In this paper, we present the optical behavior of waveguide PIN photodetectors for millimeter wave applications. The side illumination of these devices allows us to overcome the problem encountered with classical PIN top illuminated photodetectors, which is the compromise between high cut-off frequency and high responsivity. This is the reason why we have modeled PIN waveguide photodetectors grown on InP substrate. Our modeling is based on 3D and 2D FD Beam Propagation Method to describe the propagation of light in the photodetector and on the solution of classical semiconductor equations to describe the electrical behavior of the device. The model is applied to InP P+/GaInAs N-/InP N+ and InP P+/GaInAsP P+/GaInAs N-/GaInAsP N+/InP N+ structures. Cut-off frequencies up to 90 GHz can be obtained for very small devices, typically 12 micrometers for the device length and 5 micrometers for the rib width with 0.3 micrometers thickness of GaInAs absorbing layer, by neglecting parasitic effects due to boundary pads. This is also valid for structures with smaller ribs using a constant surface area. The external quantum efficiency of such a device is strongly dependent on the device structure (GaInAsP thickness, monomode or multimode structure), and also on the conditions of injection of light (width and position of the optical spot, angle of the optical beam with the device). A complete analysis of the quantum efficiency versus the influence of GaInAsP thickness, device length, GaInAs thickness, and optical injection has been performed. It was found that, using lens ended optical fiber, multimode waveguide structures are better devices compared to monomode ones, and can lead to quantum efficiency higher than 90%.