This paper presents a 94 GHz monolithic clown-converter with low conversion loss and high local oscillator (LO)-to-RF isolation using the 0.1 mu m T-gate metamorphic high electron-mobility transistor (MHEMT) technology. The down-converter consists of a one-stage amplifier and a single-balanced mixer based on the high-directivity tandem coupler structure using the air-bridge crossovers, thereby amplifying the RF signals and maximizing the LO-to-RF isolation by using an inherent S(12) isolation characteristic of the amplifier and good phase balance of the tandem coupler. The fabricated one-stage amplifier using a 30 mu m x 2 MHEMT shows a small signal gain of 7 dB at 94 GHz. The single-balanced mixer comprising two 20 mu m x 2 MHEMT Schottky diodes and the tandem Coupler with an additional lambda/4-length line exhibits the conversion loss less than 7.8 dB and the LO-to-RF isolation higher than 30 dB in a RF frequency range of 91-96 GHz. Two Circuits designed both for a 50 Omega impedance system are integrated into the clown-converter of a 2.6 x 2.5 mm(2) chip size, and it shows a low conversion loss of similar to 1 dB at 94 GHz and excellent LO-to-RF isolation above 40 dB in a frequency range of 90-100 GHz. This is the best isolation among the W-band monolithic down-converters reported to date. (C) 2009 Elsevier B.V. All rights reserved.