Si nanowires (SiNWs) were synthesized by gold (Au)-catalyzed chemical vapor deposition (CVD). To control the morphology of SiNWs, the growth parameters were systematically changed and they were studied by transmission and scanning electron microscopes and Raman spectroscopy. We found that the shape of SiNWs is strongly affected by the growth rate, which could be controlled by the total pressure. The analysis of Fano-type spectral shape of boron (B)-doped SiNWs revealed that at least as high as 1019 cm-3 active B atoms can be doped in situ during the growth. By using SiNWs as channels, back gate type field effect transistors (FETs) were fabricated on p-type silicon wafers. The best performance (ION/IOFF>107, S∼1 V/dec, Vth∼ -1 V, a hysteresis∼2 V, and mobility∼200 cm2/Vs) was obtained for SiNWs with core-shell structures. These performances are better than those of poly-Si thin film transistors.