RFID Tag Doubt

It is said that RFID tag (passive ) works at microwatts.

The voltages anywhere are in micro or atmost millivolts ,but it actually comprises of a rectifier circuit (for power generation in passive tags) which would use a diode.

My doubt is diodes usually work above 0.7 V (si ) or 0.3 V (Ge) then how is it possible to use diode in passive tags at micro or millivolts ?

Are special diode used ? or With VLSI technology it is possible to make such low power diodes in CHIP itself ?

or any other reason ?

Thanks !

The diode numbers you give are for high power levels. Their I-V curves pass through 0 with curvature. This nonlinearity is what does the rectifying at low signal levels.

In the early days of broadcasting the audience used "crystal sets' which were just the antenna, a tuned circuit, a sort of diode made from tungsten wire pressed on lead sulfide, and a crystal headphone. There was enough power rectified by the crude diode to power the headphones for stations miles away.

(1) Use higher Zo.

(2) Use diode and capacitor to form a Voltage doubler for further DC power recovery(or even higher order).