What is a metamorphic HEMT and when would I use it instead of a standard pseudomorphic HEMT?

A metamorphic HEMT (mHEMT) uses a graded composition buffer layer to accommodate a large lattice mismatch between the substrate (typically GaAs) and the channel material (InGaAs with high indium content). This allows using a higher-performance channel on a lower-cost substrate: (1) Pseudomorphic HEMT (pHEMT): the channel layer (InGaAs) is grown directly on GaAs with constrained (pseudomorphic) strain. The indium content is limited to approximately 20-25% InxGa1-xAs (beyond this: the strain exceeds the critical thickness and the layer relaxes with defects). With 20% In on GaAs: electron mobility ≈ 6,000-8,000 cm²/V·s. fT ≈ 60-120 GHz (for 100-250 nm gate). NF at 28 GHz ≈ 0.7-1.5 dB. This is the standard technology for RF amplifiers from DC to 40 GHz. (2) Metamorphic HEMT (mHEMT): a graded InAlAs buffer is grown on GaAs, slowly increasing the In content from 0% (lattice-matched to GaAs) to 40-80%. The dislocations from the lattice mismatch are confined to the buffer layer. On top of the buffer: a high-In channel (In0.6-0.8Ga0.2-0.4As) is grown. The high-In channel has: much higher electron mobility: 10,000-12,000 cm²/V·s (50-70% higher than pHEMT). Higher electron velocity (especially important at mmWave). The result: higher fT (100-300 GHz for 100-50 nm gate), higher fmax, lower NF (0.3-0.8 dB at 28 GHz), and higher gain per stage. The mHEMT on GaAs achieves performance approaching InP HEMT, but on a less expensive GaAs substrate. (3) InP HEMT: the channel (In0.53Ga0.47As or In0.7Ga0.3As) is grown lattice-matched on an InP substrate. No buffer transition needed (the InP substrate naturally accommodates high-In channels). Mobility: 10,000-15,000 cm²/V·s. fT > 300 GHz. NF is the lowest of any technology. But: InP substrates are smaller (3-4 inch), more expensive, and more fragile than GaAs. When to use mHEMT instead of pHEMT: (a) When the frequency is above 40 GHz and pHEMT gain is insufficient. (b) When the NF requirement is below 0.7 dB at 28 GHz (pHEMT cannot achieve this; mHEMT can). (c) When InP performance is needed but InP cost/availability is unacceptable.