The surface leakage current and thermal stability of the infrared detector are highly related to the sidewall surface of the mesa. This work focused on researching the sidewalls' properties of InAs/GaSb type-II superlattice middle-wavelength infrared detectors by gate-control techniques. It was found the I-V curves for samples with or without annealing showed significant difference at 80 K, and the dark current density of the annealing sample increased from 2.17×10^{-‍7 ‍}A/cm² to 6.96×10^-5A/cm² comparing with the sample without annealing at the bias of -‍0.05 V. The results of gate-control experiment proved the surface fixed charge was increased by 2.76×10¹² cm^-‍2 after annealing, which caused severe surface tunneling leakage. And the XPS showed the elemental Sb increased after annealing.