The Y₃Al_5−xGa_xO₁₂(YAGG):Ce³⁺-Cr³⁺ persistent phosphor is one of the materials in which Cr³⁺ ions act as electron traps. The possibility of electron traps by other transition metal ions (TM³⁺, TM = Sc, Ti, V, Cr, Fe) was investigated and those electron trap depth was compared with each other. In the thermoluminescence (TL) glow curves, the YAGG:Ce³⁺ samples co-doped with different TM³⁺ ions show different TL glow peak temperatures (i.e. different electron trap depth). The estimated vacuum referred binding energy of TM²⁺ from the trap depth shows a zig-zag curve, which is found to be originated from the different 3d electron energies affected by a number of d electron, nuclear charge and crystal field splitting. Utilizing the obtained TM²⁺ zig-zag curve, a new persistent phosphor of Y₃Al₂Ga₃O₁₂:Ce³⁺-Sc³⁺ and a photostimulated phosphor of Y₃Al₂Ga₃O₁₂:Ce³⁺-V³⁺ were successfully developed.