the gaas permeable base transistor (pbt) is fabricated by using overgrowth to embed a polycrystalline, submicrometer-period tungsten (w) grating completely within a single crystal of n-type gaas. recent pbts have experimentally demonstrated an extrapolated maximum frequency of oscillation (f max) of 223 ghz and an extrapolated unity current gain frequency (ft) of 50 ghz. by using a subpicosecond laser sampling technique, the turn-on risetime for a pbt has beem measured to be 5 picoseconds at room temperature. initial pbt performance results were modest, prompting a comprehensive study of the special problems involved in fabrication. extensive secondary ion mass spectrometry (sims) experiments have shown that cr, fe, cu, te, and au impurities in the w grating lines diffuse into the gaas during overgrowth. similar experiments have shown that cl preferentially incorporates from the gas phase around w gratings overgrown by ascl3 vapor phase epitaxy (vpe). se shows this same behavior in organometallic chemical vapor deposition (omscvd) overgrowth when h2se is used as the dopant gas. studies have also shown that any pre-overgrowth process-related surface contamination on the gaas or the grating that is not removed by the pre-overgrowth surface cleaning will probably be incorporated into the overgrown gaas. many of these impurities are acceptors or deep levels in gaas, and their presence in the device channels can have a strong negative effect on pbt performance