This work presents a comprehensive numerical framework for modeling the photoresponse of monolayer graphene-based photodetectors, by solving Poisson’s and current continuity equations self-consistently. The framework accurately captures both electrostatic potential and carrier transport phenomena in graphene-metal junctions and is validated against experimental data. By implementing a PIN junction architecture, a “staircase” potential profile is formed in the device leading to local electric fields on the order of 105 V/cm, significantly enhancing carrier separation and drift current. Our simulation results indicate that the PIN junction yields a 40x increase in responsivity compared to conventional sheet-based graphene devices. This highlights the potential of the PIN junction-based approach for developing advanced, tunable, broadband graphene photodetectors. The developed numerical framework offers a powerful tool for photodetector optimization, enabling systematic exploration of structural parameters and operating conditions.