How does the substrate material of a MMIC affect its noise figure at cryogenic temperatures?

The substrate material of a MMIC significantly affects its noise figure at cryogenic temperatures through three mechanisms: the semiconductor's intrinsic noise properties (the transistor noise sources are strongly temperature-dependent; InP HEMT on InP substrate achieves the lowest cryogenic noise because InP's high electron mobility and velocity increase further at cryogenic temperatures, reducing channel resistance noise; GaAs pHEMT achieves approximately 2x higher cryogenic noise than InP due to lower electron mobility in GaAs), the substrate's thermal conductivity at cryogenic temperature (the MMIC must efficiently conduct heat from the transistor junction to the mounting surface at 15-20 K; InP has thermal conductivity of approximately 70 W/m-K at 300 K but only approximately 20 W/m-K at 20 K; GaAs is similar at approximately 15 W/m-K at 20 K; poor thermal conductivity creates a temperature gradient that raises the transistor's effective operating temperature above the mounting temperature, degrading noise), and the substrate's dielectric loss at cryogenic temperatures (passive elements like matching networks on lossy substrates contribute noise; at cryogenic temperatures, dielectric loss generally decreases, reducing this contribution, but the effect varies by material). The result is that InP HEMT MMICs consistently achieve the lowest noise figures at cryogenic temperatures: 2-4 K at L-band (1.4 GHz), 5-10 K at C-band (5 GHz), and 15-30 K at Ka-band (30 GHz), representing the best technology for radio astronomy and deep space communication receivers.