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In sensor applications, millimeter wavelengths offer substantial advantages over lower frequencies. The application can benefit from relaxed bandwidth limits and wider Doppler frequency deviations. Circuit feature sizes are scaled down with the wavelength resulting in smaller and better integrated circuits than could be achieved at lower frequencies. Shorter wavelengths also enable higher spatial resolution given the same aperture. This may be irrelevant for a single sensor but could be utilized when many sensors are deployed to cover a large area in applications collectively known as "Smart Dust". Since mm-wave interconnects pose significant challenges in assembly precision, materials and therefore cost, a complete integration of all such circuits, interconnects and antenna within a single MMIC die lowers product cost and enables diverse applications. The resulting sensor can be used as a proximity and range detector and is aimed at low cost applications. Target security applications include perimeter control, intruder detection and identification. Machine vision, RFID and obstacle avoidance applications could also utilize such a sensor. This paper discusses design tradeoffs, implementation and results of one of several prototype sensor front-end devices implemented in GaAs MMIC technology.