It was shown that the growth of emitter layers of InP/InGaAs/InP double heterojunction bipolar transistors could result in significant Zn diffusion from the base and into the collector. The extent of the diffusion depended upon the n-doping level of the emitter. This behavior was explained in terms of non-equilibrium point defects which were introduced by a combination of surface pinning of the Fermi level, and n-doping. It was also shown that Zn diffusion could be greatly reduced by using AlInAs, instead of InP, as the emitter layer. The difference in behavior was shown to be at least partly due to the lower diffusivity of group-III interstitials in AlInAs. Moreover, it was shown that the introduction of only 50nm of AlInAs between emitter and base resulted in a significant reduction of Zn diffusion into the collector.

R.Bhat, M.A.Koza, J.I.Song, S.A.Schwarz, C.Caneau, W.P.Hong: Applied Physics Letters, 1994, 65[3], 338-40