Heterodyne radiometry with blackbody sources was used to determine the sensitivities of GaAs Schottky diode submillimeter receivers. At 500 μm an NEP of 10-18 watts/Hz was measured for the system. Application of these receivers to the characterization of radiation from high power pulsed submillimeter lasers has been demonstrated. Examples of the extension of this effort to planar devices, suitable for large area coherent detectors and imaging, are also presented. Recent studies have indicated that GaAs Schottky diodes could be used to construct extremely sensitive submillimeter heterodyne receivers. By carefully tailoring every phase of fabrication from material selection to optimization of the mixer environment, some of this potential performance has now been realized. In this paper we discuss heterodyne radiometry measurements which show more than two orders of magnitude improvement over our initial devices. An application of these low-capacitance, small-junction-area, Schottky diodes so frequency and linewidth measurements of a high power pulsed CH3F laser is discussed. Finally, our first test results and projections for all-planar diodes and arrays are presented. The diodes used in our studies have been recently developed at Lincoln Laboratory and are less than 1 micron in diameter and have cut-off frequencies approaching 10,000 GHz. In order to minimize waveguide losses at submillimeter wavelengths these diodes have been packaged in short sections of overmoded, reduced height, N-guide. Stripline filters, incorporated as an integral part of the coaxial studs of these diodes, supply dc bias to the diode and provide for an intermediate-frequency output. At the same time, these filters present an almost perfect reflecting circuit element at the signal and local-oscillator frequencies. The development of the complete diode package has required both theoretical calculations and experimental evaluation of scaled models. In order to determine the sensitivities of these diodes we have made heterodyne radiometric measurements of radiation from a blackbody. Three different receiver configurations were used in these experiments. The first used a horn-lens (TPX) system to couple submillimeter radiation into the diodes. A germanium beam splitter was then used to introduce local oscillator power from an optically pumped CH3F laser. The complete test setup is shown schematically in Fig. 1. Our second system replaced the hornlens with a focusing mirror and the third system used an additional orthogonal mirror to alleviate the need for the beam splitter.