What is the difference between a GaAs MMIC and an InP MMIC for millimeter wave applications?
GaAs vs InP at mmWave
The choice between GaAs and InP MMIC technologies at millimeter wave frequencies is driven by the frequency range, noise requirements, and power needs. GaAs pHEMT is the mainstream technology with mature foundry infrastructure, established reliability, and moderate cost. InP HEMT is the premium technology offering superior performance at the expense of higher cost and more limited availability.
InP's higher electron mobility (compared to GaAs) translates to higher fT and fmax (300-500 GHz for 50-100 nm InP vs 100-200 GHz for 100-150 nm GaAs). This higher speed provides more gain at mmWave frequencies: at 94 GHz, an InP MMIC can provide 15-20 dB gain per stage compared to 6-10 dB for GaAs. The gain advantage reduces the number of cascade stages needed for a given system gain requirement.
The practical frequency boundary between the two technologies is approximately 100 GHz. Below 100 GHz, GaAs offers sufficient performance at lower cost. Above 100 GHz, InP is necessary for useful gain. At 200-300 GHz (emerging THz applications), InP with 50 nm or shorter gate lengths is the only III-V technology with usable gain.
Frequently Asked Questions
What about GaN for mmWave?
GaN on SiC MMICs are emerging for mmWave power amplifiers (5G base stations at 28-39 GHz, radar at 94 GHz) where higher output power per die is needed. GaN provides 2-5× more power per die than GaAs at mmWave but with higher noise figure. GaN fills the power amplifier niche; GaAs and InP fill the LNA and general-purpose niche.
How do the costs compare?
GaAs MMIC: $5-50 per die depending on die size and complexity. InP MMIC: $20-200 per die (4-6 inch InP wafers vs 6 inch GaAs wafers, fewer foundries, lower yield). For high-volume commercial applications, GaAs is strongly preferred on cost. InP is used in defense, space, and scientific applications where performance justifies the cost.
What about SiGe for mmWave?
SiGe BiCMOS (130-55 nm) achieves useful performance to 100+ GHz with fT/fmax of 300-500 GHz. SiGe is much cheaper than III-V technologies and can integrated digital functions on the same die. SiGe is the technology of choice for 5G handset mmWave transceivers, automotive radar, and high-volume consumer mmWave products.