla's Model 3, where 48 dies were connected in parallel with the use of planar MOSFETs to achieve a 650V withhold voltage. Recently, SiC is has been considered not only for drive systems but also for charging systems. Many automotive companies, including Hyundai Motor Company, are applying SiC models. 

In the case of GaN, although it faces challenges in producing high-voltage structures compared to SiC (Silicon Carbide), it is possible to implement High-Electron Mobility Transistor (HEMT) devices that allow ultra-high-speed switching based on the two-dimensional electron gas (2DEG) layer. This makes GaN suitable for applications in 5G communication relays, X-band, K-band, and other wideband, high-frequency uses. Particularly, in RF devices where high power is required, it is easy to implement matching circuits due to GaN's high input-output impedance. In addition, GaN supports small chip areas and has better frequency characteristics than Si. Lastly, gallium oxide (Ga2O3) is introduced. Gallium oxide has a very wide band gap of4.8eV-5.3eV. The most significant difference from SiC and GaN is that gallium oxide supports the production of the material based on large-diameter wafer ingots, similar to Si. A single 4-inch SiC wafer costs close to 1 million KRW for research purposes, and a GaN wafer’s price ranges from 500,000 to 800,000 KRW. As such, they are quite expensive. In contrast, gallium oxide is advantageous in terms of productivity, but its research is in the early stages, and its actual wafer price is still very high (3 million KRW for a 2-inch wafer). Despite this, its processability and suitability for ultra-high voltage applications have led to intense research efforts both domestically and internationally. Given that gallium oxide has yet to be commercialized at present, it represents an opportunity for domestic companies to enter the power semiconductor market, which is currently heavily dependent on foreign sources. A large-scale government project is currently underway to develop technologies ranging from fundamental research to commercialization. I am also involved in developing 1200V Ga2O3 diodes and transistors, supported by the Ministry of Trade, Industry and Energy. I hope that accelerated research and development will position domestic next-generation power semiconductor technology as a global leader.

Fig. 6.(Left) 24 650V-rated SiC MOSFETs of the Main Inverter in the Tesla Model 3. (Right) 400V Tesla Model 3 System’s 650V SiC Planar MOSFET based Drive System Inverter Model for 3-Phase Motor Operation [3]

5. Conclusion

High-efficiency power conversion is becoming an essential requirement for implementing low-carbon technologies, especially as the issue of growing energy consumption is important. Power conversion is a critical technology in everything we use, from electronic devices and electric vehicles to electric aircraft and water taxis, energy storage systems, power plants, large ships, trains, and aircraft. In these technologies, the dual imperatives of environmental friendliness and maximizing energy efficiency must be met. Now is the time to invest significantly in and focus on the power semiconductor sector, an area where domestic companies have the lowest self-sufficiency, alongside basic research and comprehensive efforts. As the domestic market shifts from being heavily concentrated on memory to exploring broader opportunities, it is encouraging to see major Korean companies starting to develop power semiconductor products. I hope they achieve successful outcomes.

References

[1] https://www.semicon.sanken-ele.co.jp/en/guide/powersemicon.html

[2] https://www.yolegroup.com/strategy-insights/power-electronics-meeting-the-shift-towards-electrification-and-renewable-energy-trends/

[3] https://www.pgcconsultancy.com/post/examining-tesla-s-75-sic-reduction