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

Heavily Zn-doped graded-base AlGaAs/GaAs HBTs with base doping concentrations of 3.5×10¹⁹, 5.5×10¹⁹, and 8.1×10¹⁹ cm^-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 8.1×10¹⁹ cm^-3. The current gain decreased drastically from 96 to 42 when doping concentration increased from 3.5×10¹⁹ to 8.1×10 ¹⁹ cm^-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