Heavily Zn-doped graded-base AlGaAs/GaAs HBTs grown by MOCVD

Heavily Zn-doped graded-base AlGaAs/GaAs HBTs with base doping concentrations of 3.5*10/sup 19/, 5.5*10/sup 19/, and 8.1*10/sup 19/ cm/sup -3/ were fabricated using metalorganic chemical vapor deposition (MOCVD)-grown epitaxial wafers. The maximum measured current gain was 42 with a base doping concentration of as high as 8.1*10/sup 19/ cm/sup -3/. The current gain decreased drastically from 96 to 42 when doping concentration increased from 3.5*10/sup 19/ to 8.1*10/sup 19 /cm/sup -3/. Assuming that the emitter efficiency is unity, the minority electron lifetime in the base is estimated by the current gain by consideration of drift in the graded base. The dependence of the electron lifetime in the graded base on Zn doping concentration was similar to that in a uniform base.<>