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GaN substrate materials
Why are native GaN wafers impractical? Recall that nitrogen is a gas at room temperature, while gallium is a solid... so how could the two both exist in the liquid state and be forced to solidify into a uniform crystal?

Substrates for GaN are either silicon carbide, sapphire, or silicon. Expensive alchemy is needed to align the GaN crystal onto these mismatched substrates, using molecular-beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Four-inch (100mm) SiC substrates are just becoming available for GaN-on-SiC, four inch GaN on silicon wafers are also available with a growth path toward six inch (150mm) and larger. Most MMIC processing lines can handle either 100 mm or 150mm wafers or both, there just isn't a market that will drive toward 200 mm any time soon. Silicon wafers are dirt cheap ($10 for 200mm diameter) while silicon carbide wafers currently cost 100X more for only 100mm. Sapphire seems to have fallen by the wayside in the past few years.

Silicon carbide is an excellent heat sink, with thermal conductivity similar to the best metals (350 W-m/K around room temperature). Silicon is much lower (40 W/m-K at room temperature), so it doesn't spread the heat as efficiently and thus for a given power density will result in higher channel temperatures.


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