A theoretical analysis of the influence of carrier diffusion on the threshold characteristics of semi-conductor microdisc lasers is given. The cold-cavity modes (Whispering Gallery Modes) and their wavelengths are obtained from approximate analytical solutions of Maxwell's equations. In the steady-state, the parameters of these modes are modified in an active semiconductor medium by the presence of intensity dependent gain, refractive index and diffusion of the carriers. An increase of threshold current with decreasing carriers' lifetime is predicted. Also, the dependence of the mode pulling on the diffusion length is discussed. These effects can be explained by considering spatially varying carrier density and therefore the spatially dependent refractive index and gain.