A substrate for microelectronic radiofrequency devices includes a carrier substrate made of a first semiconductor material having a resistivity higher than 500 ohms-cm; a plurality of trenches in the carrier substrate, which trenches are filled with a second material, and defining on a first side of the carrier substrate a plurality of first zones made of a first material and at least one second zone made of a second material. The second material has a resistivity higher than 10 kohms-cm, and the first zones have a maximum dimension smaller than 10 microns and are insulated from one another by the second zone.