ABNORMAL REDISTRIBUTION OF ZN IN INP/INGAAS HETEROJUNCTION BIPOLAR-TRANSISTOR STRUCTURES

High n+ doping (> 10(19) cm-3 of Si) in the subcollector of InP/InGaAs heterojunction bipolar transistor structures is observed to induce an anomalously high Zn diffusivity and an associated broadening of the base layers. It is proposed that due to Fermi level surface pinning and a long time constant for the recovery of point defect equilibrium the subcollector acts as a continuously operating source of group III interstitials which in turn diffuse into the subsequently grown base region and enhance Zn diffusion via the kick-out mechanism. In this sense, highly n+ doped grown-in subcollector layers may be considered as having the effect of a "time bomb" in terms of generating undesirable excess point defects during subsequent further crystal growth and device processing.