Using reactive ion etching with CCl2F2/He a selectivity in etch rates between GaAs and AlGaAs of larger than 3000 has been obtained for low dc self‐bias voltages. Under isotropic etch conditions at low‐bias values this selectivity is used for the lateral etching of thin GaAs layers sandwiched between AlGaAs layers. The range of lateral etching is proportional to the square roots of the etch time and the thickness of the etched GaAs layer. Lateral etch widths of 4 &mgr;m for a GaAs layer thickness of 50 nm and larger than 20 &mgr;m for a thickness of 1 &mgr;m are achieved.