Mitsubishi Electric Corporation has developed a new structure for a silicon carbide metal-oxide-semiconductor field-effect transistor (SiC-MOSFET) embedded with a Schottky barrier diode (SBD) ¹.  The SBD was applied in a 3.3kV full SiC power module, the FMF 800 DC -66 BEW², for large industrial equipment such as railways and DC power systems. The company started giving samples on May 31. The chip’s new structure aims to help downsize railway traction systems and make them more energy efficient. Also, it aims to contribute to carbon neutrality through the increased adoption of DC power transmission.

SiC power semiconductors are attracting attention because of their capacity to significantly reduce power loss. Mitsubishi Electric commercialized SiC power modules equipped with SiC-MOSFETs and SiC-SBDs in 2010. It has adopted SiC power semiconductors for a variety of inverter systems, including air conditioners and railways.

The chip, integrated with a SiC-MOSFET and a SiC-SBD, offers a more compact mounting in a module compared to the conventional method of using separate chips; thus, it enables smaller modules, larger capacity, and lower switching loss. Its use is expected widely in in large industrial equipment, such as railways and electric power systems. Until now, the practical application of power modules with SBD-embedded SiC-MOSFETs has been difficult due to their relatively low surge-current capability³. This results in the thermal destruction of the chips during surge-current events⁴ because surge currents in connected circuits concentrate only in specific chips.

Adoption of a New Mechanism, Structure  

Recently, Mitsubishi Electric has developed the first⁵ mechanism. With the new mechanism surge current concentrates on a specific chip in a parallel-connected chip structure inside a power module. It also developed a new chip structure in which all chips start energizing simultaneously so that surge current is distributed throughout each chip. As a result, the power module’s surge-current capacity has been improved by a factor of five or more compared to the company’s existing technology. This is equal to or greater than that of conventional Si power modules, thus enabling the application of an SBD-embedded SiC-MOSFET in a power module.

¹Diode formed by the junction of a semiconductor with a metal using a Schottky barrier

²Mitsubishi Electric to Ship Samples of SBD-embedded SiC-MOSFET Module

³Current limit that a power module can withstand during a surge current event

⁴Abnormal operation in which a current exceeding the rated current flows momentarily from the circuit to a power module

⁵According to Mitsubishi Electric research as of June 1, 2023

Future Developments

The new technology will be applied in SiC power modules, leading to smaller and more energy-efficient railway traction systems. It also is expected to contribute to carbon neutrality through the use of low-loss power converter for DC power transmission, which achieves less transmission loss than AC power transmission

About SBD-embedded MOSFET

In conventional SiC power modules, SiC-MOSFETs are used for switching and SiC-SBDs are used for rectifying, with the two separately manufactured chips being connected in parallel. Conversely, Mitsubishi Electric’s SBD-embedded SiC-MOSFET integrates the two chips by periodically forming the SiC-SBD in the SiC-MOSFET unit cell.

Features

1) Technology based on breakthrough confirmation of reason for surge current on single chips

Conventionally, when surge current flows through multiple SBD-embedded MOSFET chips connected in parallel, the surge current is concentrated only on a specific chip. This structure prevents the surge current withstand capability corresponding to the number of parallel chips from being obtained.

Physical and device-simulation analyses have now revealed that surge current is concentrated on a specific chip if the dimensions of that chip’s built-in SBD vary even slightly from other chips, which is not uncommon, thereby causing that specific chip to initiate a surge current flow before the other chips. Since the size variation need only be extremely minor, such variations are basically impossible to avoid in normal chip manufacturing processes.

2) New chip structure simultaneously energizes all chips connected in parallel

The newly developed chip structure has a built-in SBD that is not placed in a unit cell that occupies less than 1% of the total chip area. Thus, it prevents surge current from concentrating on specific chips. This unit cell’s structure allows surge current to flow faster than other unit cells with SBDs, and is unaffected by dimensional variations due to the absence of SBDs. Therefore, surge current can start energizing simultaneously in the corresponding unit cells of all chips without SBDs.

Also, the surge current reduces resistance of the surrounding SiC. Thus, the energization of the surge current is also triggered in the surrounding unit cells where the surge current is energized, in a chain reaction. This phenomenon causes surge current to propagate throughout the entire chip area, starting from the unit cell where the SBD is not present. Consequently, surge current is distributed over all areas of all chips. This prevents thermal breakdown of the chip due to the concentration of surge currents on a particular chip, thereby increasing the surge current withstand capability.

3) Improved surge current capability enables SBD-embedded SiC-MOSFET power module

Using the new chip structure, the surge current capability of the SBD-embedded SiC-MOSFET in parallel connection has been improved by more than five times compared to the company’s existing technology. This is equal to or greater than that of widely used conventional Si power modules.

Furthermore, because of the chain reaction of surge current, a small portion (less than 1%) of the total chip area is sufficient for a unit cell without the built-in SBD. Also there is no effect on power module characteristics such as low ON-resistance and low switching loss due to the reduced area of the built-in SBD. As a result, chips can be connected in parallel, a requirement for power modules intended for high-power applications such as railways and electric power systems, thus allowing SBD-embedded SiC-MOSFET to be used in power modules.