Numerical simulation of the electric field distribution and photocurrent response of a planar InP/InGaAs avalanche photodiode is presented for small variations of the multiplication width. The Zn dopant diffusion front is obtained by numerically simulating the diffusion process. The simulation results indicate that while a local peak value of the electric field is observed near the device edge, it is not associated with a significant increase in the photocurrent response. Experimental photocurrent mapping of an avalanche photodiode shows a response at the edge that is enhanced by ~ 60% compared to the centre response. Scanning electron microscope images of Zn diffused structures show that the depth is enhanced by 0.03 m at the edge, compared to the centre. Simulations of devices with varied multiplication width show that the magnitude of the increase in photocurrent expected for the observed depth enhancement is consistent with the observed photocurrent enhancement along the edge the active device.