Transmission electron microscopy has been employed to characterize GaAs permeable base transistor structures. Submicrometer‐period tungsten base gratings fabricated on GaAs substrates by using either liftoff or etching techniques have been studied. Fabrication‐related problems such as electrically isolated tungsten debris and poor tungsten adhesion to GaAs have been identified. The crystal quality of GaAs layers grown by vapor phase epitaxy over the tungsten base gratings has also been investigated. Both stacking faults and voids have been found in these layers. The concentration of voids, which have an elongated‐bubble shape, depends on the orientation of the base grating on the GaAs surface. The effect of these structural imperfections on device performance is discussed.