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INDUSTRY GaN DEVICES


Slashing GaN costs with SILICON


Production costs for GaN-based devices will plummet when epilayers are formed on 200 mm silicon


BY DENIS MARCON AND YOGA SARIPALLI FROM IMEC


DEVICES BASED ON GaN can serve a vast number of applications. Transistors constructed from this wide bandgap semiconductor can increase the efficiency of power supplies, solar invertors, and base station transmitters; LEDs made from GaN are already backlight billions of screens and illuminate numerous homes and offices; and sensors fabricated from this material can detect gases such as nitrogen dioxide, a source of air pollution. With all these GaN devices, if the cost of their manufacture falls, their deployment will rise.


One way to drive down costs is to switch the growth of the GaN-based epitaxial structures to a cheaper foundation − the best in this regard being silicon with a diameter of 200 mm. Epiwafers formed in this manner can be processed at very low costs, by shipping material to under-utilized, fully depreciated 8-inch silicon fabs.


Producing GaN devices in this manner is appealing, but challenging. It is far from easy to obtain wafers that have a small, controlled bow, are free of cracking or surface pits, and are made up of layers with a low density of defects. That’s partly because the thermal expansion mismatch between silicon and GaN can cause the build-up of excessive tensile stress, which is alleviated by film cracking; and it is partly because the lattice mismatch during growth, plus the thermal mismatch during cool down from higher temperatures, can give rise to a high density of defects.


These lattice and thermal mismatches can also cause the wafer to bow, and if exceeds just 50 μm for a 200 mm wafer, this can prevent it from being processed in a silicon line. Note that wafer bow tends to increase with increasing wafer size, making up-scaling difficult.


48 www.compoundsemiconductor.net October 2014 Copyright Compound Semiconductor


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