In this work, the surface treatment of InAs/GaSb type-II super-lattice long-wavelength infrared detectors is studied. An optimizing process of N₂O plasma treatment and rapid thermal annealing was developed, which can improve the performance of long-wavelength detector with λ_{50% ?cut-off}=12.3μm from 5.88×10^-1A/cm² to 4.09×10^-2A/cm² at liquid nitrogen temperature, -0.05V bias. Through variable area device array characterization, the sidewall leakage current was extracted. Under zero bias, the surface resistivity improved from 17.9Ωcm to 297.6 Ωcm. However, the sidewall leakage couldn’t be ignored under large inverse bias after optimizing process, where surface charge might induce the surface tunneling current. It is verified by gate-control structure that there are two main leakage mechanisms in long-wave device: pure sidewall parallel resistance and surface tunneling. At last, the surface charge was calculated to be 3.72×10¹¹cm^-2 by IV curve fitting after optimizing process.