The electrical properties and the Franz-Keldysh effect at the direct band edge of Ge-on-Si p-i-n photodiodes for operating conditions in a wide temperature range are studied. Between 233 K and 343 K the reverse dark current at -1 V increases at an average of 5% every 1 K. At an operating temperature of 343 K the dark current is 7.4 times larger than at 293 K which also affects the electro-optical performance of the device. The dark current in the investigated diodes is caused by the generation/recombination of the trap levels in the bandgap as concluded from the magnitude and electric field dependence of the reverse current and the ideality factor of the forward current. Activation energies derived from the temperature dependence of dark current suggest a trap energy of about 100 meV off the intrinsic level. Subsequently the influence of temperature on electro-optical properties particularly with regard to integrated detection and modulation devices is presented. Optical responsivity spectra demonstrate a distinct extension of absorption range of 1 nm/K to the infrared for higher temperatures. Despite of increasing dark current we demonstrate the electro-optic Franz-Keldysh effect and its typical modulation spectra for temperatures up to 359 K. At its maximum the change of absorption Delta alpha for a voltage swing of 2 V between on-state and off-state reaches 777 cm(-1) constantly for all temperatures from 300 K to 359 K. (c) 2012 Elsevier B.V. All rights reserved.