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The article presents the results of experimental studies of the dc and high-frequency electrical characteristics of planar microwave diodes that are fabricated on the base of the <i>n</i>-Al<i>_x</i>Ga_1-<i>x</i>As layer (<i>x</i> = 0, 0.15 or 0.3), epitaxially grown on a semi-insulating GaAs substrate. The diodes can serve as reliable and inexpensive sensors of microwave radiation in the millimeter wavelength range; they sense electromagnetic radiation directly, without any external bias voltage at room temperature. The investigation revealed a strong dependence of the detection properties of the microwave diodes on AlAs mole fraction <i>x</i>: in the K_a microwave frequency range, the median value of voltage responsivity is several volts per watt in the case of GaAs-based diodes (<i>x</i> = 0), and it substantially increases, reaching hundreds of volts per watt at higher <i>x</i> values. Also, a model enabling us to forecast the responsivity of the sensor in other frequency ranges is proposed.