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SiC Developments

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December 15, 2014, International Electron Devices Meeting, San Francisco—John Palmour from Cree talked about the state of the industry in the silicon carbide power area. The various developments in technologies allows these power devices to address more applications.

Silicon carbide is being adapted for many applications, such as GaN on SiC substrates for LEDs, and SiC devices on SiC substrates for power control. The materials are moving into new areas while growing into new areas like the LED lights on the Bay Bridge. The new apps are calling for increasing volumes of SiC substrates, and the manufacturers of the substrates are changing the wafers from 4-inches to 150 mm. Current volumes of substrates is over 16 metric tons a year.

The challenges for the materials has been micro-pipes, a fatal defect in the materials. These defects greatly affect yield, but improvements in manufacturing have resulted in defect densities approaching 0.3 micropipe per cm2. The move to the larger wafers has also resulted in improved thickness and doping uniformity.

The electrical characteristics of SiC include a breakdown voltage 10 times that of silicon as well as much lower on resistance. Devices in this technology can operate at levels above 1 KV without needing to be in an IGBT (insulated gate bipolar transistor) configuration as in silicon. This topology contributes to high switching losses due to the minority carrier operation. SiC has lower switching losses and on resistance than the silicon devices.

Research is ongoing to improve energy efficiency. One new device is a SiC MOSFET that is capable of operating at 1200 V for currents from 10 -60 amps. The device uses a standard planar DMOS structure that operates in enhancement ( normally off) mode. reliability tests indicate an estimated 30 M hour lifetime to failure even with double the nominal gate voltage. The threshold voltage is stable in operations at 175 °C over the applied test conditions.

The driver for changing from a Si to SiC device is the increased operating frequency allows for a reduction in total manufactured costs for power converters. This reduction is due to smaller external components, higher operating temperature, and reduced switching losses, even though the SiC transistor is much more expensive than its silicon counterpart. The SiC devices can operate as a 10 KV 120 A switching converter at 20 kHz, much higher than the equivalent IGBT design.

A figure of merit for power switching devices is the change in on-resistance over temperature. A SiC transistor only increases 50 percent up to an operating temperature of 150° C. higher voltage devices have higher on-resistance and have an on-resistance increase over temperature of 2-3 X. Next generation devices will have even lower on resistance due to die shrinks that reduce the channel length and higher doping of the channel. Other channel engineering is working to increase mobility with a BaO2 barrier layer to improve on-resistance even more.

Even higher operating voltages are possible with a bipolar structure like a gate-turn-off topology. Current work is on device optimization to address mobility and carrier lifetime changes. The goal is to manufacture 27.5 kv 20 A devices for very high voltage transmission systems.


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