GROWTH OF PNP HETEROJUNCTION BIPOLAR-TRANSISTOR STRUCTURES BY METALORGANIC MOLECULAR-BEAM EPITAXY

Fabrication of high-quality Pnp heterojunction bipolar transistors has traditionally been difficult due to the inability to achieve and confine high p- and n-type doping levels using common dopants such as Be and Si. In this paper we discuss how carbon and tin can be incorporated during growth by metalorganic molecular beam epitaxy in order to produce Pnp structures. In particular it has been found that carbon introduced from trimethylgallium can be used to produce abrupt, thermally stable profiles in AlGaAs and that incorporation at concentrations up to mid-10(19) cm-3 does not adversely affect the optical or structural quality of the material. In addition, we have found that the use of tetraethyltin (TESn) for tin doping of the GaAs base layer allows for higher doping and better confinement of the dopant than can be obtained with elemental Sn. Consequently, large-area (90-mu-m diameter) Pnp transistors fabricated from material grown with TESn show higher gain than those grown with elemental tin, in spite of the higher base dopant concentration. The gain obtained with TESn, 45, is the highest yet reported for an abrupt-junction, uniformly-doped Pnp structure. Furthermore, because of the low parasitic resistances which result from the use of carbon and tin doping, the I-V characteristics obtained in this study show superior performance relative to previously published reports.