The arsenic doping and activations on HgCdTe (mercury cadmium telluride) p+-on-n heterojunctions grown by MBE (molecular beam epitaxy) have got a lot of attention in fabricating high performance long-wavelength IRFPAs (infrared focal plane arrays). In this paper, the performances of HgCdTe diodes with different arsenic doping concentrations are presented. According to the I-V results and dark current mechanism, the effect of arsenic concentration on the trap-assisted tunneling current was calculated and analyzed. To achieve reproducible doping and activation process, the dependence between arsenic doping efficiency and Hg/Te condition was reported. As activation ratio could be higher than 60% under activated annealing at 300, 420 and 240 degree Celsius under an Hg saturated vapor pressure, which was demonstrated by means of Hall-effect measurements and SIMS (Secondary Ion Mass Spectrometry).