Enhanced Fracture Resistance Yields Dramatically Improved Reliability

ROHM and Nissan Develop a New SiC Diode Design

Kyoto, Japan - Semiconductor manufacturer ROHM CO., LTD. together with Tokyo-based Nissan Motor Co., Ltd. has developed a new SiC diode design offering dramatically improved reliability. The new diode, which combines ROHM's highly reliable device development technology with a new heterojunction diode (HJD) design developed by Nissan, delivers avalanche energy and fracture resistance that exceed the performance of previous designs by a factor of 10.

Inverter applications such as electric automobiles (in HEV, FCV, and EV configurations)—a market segment that has experienced rapid growth in the last several years—demand diodes with high breakdown voltage and current capacity. Silicon (Si) diodes, the conventional choice for such applications, suffer from high on-resistance ranging from 5 mΩcm² to 10 mΩcm², resulting in power loss and necessitating heat dissipation countermeasures. Thanks to high breakdown voltages and superior power efficiency made possible by on-resistance values of less than 1 mΩcm², SiC devices are expected to play a key role in the drive to reduce power consumption. However, the technology's relatively low avalanche energy compared to Si designs, exemplified by its weaker fracture resistance under overvoltage conditions, has hindered its adoption in automotive inverters and other applications where devices must endure low temperatures and voltage noise at startup.

The new diode pairs a novel HJD design developed by Nissan with an enhanced termination structure and other improvements, delivering 10 times the avalanche energy of conventional SiC diodes and surpassing the reliability of existing silicon diodes. With a breakdown voltage of 900V and On-resistance of 0.85 mΩcm², the new 5-mm square device will enable high-reliability SiC diodes to be used in automotive inverter circuits. ROHM and Nissan engineers believe that by improving inverter circuit energy efficiency by some 20% and simplifying heat sinks and other cooling equipment the new technology has the potential to dramatically reduce the size and weight of existing inverter systems.

ROHM has been shipping HJD engineering samples along with proprietary high-power SBDs and MOSFETs for the last two years, during which time the company has worked aggressively to improve underlying technologies for commercialization. The company believes the technology can potentially contribute to improved efficiency in a range of applications that require power conversion devices, including household appliances, industrial equipment, and power transmission infrastructure. ROHM plans to develop products for use as environmentally friendly “eco devices” in a variety of fields and applications.

·Terminology

• Heterojunction
  A junction consisting of an abrupt interface between two different substances, or such an interface. ROHM's new SiC device incorporates a polysilicon-SiC heterojunction.
• Avalanche energy
  Avalanche breakdown is one phenomenon by which an electric field breakdown current can flow when a large reverse breakdown bias is applied to a semiconductor junction. Avalanche resistance is a measure of the amount of energy that can be absorbed by the element when this breakdown phenomenon occurs.
• Inverter
  A power supply circuit capable of electrically generating (converting) AC power from DC power, or a power conversion device incorporating such circuitry. Inverters can be combined with control devices to deliver energy-saving benefits, and recently are being used in an increasing number of fields.
• On-resistance
  The resistance in a device when it is powered up. Lower on-resistance values result in less steady loss.
• EV
  Electric Vehicle. An automobile powered by electricity. EVs use rechargeable (rarely, non-rechargeable) batteries as a power source and obtain drive power by either replacing or recharging their batteries from an external source.
• HEV
  Hybrid Electric Vehicle. HEVs combine an internal combustion engine with one or more electric motors, enabling them to switch between single and multiple power sources as circumstances dictate. The overall efficiency of hybrid vehicles is on par with electric and fuel cell vehicles, and hybrid designs are gaining recognition for their low environmental impact.
• FCV
  Fuel Cell Vehicle. Fuel cells are one type of device (chemical cell) that can produce power by means of an electrochemical reaction. They are capable of producing power continuously with a constant supply of a fuel such as hydrogen and an oxidant such as oxygen. Fuel cells benefit from high generation efficiency as well as low levels of noise and vibration. Automobiles powered by fuel cells are known as fuel cell vehicles.
• SBD
  Schottky Barrier Diode. The barrier formed when a metal and semiconductor are brought into contact is known as a Schottky barrier, and rectifying elements that exploit the physical properties of this barrier are known as Schottky barrier diodes.

■HJD Forward Characteristics	■Comparison of HJD and SBD Avalanche Resistance
■Appearance of 5-mm Square HJDs	■Package Photograph (TO220)
■Comparison of HJD and SBD Structure
HJD Diagram	SBD Diagram