In RFIC, spiral inductor behaves as capacitor at some hight frequency . Why so?

If we plot the curve between Q (quality factor) Vs f (frequency), we will find out that after a certain frequency in Ghz , Q start becomes negative which signifies that this inductor is turning to capacitor. Kindly explain the effect with respect to on Chip spiral inductor

Due to parasitic effects on element, it behaves differently than expected. For inductor,due to parasitic capacitance between windings, coupling to ground and series resistance of the wire, circuit model is not a single inductor but combination of inductor capacitor and resistors.
http://www.microwavejournal.com/arti...ductor-filters

As metu explained, the effect is caused by (a) parasitic capacitance between the turns and (b) shunt capacitance to the silicon substrate. Self resonance goes up in frequency when you increase the distance between inductor and substrate, because that reduces (b)

You can see the equivalent circuit model for a center tapped RFIC inductor here:
http://muehlhaus.com/products/equiva...rfic-inductors