Indium phosphide HEMT has broken all of the upper frequency records, on the way to terahertz devices. However, there are serious drawbacks to this technology, not the least of which is its high cost. For this reason, InP is more regarded as a lab curiosity rather than a production process.
The actual semiconductor that is doing the work in so-called InP is actually InGaAs. Indium phosphide is merely the substrate that it is grown onto. The reason for this is that InGaAs shares the same lattice constant with InP, 5.87 angstroms.
InP substrates are small (3" typical, 4" are available but remember bigger is not always better when something is brittle). ER=12.4, close to that of GaAs. A huge drawback of indium phosphide technology is that InP wafers are extremely brittle compared to other semiconductors. Try shipping an InP wafer sometime. Silicon is the least brittle (think Frisbee!), and GaAs is somewhere in the middle.
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