<p>Defect engineering is pivotal in comprehending physical mechanisms that govern carrier transport and device performance. The defect evolution and carrier manipulation in Sn-doped Ga₂O₃ bulk crystals subjected to different thermal treatments were investigated, utilizing depth-profiled deep-level transient spectroscopy (DLTS) and frequency-dependent capacitance-voltage (<i>C-V-f</i>) techniques. In untreated Sn-doped Ga₂O₃ with an electron concentration of 6.37×10¹⁷ cm⁻³, two dominant electron traps, E_T1 (<i>E</i>_C −0.68 eV) and E_T2 (<i>E</i>_C −0.76 eV), were identified, corresponding to gallium vacancy (V_Ga) and the neutral complex of V_Ga-V_O, respectively, and characterized as bulk traps. Fe_Ga-related defects, E_T3 (<i>E</i>_C ∓0.84 eV), were concentrated near surface. Nitrogen annealing significantly reduced E_T1, increased E_T2 density from 6.13×10¹⁵ to 1.1×10¹⁶ cm⁻³, and raised the interfacial state density (<i>D</i>_it) to 3.36×10¹⁵ eV⁻¹ cm⁻², accompanied by an elevated electron concentration of 7.48×10¹⁸ cm⁻³. In contrast, air annealing enhanced E_T1, with a density of 1.42×10¹⁶ cm⁻³, suppressed of E_T2/E_T3 traps, resulting in a lower <i>D</i>_it of 1.74×10¹⁴ eV⁻¹ cm⁻², and a reduced electron concentration to 3.01×10¹⁶ cm⁻³. The findings reveal that a reducing environment induces V_O formation and converts discrete V_Ga acceptors into neutral V_Ga-V_O complexes, leading to downward surface band bending and electron accumulation. Conversely, V_Ga-V_O complexes are dissociated into V_Ga acceptors in oxidizing conditions, leading to an upward surface band bending and electron compensation. This work underscores the carrier concentration manipulation by defect engineering in Ga₂O₃, offering insights essential for developing high-performance gallium oxide electronics.</p>