We report the growth of silicon nanowires by plasma assisted metal organic chemical vapor deposition. Silicon nanowires grew as three-dimensional networks in which electrical charges and heat can travel over the distance much longer than the mean length of the constituent nanowires. We studied the dependence of thermoelectric properties on two factors; nominal doping concentrations and geometrical factors within the silicon nanowire networks. The silicon nanowire networks show Seebeck coefficients comparable with that of bulk silicon for a given nominal doping concentration, allowing us to control Seebeck coefficients by tuning the doping concentrations. Rather than studying single nanowires, we chose networks of nanowires formed densely across large areas required for large scale production. We also studied the role played by intersections where multiple nanowires were fused to form the nanowire networks. Structural analysis, transport measurement, and modeling based on finite-element analysis were carried out to obtain insights of physical properties at the intersections. Understanding these physical properties of three-dimensional nanowire networks will advance the development of thermoelectric devices.