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.
Source:Ieee
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