a complete gaas device fabrication process may involve epitaxial growth, ion implantation, lithography, metallizations, alloying and annealing steps, as well as a number of etching and surface preparation steps. in fabricating lateral devices, etching is used for mesa isolating a device (although ion beam isolation is also available) and for recess etch prior to gate metallization in mesfets. etching is used also for the fabrication of vertical devices such as the permeable base transistor and the vertical fet after etching, one needs to remove surface oxides in preference to the gaas underneath prior to metallization. the presence of a thin interfacial dielectric layer causes poor ohmic contact formation and also causes non-reproducible, non-ideal schottky barrier characteristics to appear. the authors describe the physical and chemical nature of the different etching-induced structuring processes available so that useful choices can be made for a given fabrication requirement