An overview of recent results on high-speed germanium-on-silicon-on-insulator (Ge-on-SOI) photodetectors and their prospects for integrated optical interconnect applications are presented. The optical properties of Ge and SiGe alloys are described and a review of previous research on SOI and SiGe detectors is provided as a motivation for the Ge-on-SOI detector approach. The photodetector design is described, which consists of lateral alternating p- and n-type surface contacts on an epitaxial Ge absorbing layer grown on an ultrathin-SOI substrate. When operated at a bias voltage of <formula formulatype="inline"><tex>$-0.5$</tex></formula> V, <formula formulatype="inline"><tex>$10,mu$</tex></formula>m<formula formulatype="inline"><tex>$,times 10 mu$</tex> </formula>m devices have dark current <formula formulatype="inline"><tex>$I_{hbox{dark}}$</tex></formula>, of only <formula formulatype="inline"><tex>$sim$</tex></formula>10 nA, a value that is nearly independent of finger spacing <formula formulatype="inline"><tex>$S$</tex></formula>, between <formula formulatype="inline"><tex>$S = 0.3,mu$</tex> </formula>m and <formula formulatype="inline"><tex>$1.3 ,mu$</tex></formula>m. Detectors with <formula formulatype="inline"><tex>$S = 1.3, mu$</tex></formula>m have external quantum efficiencies <formula formulatype="inline"><tex>$eta$</tex></formula>, of 52% (38%) at <formula formulatype="inline"><tex>$lambda = 895$</tex></formula> nm (850 nm) with corresponding responsivities of 0.38 A/W (0.26 A/W). The wavelength-dependence of <formula formulatype="inline"><tex>$eta$</tex></f- ormula> agrees fairly well with expectations, except at longer wavelengths, where Si up-diffusion into the Ge absorbing layer reduces the efficiency. Detectors with 10 <formula formulatype="inline"><tex>$mu$</tex></formula>m<formula formulatype="inline"><tex>$,times {hbox{10}} mu$ </tex></formula>m area and <formula formulatype="inline"><tex>$S= 0.6, mu$</tex></formula>m have <formula formulatype="inline"><tex>$-3$</tex></formula>-dB bandwidths as high as 29 GHz, and can simultaneously achieve a bandwidth of 27 GHz with <formula formulatype="inline"><tex>$I_{hbox{dark}} =24$</tex></formula> nA, at a bias of only <formula formulatype="inline"><tex>$-$</tex></formula>1 V, while maintaining high efficiency of <formula formulatype="inline"><tex>$eta= 46{%} (33{%})$</tex></formula>, at <formula formulatype="inline"><tex> $lambda = 895$</tex></formula> nm (850 nm). Analysis of the finger spacing and area-dependence of the device speed indicates that the performance at large finger spacing is transit-time-limited, while at small finger spacing, <formula formulatype="inline"><tex>$RC$</tex></formula> delays limit the bandwidth. Methods to improve the device performance are presented, and it is shown that significant improvement in the speed and efficiency both at <formula formulatype="inline"><tex>$lambda = 850$</tex></formula> and 1300 nm can be expected by optimizing the layer structure design.