Using MBE and MOCVD, High Electron Mobility Transistor (HEMT) structures were grown on two-inch GaAs wafers. The vertical structure included three GaAs layers and two thin (3nm) Al0.3Ga0.7 As etch stop layers to control precisely the depth of the etched recess and hence the threshold voltage of the devices.
The structure below the second etch stop layer depends on the particular device to be fabricated, but is irrelevant for the etching process. The selective reactive ion etching of GaAs on AlGaAs is done by a Leybold MPE 3003 Plasma Etcher. The etch gas used is CF2Cl2 for GaAs and Ar for AlGaAs. The pressure lies between 30 µbar and 100 µbar and the self bias voltage between 47 V and 250 V. The standard deviation for enhancement FETs is 12 mV for Vt = +0.13 V and for depletion FETs 18 mV for Vt = -0.48 V.