Improved RF Power Extraction from 1.55�m Ge/Si n-i-p Photodiodes with Load Impedance Optimization

High power photodiodes (PDs) improve signal fidelity in high-speed optoelectronic communications links by eliminating a post amplifier stage, which reduces noise figure, decreases RF insertion loss, and increases spur-free dynamic range [1][2]. Recent efforts have focused on developing photodiodes with high power handling capabilities, and a photodiode with a compression current of 199 mA has been reported [3]. Other approaches have included coherently combining the RF outputs of multiple photodiodes [4] and modifying the modulation format to compensate for the compression that occurs within the photodetector [5]. Here, we increase the power delivered to the load by increasing the impedance seen by the photodetector. A cross section schematic of the Si-Ge photodetector studied is shown in Figure 1a. Light is evanescently coupled from the Si-waveguide layers into the Ge-absorber layer, which has a larger index of refraction than Si. These photodiodes show up to 31GHz bandwidth for a device cross section of 7.4�m x 50�m (width x length) and an internal responsivity of 1.1A/W for absorber lengths greater than 100�m [6]. The fabrication and epitaxial growth of these detectors is described in [6]. These devices have been shown to be capable of dissipating a large amount of electrical power before failure (1W), which is primarily due to the relatively high thermal conductivities of the Ge and Si used [7].