Highly selective reactive ion etching applied to the fabrication of low-noise AlGaAs GaAs FET´s

Selective dry etching of GaAs to AlGaAs (x = 0.25) using pure CCl2F2etching gas has been achieved. During reactive ion etching (RIE), the discharge has been analyzed by optical emission and mass spectroscopy. A high-selectivity ratio up to 1000, associated with a clean and anisotropic etching or an undercut of GaAs can be obtained by adjustment of the pressure. Selective RIE has been used to etch the n⁺GaAs cap layer of an n⁺GaAs/AlGaAs/GaAs heterostructure and to define the gate recess of discrete two-dimensional electron-gas FET´s (TEGFET´s). Results on low-noise TEGFET´s fabricated by this technique are reported for the first time. Thanks to the reduction of side etching, very low source resistances have been obtained (less than 1 Ω . mm) for a source-to-gate distance up to 2 µm. Noise figures of 2 dB have been measured at 12 GHz with an associated gain of 8.3 dB for a gate length of 0.7 µm.