Faced multiple device failure(GaAs MESFET, depletion type MOSFET) until final product was produced by controlling the electrons in the superlattice by introducing a Schottky barrier over the single heterojunction
HEMT’s replaced GaAs MESFET’s because of the shrink in necessary size of the antenna by .5 or more
In the 90’s these entered into the satellite receivers and mobile phone applications, improvements in cost and processes