Improved RF Power Extraction from 1.55um Ge-on-SOI PIN Photodiodes with Load Impedance Optimization

VLSI miniaturization has created the need for high-density, low-cost, monolithically-integrated optical interconnects. High output power photodetectors are needed to directly drive load circuitry, which improves the noise performance and dynamic range of optical communications links by eliminating a post amplifier stage. Elimination of the post amplifier also reduces circuit cost and complexity. A new Si-Ge PIN waveguide photodiode with 31GHz bandwidth and 93% quantum efficiency at 1550nm has been developed by Yin et al., which was fabricated using standard CMOS processes on a Silicon substrate. This thesis demonstrates a method for improving the RF power extraction from these photodiodes by increasing the impedance of the load. An RF output power improvement of 5.5dB is obtained by increasing the load resistance from 50 ohms to 177 ohms with 15MHz modulation. The maximum obtainable RF power of all devices tested using 50 ohm and 100 ohm loads at 15MHz is 15.73dBm and 17.83dBm, respectively. The maximum obtainable RF power using a 177 ohm load for all devices tested is 17.67dBm, which is slightly smaller than that obtained with a 100 ohm load. A measurement procedure for RF power extraction at microwave frequencies is also described. Quarter-wavelength 70.71 ohm thin film coplanar waveguides are designed to transform 50 ohms to a higher impedance of 100 ohms for measurements of improved RF power extraction at 3GHz and 7GHz.