Experiments for silicon biosensors with gate lengths in the range of 200nm to 500nm have not been extensively carried out. In this paper, simulations were performed for gate lengths proportionally smaller and greater than regular experimental gate lengths. The sensitivity of the biosensors was simulated using a 2D drift-diffusion model in cylindrical coordinates using the Prophet simulator. In this study simulated conductance results and the respective experimental values  are compared. The good agreement between simulation and experiment enables us to predict and optimize the sensitivity of the DNA sensors. The sensitivity was studied in terms of conductance by varying the gate length, probe spacing, binding efficiency and angle of probe from normal.