A Self-Aligned AlGaAs/GaAs Heterostructure Bipolar Transistor With Non Alloyed Graded-Gap Ohmic Contacts To The Base And Emitter

A nonalloyed graded-gap scheme for obtaining ohmic contacts to ntype GaAs, by first growing a graded transition from GaAs to lnAs and then making a nonalloyed metallic contact to the InAs, was proposed by Woodall et al. [l]. The underlying idea was as follows. A metal-to-lnAs interface acts as an ideal negative-barrier ohmic contact because the Fermi level is pinned inside the lnAs conduction band, as shown in Fig. 1. However, if the GaAs-to-lnAs transition were not graded, it would act as a quasi-Schottky barrier, and the contact would be poor overall. Sufficient grading flattens out the heterojunction barrier and leads to an excellent ohmic contact with properties that make it an attractive alternative to the widely used Au/Ge/Ni/Au alloyed system. For p-type GaAs, the Ga(As,Sb) system could be similarly used, as proposed by Chang and Freeouf [2].