GaN or SiC, how should electrical engineers choose?

[Introduction]As the peerless double pride of the third-generation power semiconductors, GaN transistors and silicon carbide MOSFETs are increasingly attracting the attention of the industry, especially electrical engineers. The reason why electrical engineers place so much importance on these two power semiconductors is that their materials have many advantages over traditional silicon materials, as shown in Figure 1. The larger band gap and higher critical field strength of gallium nitride and silicon carbide materials make power semiconductors based on these two materials have excellent characteristics such as high withstand voltage, low on-resistance, and small parasitic parameters. When applied in the field of switching power supply, it has the advantages of low loss, high operating frequency and high reliability, which can greatly improve the efficiency, power density and reliability of switching power supply.

1 Introduction

As the peerless double pride of the third-generation power semiconductors, GaN transistors and silicon carbide MOSFETs have increasingly attracted the attention of the industry, especially electrical engineers. The reason why electrical engineers place so much importance on these two power semiconductors is that their materials have many advantages over traditional silicon materials, as shown in Figure 1. The larger band gap and higher critical field strength of gallium nitride and silicon carbide materials make power semiconductors based on these two materials have excellent characteristics such as high withstand voltage, low on-resistance, and small parasitic parameters. When applied in the field of switching power supply, it has the advantages of low loss, high operating frequency and high reliability, which can greatly improve the efficiency, power density and reliability of switching power supply.

Figure 1: Silicon, Silicon Carbide, Gallium Nitride

Comparison of key properties of three materials

Due to the above excellent characteristics, gallium nitride transistors and silicon carbide MOSFETs are being used more and more in industrial fields, and will be applied on a larger scale. Figure 2 shows the two power semiconductor application fields and their sales forecasts given by IHS Markit. With the expansion of application fields, the sales of GaN transistors and SiC MOSFETs will also increase substantially. Figure 3 shows the sales forecast for these two power semiconductors provided by IHS Markit.

Figure 2: Gallium Nitride Transistors and Silicon Carbide MOSFETs

Application areas and sales forecast

Figure 3: Gallium Nitride Transistors and Silicon Carbide MOSFETs

Sales forecast

In Chapter 2 of this paper, the structure and characteristics of GaN transistors, especially the GaN transistor products of Infineon Technologies Co., Ltd., will be introduced in detail. Chapter 3 will give a detailed introduction to the structure and characteristics of silicon carbide MOSFETs, especially the silicon carbide MOSFET products of Infineon Technologies Co., Ltd. In Chapter 4, the two power semiconductors will be used in the same circuit for a comparative analysis, so as to more clearly explain the similarities and differences between the two applications, and finally the full text will be summarized.

2. GaN transistor structure and its characteristics

2.1 Structure of GaN transistors

Unlike power semiconductors made of silicon, GaN transistors conduct electricity through a two-dimensional electron gas (2DEG) formed by the piezoelectric effect of two materials with different band gaps (usually AlGaN and GaN) at the interface, as shown in Figure 4. Show. Since the 2D electron gas conducts only with a high concentration of electrons, the problem of minority carrier recombination (ie, body diode reverse recovery) of silicon MOSFETs does not exist.

Figure 4: Schematic diagram of the conduction principle of gallium nitride

The basic GaN transistor structure shown in Figure 4 is a depletion-mode high electron mobility transistor (HEMT), which means that no voltage is applied between the gate and source (VGS =0V) in the case of conduction between the drain of the GaN transistor and the element, that is, a normally-on device. This is completely different from the traditional normally-off MOSFET or IGBT power switch, which is very difficult to use in industrial applications, especially in the field of switching power supplies. In order to deal with this problem, the industry usually has two solutions, one is to use a cascade (cascode) structure, and the other is to add P-type gallium nitride at the gate to form an enhancement mode (normally closed) transistor. The two structures are shown in Figure 5.

Figure 5: Two structures of GaN transistors

The gallium nitride of the cascade structure is depletion-mode gallium nitride and a low-voltage silicon MOSFET cascaded together. The advantage of this structure is that its driving is exactly the same as that of a traditional silicon MOSFET (because the driving is a silicon MOSFET), However, this structure also has great disadvantages. First of all, the silicon MOSFET has a body diode, and there is a reverse recovery problem of the body diode when the GaN reverse conducts current. Secondly, the drain of the silicon MOSFET is connected to the source of the depletion-mode gallium nitride. The oscillation of the drain to the source during the turn-on and turn-off of the silicon MOSFET is the oscillation of the gallium nitride source to the gate. Inevitably, then there is the possibility that the GaN transistors are turned on and off by mistake. Finally, because two power devices are cascaded together, the possibility of further reducing the on-resistance of the entire GaN device is limited.

Due to the above problems in the cascade structure, the mainstream technology of GaN transistors in the power semiconductor industry is enhancement mode GaN transistors. Taking the gallium nitride transistor CoolGaN™ of Infineon Technologies Co., Ltd. as an example, its detailed structure is shown in Figure 6.

Figure 6: Schematic diagram of CoolGaN™ structure

As shown in Figure 6, the current GaN transistor products in the industry have a planar structure, that is, the source, gate and drain are in the same plane, which is similar to the vertical structure of silicon MOSFETs represented by Super Junction technology. different. The P-GaN structure below the gate forms the enhancement mode GaN transistor described earlier. The other p-GaN structure next to the drain is to address the current collapse problem often seen in gallium nitride transistors. Infineon Technologies Co., Ltd.’s CoolGaN™ products use silicon as the substrate, which can greatly reduce the material cost of GaN transistors. Since the thermal expansion coefficients of silicon materials and gallium nitride materials are very different, many transition layers are added between the substrate and GaN, so as to ensure that gallium nitride transistors can withstand high and low temperature cycles, high and low temperature shocks and other harsh operations. In this case, there will be no failure problems such as wafer delamination.

2.2 Characteristics of GaN transistors

Based on the structure shown in Figure 6, CoolGaN™ has the properties and benefits shown in Table 1.

Table 1: Features of CoolGaN™ and the benefits it brings

From the characteristics shown in Table 1, it can be seen that GaN transistors have no body diode but can still conduct reverse current, so they are very suitable for circuits that require reverse current flow of power switches and will be hard-commutation, such as Very high reliability and efficiency can be achieved in a totem-pole bridgeless PFC in current continuous mode (CCM). The schematic diagram of the circuit topology is shown in Figure 7. In the figure, Q1 and Q2 are gallium nitride transistors, and Q3 and Q4 are silicon MOSFETs.

Figure 7: Using Gallium Nitride Transistors

Totem pole PFC topology diagram

It can also be seen from Table 1 that the switching speed of gallium nitride is extremely fast and the driving loss is small, so it is very suitable for high frequency applications. High-frequency switching power supplies using GaN transistors have the advantages of high power density and high efficiency. Figure 8 shows a 3.6KW LLC topology DC-DC converter designed by Infineon. The LLC resonant frequency is 350KHz. The converter has a power density of 160W/in^3 and a maximum efficiency of over 98%.

Figure 8: 3.6KW LLC conversion circuit with CoolGaN™

It can be seen from the above analysis that GaN transistors are suitable for applications requiring high efficiency, high frequency and high power density.

3. Silicon carbide MOSFET structure and its characteristics

3.1 Structure of SiC MOSFET

The structure of a common planar silicon carbide MOSFET is shown in Figure 9. To reduce channel resistance, such structures are typically designed with very thin gate oxides, thereby presenting a reliability risk of the gate oxide at higher gate input voltages. In order to solve this problem, the SiC MOSFET product CoolSiC™ of Infineon Technologies Co., Ltd. uses a different gate structure, which is called Trench SiC MOSFET, and its gate structure is shown in Figure 10. After adopting this structure, the channel resistance of the silicon carbide MOSFET is no longer strongly related to the gate oxide layer, so the on-resistance can still be extremely low while ensuring the reliability of the gate electrode.

Figure 9: Schematic diagram of the planar silicon carbide MOSFET structure

Figure 10: CoolSiC™ Trench Gate Structure

3.2 Characteristics of SiC MOSFETs

Similar to gallium nitride transistors, silicon carbide MOSFETs also have the characteristics of low on-resistance and low parasitic parameters, and their body diode characteristics are also greatly improved compared to silicon MOSFETs. Figure 11 is the comparison of the two main indicators RDS(on)*Qrr and RDS(on)*Qoss of Infineon’s silicon carbide 650V withstand voltage MOSFET CoolSiC™ and the current industry’s best silicon material power MOSFET CoolMOS™ CFD7 , the former is an index to measure the reverse recovery characteristics of the body diode, and the latter is an index to measure the amount of charge stored on the output capacitor of the MOSFET. The smaller the values ​​of these two items, the better the reverse recovery characteristics and the lower the stored charge (in the soft switching topology, the shorter the dead time required for the upper and lower power transistors of the half-bridge structure). It can be seen that, compared with the silicon MOSFET with similar on-resistance, the reverse recovery charge of the silicon carbide MOSFET is only about 1/6, and the charge on the output capacitor is only about 1/5. Therefore, SiC MOSFETs are particularly suitable for topologies where the body diode is hard turned off (such as current continuous mode totem pole bridgeless PFC) and soft switching topologies (LLC, phase-shifted full bridge, etc.).

Silicon carbide MOSFETs also have an outstanding feature: short-circuit capability. Compared with silicon MOSFET, the short circuit time is greatly improved, which is very important for motor drive applications such as inverters. Figure 12 shows a comparison of the short circuit capabilities of Infineon CoolSiC™, CoolMOS™ and competitors. It can be seen from the figure that CoolSiC™ has achieved excellent characteristics such as long short-circuit time and small short-circuit current, and the reliability in short-circuit state is greatly improved.

Figure 11: Performance comparison of silicon carbide MOSFETs and silicon MOSFETs

Figure 12: Comparison of short-circuit capability of SiC MOSFETs

4. Comparison of GaN and SiC MOSFETs

4.1 Comparison of electrical parameters

Table 2 compares the key parameters of the two power semiconductors based on Infineon Technologies’ gallium nitride transistor CoolGaN™ and silicon carbide MOSFET CoolSiC™.

Table 2: Comparison of key parameters of CoolGaN™ and SiC MOSFET CoolSiC™

It can be seen from Table 2 that the dynamic parameters of GaN transistors are lower than those of SiC MOSFETs, so the switching loss of GaN transistors is lower than that of SiC MOSFETs, and the advantages at high operating frequencies will be more obvious. The voltage drop of a GaN transistor when the current flows in the reverse direction (source to drain) is related to its gate-to-source drive voltage, which needs to be compared according to the application. For the last threshold voltage Vgs(th), the value of the GaN transistor is very small, which means that great attention should be paid to the driving design of the GaN transistor. If the noise on the gate is large, it may cause the GaN transistor wrong opening. At the same time, CoolGaN™ is a current mode drive mode, which is different from the traditional voltage mode drive. The threshold voltage of silicon carbide MOSFET is much higher, and its driving requirements are very close to IGBT driving.

Figure 13 provides a comparison of another important parameter, the rate of change of on-resistance RDS(on) with temperature. It is well known that the on-resistance of a power semiconductor switch has a positive temperature coefficient, that is, the higher the junction temperature, the greater the on-resistance. It can be seen from Figure 13 that the temperature rise coefficient of silicon carbide MOSFET is much smaller than that of silicon nitride transistor and silicon MOSFET, and the difference has reached 30% and 50% when the junction temperature is 100°C. As can be seen from Figure 13, assuming that the on-resistance of the silicon carbide MOSFET and the gallium nitride transistor is the same at 25°C junction temperature, it means that the conduction loss of the two in the same application circuit (〖I_Drms〗^2*R_( DS(on))) is the same, but when the junction temperature of both rises to 100°C, the conduction loss of the silicon carbide MOSFET is only 70% of that of the silicon nitride transistor, which is demanding for those environments and also requires The use case for maintaining high efficiency is very attractive.

Figure 13: Silicon carbide MOSFETs, GaN transistors and

Silicon MOSFET on-resistance vs. junction temperature curve

4.2 Application comparison

First, the effects of GaN transistors and SiC MOSFETs on conversion efficiency were tested on the current continuous mode (CCM) totem-pole bridgeless PFC circuit shown in Figure 7. The test conditions are shown in Table 3. Show.

Table 3: PFC Circuit Test Conditions

Two on-resistance devices were tested for each power switch in the test, with RDS(on) of 35mohm and 45mohm for GaN transistors and 65mohm and 80mohm for SiC MOSFETs. The test results are shown in Figure 14. Since the switching losses of the power switch are higher than the conduction losses at light loads, the efficiency of GaN transistors is significantly higher than that of SiC transistors. When the load is gradually increased, the conduction loss accounts for a higher proportion of the total loss than the switching loss. At the same time, due to the increase of the load, the temperature rise of the power switch increases, and according to the rate of change of the on-resistance with the junction temperature in Figure 13, it can be seen that the on-resistance of the silicon carbide transistor increases less with the temperature. The efficiency difference of the switch is already very small, although the on-resistance at 25°C of the SiC transistor is higher than that of the GaN transistor.

Figure 14: Silicon Carbide MOSFET, Gallium Nitride Transistor

Efficiency curves at the PFC stage

Next, the calculated efficiencies of GaN transistors and SiC MOSFETs in the circuit topology of the two-phase interleaved parallel half-bridge LLC for 3KW output power at different operating frequencies are compared, ignoring the frequency rise in the calculation. The effect of increasing losses in magnetic components (including resonant inductors, main power inductors). The circuit topology is shown in Figure 15. The type of GaN transistor selected is IGOT60R070D1 (the maximum RDS(on) at 25°C is 70mohm), a total of 8 pieces. The model of silicon carbide MOSFET selected is IMZA65R048M1H (the maximum RDS(on) at 25°C is 64mohm), a total of 8 pieces.

Figure 15: Schematic diagram of two-phase interleaved parallel LLC circuit

Under 50% load (1500W) and normal temperature working environment, the efficiency comparison under different working frequencies is shown in Figure 16. When the operating frequency is low (99.2%) efficiency. When the operating frequency is increased to 300KHz, Due to its very small parasitic parameters, gallium nitride has a low ratio of switching loss to total loss, so its efficiency drop is small (0.08%), while the efficiency of silicon carbide MOSFET will drop by 0.58% (99.28%-98.7%) . When the operating frequency rises to 500KHz, the efficiency gap between the two is very large (1%). Of course, if for an actual circuit, considering that the increase in frequency will cause a sharp increase in the loss of magnetic components, the efficiency difference between the two will not be so large, but the trend of efficiency changes is the same.

Figure 16: Efficiency comparison of two power devices at different operating frequencies

5. GaN and SiC MOSFET application recommendations

(1) For some reasons, the applied system must work at frequencies above 200KHz. Gallium nitride transistors are the first choice, and silicon carbide MOSFETs are the second choice; if the operating frequency is lower than 200KHz, both can be used;

(2) The applied system requires extremely high efficiency from light load to half load. Gallium nitride transistors are the first choice, and silicon carbide MOSFETs are the second choice;

(3) The maximum ambient temperature of the applied system is high, or the heat dissipation is difficult, or the full load requires extremely high efficiency, the silicon carbide MOSFET is the first choice, and the gallium nitride transistor is the second choice;

(4) The noise interference of the applied system is relatively large, especially the gate drive interference is relatively large, the silicon carbide MOSFET is the first choice, and the gallium nitride transistor is the second choice;

(5) The applied system requires a large short-circuit capability of the power switch, and silicon carbide MOSFET is preferred;

(6) For other application systems without special requirements, which product to choose is determined according to factors such as heat dissipation method, power density, and the designer’s familiarity with the two.

6. Summary

In this paper, the structure, characteristics, performance differences and application suggestions of the wide-bandgap power semiconductors that have appeared in recent years, namely GaN transistors and SiC MOSFETs, are introduced in detail. Because wide-bandgap power semiconductors have many performance advantages that silicon-based semiconductors cannot match, the industry is increasingly turning to them.

With the increasing familiarity and application experience of the two in the industry, the usage of the two will increase sharply, which will drive the price of the two to decline, which will in turn promote the wide-bandgap power semiconductors to be used on a larger scale. use, forming a virtuous circle. Therefore, it is very important for electrical engineers to master and use wide-bandgap power semiconductors as soon as possible to improve the competitiveness of their products, improve product visibility and their own capabilities. I believe this article has great reference and reference significance for electrical engineers to be familiar with and use wide bandgap power semiconductors.

Source: Infineon Power and Sensing Community, Author: Song Qingliang.

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