The conventional analysis of surface plasmon modes on dielectric-metal interfaces requires clearly defining the permittivity discontinuity at the interface. A pivotal assumption of such an analysis is that the formation of the dielectric-metal interface does not change the material properties and the materials forming the interface have identical permittivities before and after the formation of the interface. However, this assumption breaks down if an interface is made between a metal and a semiconductor which is commonly known as a Schottky junction. Under certain conditions, such an interface can sustain a surface plasmon polariton (SPP) mode. It is also possible to change the properties of the media surrounding the Schottky junction interface by applying an external potential difference across the junction. Central to the understanding of the SPP mode behaviour in such a complex morphological interface is the dispersion relation which defines the feasible SPP modes and their characteristics. Here, we carry out a detailed analysis to derive an analytical expression for the dispersion relation for a Schottky junction. Our analysis takes into account the space charge layer formed due to the charge distribution across the Schottky junction and resulting new boundary conditions.