You must know the barrel theory: how much water a bucket can hold depends on the shortest piece of wood. For those who do research, just one point is good; for applications, the overall performance must always be considered and find the most suitable one for the market. Various silicon carbide properties are convenient to match different demands. More specific information about the silicon carbide properties, please refer to 1.11 SILICON CARBIDE MATERIAL PROPERTIES on Xiamen Powerway Advanced Material Co., Ltd.(PAM-XIAMEN).
According to the band gap, semiconductor materials are divided into first-generation semiconductors, second-generation semiconductors, and third-generation semiconductors. The material involves band gap, band gap type, breakdown field strength, electron mobility, hole mobility, saturated electron drift rate, thermal conductivity, dielectric constant, hardness and other properties.
However, what the market needs is not the properties. The market really needs the devices with the highest frequency performance, like the inverter, rather than the performance of semiconductor materials. But the properties in silicon carbide is the basis for realizing the performance of electronic circuit devices. You can combine the relationship between demand and performance to get the final material needed. It is worth noting that the silicon carbide wafer properties may affect the performance of multiple devices; similarly, the realization of the performance of a device also requires the satisfaction of the performance of multiple materials.
For example, if the structure of the energy band is a direct band gap, the probability of electrons transitioning from a high energy level to a low energy level to emit light is greater, rather than turning into heat, which is more suitable for LEDs or lasers as a working material. With high thermal conductivity , Which means the same heat generation, the material can quickly conduct heat to the surrounding environment.
To specifically introduce silicon carbide properties, we start from the analysis of the requirements of the device. There is a simple model to describe the requirements: more devices, high efficiency, good technology and cost saving.
- More devices: the devices should be small enough, so that there will be enough devices;
- High efficiency: the technology can be realized in time;
- Good technology: the technology can meet the market demands, and there are enough sub-markets. The specific demands of this point is like the four major requirements of the charger: small size, fast charging, low loss, and safety;
- Cost saving: the cost is low enough, so that profits can support the continuous development of the enterprise.
1. SiC MOSFETs Replace Si IGBTs Baesd on Analysis of Silicon Carbide Properties
Why use SiC MOSFETs to replace Si IGBTs for devices? Reasons will be explained through silicon carbide properties on the simple model as follows.
1.1 Good Technology
For power converters, the frequency requirements and withstand voltage requirements must be met, and the standard to meet is loss. The semiconductor device works in the switching state, that is, it is either on or off. The ideal voltage and current waveforms are shown in the left figure below. Current flows in the on state, the voltage drop is 0, and the current in the off state is zero.
But in fact, there are four kinds of losses as shown below:
* There is leakage current IL when it is turned off, which also produces off-state loss;
* In the process of switching on and off, the voltage and current need a certain amount of time to change, which is the switching time. The voltage and current overlap during the switching process, resulting in switching losses.
* When the circuit is turned on, the voltage is not zero, and there is a certain saturation voltage drop VF. At this moment, according to the power formula W=Uit, there is a on-state loss;
* The same switching loss is cut off at this time, corresponding to cut-off loss.
Loss = static loss + switching loss. Static loss = on-state loss + off-state loss; switching loss / dynamic loss = conduction loss + cut-off loss.
Generally, the off-state loss is extremely small, so there is no need to consider it. Because the use mode is fixed, the device performance that determines the on-state loss is the saturation voltage drop, and the silicon carbide electrical properties in the devices that determines the switching loss is the switching time.
As shown in the figure below, as the switching frequency rises, the time for turn-on and turn-off must be shorter, and the proportion of the on-state loss in the total loss is also constantly decreasing; the switching loss-the number of switching times is rising, making the total switching time rise. It is the electron mobility that determines the high-frequency operating performance under low-voltage conditions, and the saturation drift rate determines the high-frequency operating performance under high-voltage conditions.
When Si MOSFETs come to the market, they directly meet the market demand of low-frequency and low-voltage. However, there is a problem in Si MOSFET: if the withstand voltage capability is to be improved, the chip must be thicker accordingly, resulting in high on-state loss. That is, the withstand voltage is doubled, and the on-resistance will be 5 to 6 times than the original. Therefore, the high-voltage Si MOSFET on-state loss is very large, which restricts the application of MOSFET in high-voltage occasions. It is the reason that the Si IGBT structure (insulated gate bipolar transistor) is proposed to improve the voltage resistance of Si MOSFET.
Compared with MOSFET, IGBT has an extra layer of P-doped layer, which is transformed into a bipolar device. Its conductance modulation effect can significantly reduce the resistance, so the high-voltage IGBT can still maintain a relatively low on-state voltage drop, thereby reducing the on-state loss significantly. However, the conductance modulation effect has both positive and negative sides. When turning off, minority carriers need to recombine naturally, and there is no external electric field in this process, which leads to the existence of current tailing. The switching loss is very large, which restricts the IGBT’s applications in high-frequency applications. Generally, the operating frequency can only be at the level of a few kHz.
The introduction of silicon carbide properties crystal has improved the voltage resistance of MOSFEETs from another direction. Since the breakdown field of SiC is strong, the chip will be very thin under the high withstand voltage. The breakdown field strength is related to the band gap width. In general, wide band gap semiconductors are more withstandable than Si. And this thinness also reduces the on-resistance, thereby overcoming the defect of large switching losses of the IGBT.
|Devices and materials||Low voltage <300V||High voltage 300-900V||Ultra high voltage ＞900V|
|Low switching frequency 10kHz||Si Trench||Si SJ||Si IGBT||Si IGBT|
|Medium switching frequency 100kHz||Si Trench||–||–||SiC|
|High switching frequency||GaN||GaN||SiC||SiC|
Therefore, silicon carbide properties can help the devices achieve higher doping concentration and thinner devices, obtaining a relatively low on-resistance under the condition of high withstand voltage.
1.2 More Devices
The advantage of SiC wafer is not only reduced conduction losses. For power switches, we need to focus on the heat and heat dissipation. The silicon carbide thermal properties are large, so SiC wafer heat dissipation will be easier to achieve. This greatly reduces the use of cooling components, together with a thinner structure, promoting the miniaturization of the device. This makes SiC wafer substrate dominate in high-power applications. When the power is a bit lower, GaN has higher electron mobility, so it can have a higher switching speed than SiC or Si. In low-power high-frequency applications, GaN has advantages.
1.3 High Efficiency
With the development of SiC technology, SiC MOSFETs can replace some Si IGBTs in the situation that the power is between 100kW-10MW and the operating frequency is between 10kHz-100MHz. Especially for some applications, those require high energy efficiency and space size, such as chargers and electric drive systems, charging piles, photovoltaic micro-inverters, high-speed rail, smart grids, and industrial-grade power supplies.
1.4 Cost Saving
Cost saving depends on the price of the whole device, not the price of a component. The price of SiC products is 5-6 times that of the Si products, declining at a rate of 10% per year. With the expansion of upstream materials and devices, the market supply will increase in the next 2 to 3 years, and the price will fall further. It is estimated that when the price reaches 2~3 times of the corresponding Si product, the advantages brought by the reduction of system cost and the improvement of performance will promote silicon carbide devices to gradually occupy the market of silicon devices.
Multiple indicators that SiC MOSFETs need to meet:
|Silicon carbide properties on MOSFETs||Firmness and production stability|
|Static characteristics||Threshold voltage|
|Gate oxide reliability|
|Dynamic characteristics||Ease of use|
|Chip production stability|
2. Why not Use SiC Wafer as IGBT?
Now MOSFET on silicon carbide properties crystal can achieve 6kV withstand voltage, which can already cover the current withstand voltage level of Si IGBT. The chip structure of MOSFET is simpler than IGBT. Thus, there is no need to use silicon carbide on a large scale to make IGBT, which will waste cost. Now, there are only a few occasions using the 10kV level high withstand voltage switches, such as some converter stations and traction stations.