Submillimeter InP MMIC Low-Noise Amplifier Gain Stability Characterization

Millimeter and submillimeter indium phosphide (InP) microwave monolithic integrated circuits (MMICs) are increasingly used in applications spanning Earth science, astrophysics, and defense. In this paper, we characterize direct detection and heterodyne gain fluctuations of 35-, 30-, and 25-nm gate-length InP MMIC low-noise amplifiers (LNAs) designed for the 200-670-GHz frequency range. Of the twelve MMIC LNAs, five pairs have also been measured in multistage or cascaded configuration. In direct detection mode, the MMICs room temperature (RT) 1/f noise spectrum and responsivity were measured. From these the power spectral density, the noise equivalent temperature difference (NETD), equivalent system noise temperature (T ^DD _sys), and low-frequency normalized gain fluctuations (ΔG/G) are derived. On the same set of MMIC LNAs, using a heterodyne down conversion technique, the Allan variance method is applied to obtain the Allan stability time and normalized 4-8 GHz gain fluctuation noise at both RT and two cryogenic temperatures. We find in the case of 35-, 30-, and 25-nm gate-length InP MMIC LNAs that the derived direct detection and heterodyne gain stability is highly process dependent with only a secondary dependence on gate periphery, the number of gate fingers, and/or gain stages. This observation confirms the underlying solid-state physics understanding that gain fluctuation noise is the result of a temporal distribution of the generation and recombination of electron free carriers due to lattice defects and surface impurities. Upon cooling below ∼ 66 K, it is observed that on average gain fluctuations increase by ≳2.2× and the Allan stability time decreases by ∼2.5×. The presented measurement results compare favorably to the ALMA system gain specification of ΔG/G ≤ 1.4E-4 from 0.05-100 s, and offers guidance for application of InP LNAs for RT and cryogenic direct detection and heterodyne systems.