This article presents the design and characterization of In0.22Ga0.78As Schottky-barrier diode mixer realized on a 2-µm-thin AlGaAs membrane at 3.4 THz. This work is a key step toward room-temperature THz front-end systems for limb-sounding instruments to detect trace gases in the Earth’s atmosphere. A 3.4 THz quantum-cascade laser pumps the mixer, a mylar beam splitter spatially superimposes the radio and LO signals, and an integrated pyramidal feedhorn antenna couples the signals to the mixer. A double-sideband (DSB) receiver atmosphere-corrected noise temperature of 27 600 K was measured at 3.4 THz for an intermediate frequency of 1.5 GHz with 2-mW of LO power coupled to the mixer.