This work investigates improved performance of dilute-antimony-channel In0.2Ga0.8AsSb/GaAs high-electron mobility transistors (HEMTs), grown by a molecular beam epitaxy system, with (NH4)(2)S-x or silicon nitride (SiNx) passivations. With the advantageous dilute-antimony-channel design to effectively improve the carrier transport property and carrier confinement capability and the sulfur passivation technique to annihilate the surface states, the proposed (NH4)(2)S-x-passivated InGaAsSb/GaAs HEMT has demonstrated superior amplification, breakdown, and power performances, with high-temperature threshold stability. It is suitable for high-gain and high-power millimeter-wave integrated circuit applications.