I'm looking for a datasheet of the component HP HFET-102 GaAs FET
I've been looking everywhere, google, HP site, ... other transistors site, but no success.
Is it some kind of fake/invented transistor made for some college/university excercises?
Probably not fake but certainly very old and most likely obsolete and no longer manufactured. Hewlett Packard spun-off businesses, including their semiconductor operation, in 1999. That part of the business is now Avago Technologies. All the GaAs FETs they currently support are listed here: