The annealing characteristics of GaAs tunnel junctions using carbon (C) as a p-type dopant were studied through current-voltage measurement and secondary-ion mass spectroscopy (SIMS). After annealing (750 degrees C, 20 min), diodes fabricated with a C-doped p(+)-GaAs/Si-doped n(+)-GaAs tunnel junction showed an excellent tunnel peak current density (J(p)) of 114mA/cm(2), which is a larger value than ever reported for structures using Be or Zn as a p-type dopant. Degradation of J(p) in diodes after annealing was found to drastically improve when the tunnel junction was sandwiched between p- and n-AlGaAs layers. The inserted n-AlGaAs layer was more effective in suppressing J(p) degradation than the p-AlGaAs layer. SIMS results revealed that Si diffusion, which causes J(p) degradation, was suppressed at the interfaces of the p(+)-GaAs/n(+)-GaAs tunnel junction and the GaAs/AlGaAs heterojunction. An excellent J(p) value of 1.7 A/cm(2) at 55 mV was obtained after annealing (750 degrees C, 20 min) through structural optimization.