preface

In recent years, words such as carbon neutral and SDGs have become commonly used in business settings.
Macnica 's semiconductor business is also a field closely related to carbon neutrality.
In this article, we will discuss why wide bandgap (WBG) semiconductors such as SiC and GaN are called green. We will also introduce the reasons why efforts are being made to achieve carbon neutrality.

The details are explained below.

・ Achieving carbon neutrality

・ Why are wide bandgap (WBG) semiconductors attracting attention?

・ Improved efficiency at high power levels

・ SiC introductionIssues and countermeasures for

Toward achieving carbon neutrality

First of all, what is carbon neutral?
Carbon neutral means "reducing greenhouse gas emissions to zero overall."

As a global trend, governments around the world are aiming for a sustainable society by achieving carbon neutrality.
The reason why we are trying to achieve this is due to the problem of global warming. There are various theories about the causes of global warming, but one factor is the increase in greenhouse gas emissions.
One of the greenhouse gases is carbon dioxide CO2. As CO2 increases, global warming progresses, and the need for carbon neutrality as a countermeasure is increasing.

The Japanese government is also trying to achieve carbon neutrality by 2050. The Ministry of Economy, Trade and Industry has compiled measures for this purpose, and semiconductors are among those measures. There are many types of semiconductors, but wide bandgap (WBG) semiconductors, which are green semiconductors, are attracting attention.

Source: Ministry of Economy, Trade and Industry Green Growth Strategy for Carbon Neutrality in 2050

Why are wide bandgap (WBG) semiconductors attracting attention?

This is because SiC and GaN can achieve the functions of"high breakdown voltage,"​"low on-resistance,"and"high-speed switching"that were not possible with the conventional semiconductor material silicon.
We will introduce an example of MOSFET, which is a type of semiconductor device.
MOSFETs play the role of on/off switches in electrical circuits.

MOSFETs have an electrical property called"on-resistance,"and on-resistance decreases as the chip area increases.
Here, we will discuss on-resistance per unit area. There is a trade-off relationship between on-resistance per unit area and breakdown voltage. In a vertical MOSFET, the source and drain are on the front and back sides of the chip, and to increase the withstand voltage, it is necessary to increase the thickness of the chip.
Therefore, the higher the voltage resistance of the device, the higher the on-resistance per unit area.

Figure 1 shows comparison data between silicon and other materials.
The horizontal axis shows the breakdown voltage, and the vertical axis shows the theoretical on-resistance value per unit area.

Figure 1: Source TND6239/D
(Note) References: The following documents are referred to.
・ Comparative Study of Optimally Designed DC-DC Converters with SiC and Si Power Devices

When compared at the same breakdown voltage, it can be seen that SiC and GaN have a smaller theoretical on-resistance per unit area than silicon. Lower on-resistance reduces MOSFET loss, leading to a reduction in power loss. (Low on-resistance)

High breakdown voltage can be achieved because SiC and GaN have a larger band gap energy than silicon. (High pressure resistance)
A high bandgap energy allows a higher breakdown voltage to be achieved with a thinner chip thickness, and thus a lower on-resistance (as the chip thickness decreases, so does the on-resistance). Materials with large bandgap energy are called wide bandgap (WBG). The characteristics of silicon, SiC, and GaN are listed below.

Source TND6239/D

In the table, the electron saturation velocity is basically the average velocity of the current that carries electrons within the material. Generally, the higher the number, the faster the switching speed and the lower the on-resistance.
Faster switching speeds reduce switching losses that occur during MOSFET turn-on and turn-off, making it possible to suppress power loss even when driven at higher frequencies. As the frequency increases, peripheral components such as inductors and capacitors can be selected with small constants, and the size can be reduced. (fast switching)
Additionally, if MOSFET power loss is low, the heat sink for heat dissipation can also be made smaller, which leads to smaller final products.

SiC is attracting attention because its material properties allow it to achieve both "high breakdown voltage," "low on-resistance," and "high-speed switching."
These features are clearly visible in actual semiconductor products, and SiC MOSFETs can achieve characteristics that were previously not possible with silicon MOSFETs and IGBTs.

*The table shows Onsemi's product lineup.

Silicon MOSFET products with a withstand voltage of 1200V are not often found on the market, but there is a large lineup of SiC MOSFETs.
​SiC MOSFETs are especially expected to be useful in fields that require high voltage resistance of 1200V or higher. In addition, SiC MOSFETs can handle high frequency switching of 100kHz or higher, which is not practical with IGBTs due to large switching losses.

Increased efficiency in high power

Earlier, I explained that low on-resistance can reduce power loss. Then why does reducing power loss make it carbon neutral?
By lowering power losses, less electricity is needed, which in turn leads to less fossil fuels being needed. (*If the electricity is thermal power generation)
Fossil fuels are the source of CO2 emissions, so reducing the amount of fossil fuels used will also reduce CO2, leading to carbon neutrality.

So, what kind of products are wide bandgap (WBG) being considered for? These are motor (inverter) and power supply related applications that use a lot of electricity. Figure2 illustrates example applications where SiC and GaN are used.

Figure 2: Source TND6299/D

The vertical axis shows the power band handled, and the horizontal axis shows the switching frequency. When the frequency exceeds 1MHz, GaN is the main choice, but when dealing with power of several kilowatts or more, SiC is considered.

GaN is often used in USB power adapters, etc. Due to the reduction in switching loss, there are many products on the market that are small adapters that can handle tens of watts, which was not possible with conventional products.
SiC is used in traction inverters that turn automobile wheels. Traction inverters handle large amounts of power, around 100kW.

For example, if we assume that by changing from silicon IGBT to SiC​ ​MOSFET in a 　100kW product, efficiency can be improved by 1 % (loss reduced by 1 %), 1kW of power that was previously wasted as heat will be eliminated, making it carbon neutral. Contribute to.

According to data from the Ministry of Economy, Trade and Industry, motors account for nearly half of the world's electricity demand. Motors that handle large amounts of power are driven by inverter circuits that use power semiconductors. You can see that using SiC to increase the efficiency of the inverter circuit is important for carbon neutrality.

Source: Ministry of Economy, Trade and Industry “Construction of next-generation digital infrastructure” Outline of R&D and social implementation plan (draft) for the project

Challenges and countermeasures for SiC introduction

In this case, I think it would be a good idea to use SiC in all products, but there are two issues when introducing SiC MOSFETs.

- Requires design change of gate voltage (VGS). Silicon VGS = 10V/0V ⇒ SiC VGS = 18V/-3V

- Expensive compared to silicon MOSFET​

There are countermeasures for each issue.

Adjusting gate voltage

Conventionally, silicon MOSFETs can sink with a gate voltage of 0V, but SiC MOSFETs may require a voltage of-3V(minus 3V).
ONSEM's SiC gate driver NCP51705 has a built-in charge pump circuit that generates a negative voltage, so it can easily prepare a voltage of -3V.​

Depending on the state of the VEEset pin, the sink voltage can be selected from 0V,-5V,-3.4V, and-8V.

lastly

Onsemi handles SiC MOSFETs, SiC Diodes, and SiC modules.

・ ONCEMI product page

・ What is Macnica SiC MOSFET? Introducing the challenges of silicon and the benefits and features of using SiC!
・ About Onsemi's activities towards carbon neutrality

Inquiry

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