The spectral response of resonantly enhanced photodiodes is analyzed theoretically and verified experimentally. Comprehensive design guidelines and formulas are given for device structures contain- ing a metal reflector, a contact layer, an optional grading layer, an absorbing layer, and a quarter-wave stack (QWS). The analysis shows, for instance, that the quantum efficiency of a Schottky photodiode with a 162-nm GaInAs absorbing layer can be enhanced 3.7 fold by using a 41-layer AlInAs/AlGaInAs QWS. The number of layers required could be much lower for other material systems and/or if the substrate is removed. Experimentally, 50% enhancement is demonstrated for a 475-nm thick absorbing layer at 1.52 µm by using a 16-layer QWS. The resonance width is ~4.4%. This type of structures should be very valuable for very small size ultrafast photodiodes and for optoelectronic integration.