The rapid emergence of high-performance optical systems has accentuated the need for photodiodes with enhanced performance and functionality. In this paper we will describe a new class of photodiodes that utilize novel resonant-cavity structures to achieve high speed, high quantum efficiency, and a narrow spectral response which may prove useful for some wavelength division multiplexing applications. The resonant-cavity photodiode consists of a thin absorbing layer sandwiched between two dielectric mirrors. One advantage of this structure is that it can be utilized to circumvent the responsivity/bandwidth tradeoff inherent to conventional PIN photodiodes structures. For the typical normal-incidence photodiode a wide bandwidth necessitates a thin absorption layer which, in turn, results in low quantum efficiency. The resonant-cavity structure, on the other hand, effectively decouples the responsivity from the transit-time component of the bandwidth because the optical signal makes multiple passes across the thin absorbing layer inside the microcavity. The resonant- cavity approach has been utilized for p-i-n photodiodes, phototransistors, dual-wavelength photodetectors, avalanche photodiodes (APDs), and Schottky barrier photodiodes. In this paper we will concentrate on two specific devices, a Si1-x/Gex resonant-cavity p-i-n photodiode and a resonant-cavity APD with separate absorption and multiplication regions.