This paper presents the design of a rectenna integrated on silicon, using a multilayer substrate composed of silicon and air. The rectenna is designed to harvest microwave energy at 2.45 GHz. The substrate structure is composed of three layers of high-resistivity silicon (HRSi, εr = 11.7) where the central layer presents an air cavity obtained through etching process. The chosen feeding structure for the antenna is based on a planar microstrip feeding line while for the rectifier a planar matching network is adopted to allow the maximum power transfer. The adopted rectifier topology is a voltage doubler, consisting of a pair of Skyworks SMS7630-079LF Schottky diodes. The synthetized low-permittivity substrate is compatible with on-chip systems silicon and assure a 84 % radiation efficiency with a maximum gain of 7.76 dBi, for the patch antenna. The overall efficiency of the rectenna is 45% for a received power level of 7 dBm.