Back gate triggered silicon controlled rectifiers (SCR) and methods of manufacture are disclosed. The method includes forming a first diffusion type and a second diffusion type in a semiconductor layer of a silicon on insulator (SOI) substrate. The method further includes forming a back gate of a first diffusion type in a substrate under an insulator layer of the SOI substrate. The method further includes forming raised diffusion regions of a first dopant type and a second dopant type, adjacent to the second diffusion type and the first diffusion type, respectively. The back gate is formed to cover the second diffusion type, the first diffusion type and the second dopant type of the raised diffusion regions.