SiC and GaN – The Potential of Wide Bandgap Semiconductors
Since the first FET was presented in the 1950s, transistors have made significant development steps roughly every 20 years: starting with bi-polar transistors, through MOSFETs and IGBTs to the most recent development of wide bandgap semiconductors (WBG) like SiC (silicon carbide) and GaN (gallium nitride).
Today, WBGs are no longer a topic of basic research, the time for applications is definitely here: The market for SiC and GaN power semiconductors is expected to grow strongly in the coming years. Experts expect the global SiC market alone to exceed the 1 billion dollar mark by 2021. The largest share is accounted for by power supply applications, such as power conditioners, battery chargers for electric vehicles and the power grid.
Outperforming silicon
The properties of WBGs promise comprehensive application possibilities in power electronics. A large breakdown electric field and high electron saturation velocity (table) have the potential to contribute to energy savings. WBG-based voltage converters have significantly less losses than conventional silicon-based converters. WBGs also enable significantly smaller modules, components and systems than silicon.
| Si | 4H-SiC | GaN | |
| Crystal Structure | Cubic | Hexagonal | Hexagonal |
| Lattice Constant | 0.5431 nm | a=0.3073 nm, c=1.005 nm | a=0.3186 nm, c=0.5186 nm |
| Density | 2.33 g/cm3 | 3.211 g/cm3 | 6.15 g/cm3 |
| Band Gap (eV) | 1.12 | 3.2 | 3.4 |
| Dielectric Constant | 11.7 | 9.66 | 8.9 |
| Breakdown Field (MV/cm) | 0.3 | 3 | 3.3 |
| Electron Saturation Velocity (107 cm/s) | 1 | 2 | 2.5 |
| Electron Mobility in the Bulk (cm2/Vs) | 1350 | 720 | 900 |
| Thermal Conductivity (W/cm K) | 1.5 | 4.5 | 2-3 |
Table. Comparison of the Wide-Bandgap-Semiconductors SiC and GaN to silicon.
Generally speaking, both common WBGs (SiC and GaN) show the same advantages. But in the details, there are big differences between the two materials. Especially the different production processes lead to most manufactures focusing on just one WBG. ROHM Semiconductor for example chose SiC early on. In 2010 ROHM started the mass production of SiC power components such as SiC Schottky diodes and SiC planer MOSFETs. In addition, ROHM was the first supplier to produce complete SiC power modules and SiC trench MOSFETs. The company has introduced a vertically integrated production system throughout the group. This means that the company covers the entire manufacturing process from the SiC wafers through the devices to the packaging.
To further boost the production capacity for SiC, ROHM is improving the production efficiency by increasing the wafer size and using the latest equipment. In addition, the company is planning a new production building at the Apollo Plant in Chikugo, Japan. It will increase the production area by approximately 20,000m2(plan). Detailed planning has just begun, construction is scheduled to begin in February 2019 and to be completed by the end of 2020.
The advantages of GaN
But the future of WBGs will not belong to SiC alone, GaN will play an important role, too. SiC is better suited for some applications, but in other applications GaN wins, thanks to the different properties of the two materials. Let’s have a look at the device structures used in WBG transistors. For SiC-MOSFETS it is more or less the same as for silicon-MOSFETS. But GaN HEMTs (High Electron Mobility Transistors) are different. They use a lateral structure as opposed to the vertical structure used in silicon and SiC devices (figure). As a result, GaN HEMTs do not work with high voltages like silicon and SiC MOSFETs and can be used as high-speed devices.
Fig. Comparison of the different device structures for Si/SiC-MOSFETs (left) and GaN-HEMTs (right).
A company which really mastered GaN HEMTs is GaN Systems. With their patented island technology, they achieve higher currents and higher yields than the competition. Their special GaNPX Packaging enables faster switching, more current, ultralow inductance and small size. Market acceptance and implementation of their products is another testament to the importance of GaN in the marketplace.
Just recently ROHM announced a collaboration with GaN Systems. The companies have agreed to jointly develop form-, fit-, and function-compatible products using GaN semiconductor dies in both GaN Systems’ GaNPX packaging and ROHM’s traditional power semiconductor packaging. GaN Systems and ROHM customers will now have the advantage of having two possible sources for package-compatible GaN power switches, presenting the widest selection of dual-sourced GaN devices.
Summary
The WBGs SiC and GaN have finally passed the research state. Their utilization in voltage converters leads to significantly reduced losses compared to conventional silicon-based converters. WBGs also enable significantly smaller modules, components and systems at lower cost. ROHM Semiconductor is leading in the production of SiC components, modules and systems. To further increase the lead, the company plans a new building for additional production capacity. Additionally, ROHM recognizes the advantages of GaN. To offer these advantages to its customers as well, ROHM collaborates with GaN Systems, the global leader in GaN power semiconductors.
