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The electrical and optical characteristics of a 1.3 mum GaNAsSb-GaAs p-i-n waveguide photodetector (WGPD), consisting of GaAs inner and AlxGa1-xAs outer cladding layers, were simulated using a lumped-element model and finite difference beam propagation method (BPM). The effect of multiple cladding layers' thicknesses and aluminum composition on the quantum efficiency was investigated to provide design guide and deduce the range of parameters needed for achieving quantum efficiency higher than 89%. The simulation shows that a p-i-n waveguide structure with 10 mum length, 4 mum ridge width, 2.4 mum ridge height and 0.4 mum GaN0.033As0.887Sb0.08 core layer thickness can exhibit a theoretical cut-off frequency limit of over 80 GHz. Maximum quantum efficiency of 91.5% can be obtained for this structure with cladding layer thickness of 0.5 mum for GaAs and 0.5 mum for Al 0.7Ga0.3As.