This article presents the design of high-frequencydown-conversion mixers using a carbon nanotube fieldeffecttransistor (CNTFET). An experimentally-calibrated semiphysicalcompact model for high-frequency CNTFET technologieshas been used. The designed mixer is proposed using atopology composed of a 90º hybrid coupler at a frequencyof 25 GHz with a coupling factor of 3 dB, a CNTFET andmicrostrip-based coupling networks. The mixer operates in theupper band for 5G communications.The RF signal is used at afrequency of 25 GHz for an intermediate frequency conversion(IF) of 1 GHz using a LO of frequency 24 GHz. The designyields a conversion gain of 20 dB whereas the RF/LO isolationis -18.9 dBm and the LO/IF isolation is -25.6 dBm with ananotube density of 40 CNT/μm. By increasing the densityof nanotubes to 100 CNT/ μm, the conversion gain and portisolation RF/LO figures of merit improve to 26 dB and -20.7dB, respectively. The mixer consumes 9.52 μW from a -1.5 V supply.