Waveguide photodetectors, in which the direction of the incident light is perpendicular to direction of the carrier collection, are characterised by their high speed and high quantum efficiency, but also the possibility of bias-free operation. Basic design equations for waveguidepinheterojunction photodetectors are derived, and criteria for optimum operation in terms of external quantum efficiency and bandwidth are discussed. The analysis shows that bandwidths of more than 150 GHz with a quantum efficiency of 50%, limited by the fibre-to-detector coupling efficiency, is achieved at zero-bias operation.