That article must be either very old (older than Wikipedia itself??), or the author is rather uninformed. Certainly Schottky diodes are useful to well beyond 5GHz. For example, from the Avago website (where you can also find quite a few application notes on Schottky signal diodes),
"The HSCH-533x family of low barrier Beam Lead Schottky diodes are ideally suited for mixer and detector applications from 1-26GHz."
I suppose without much trouble you can find Schottkys intended for use to 40GHz and higher.
Cheers, Tom
On Dec 28, 11:23 am, karthikbalaguru wrote (quoted from a Wikipedia article): ...
Small-area Schottky diodes are the heart of RF detectors and mixers,
> which often operate up to 5 GHz.