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The p-channel impact-ionization nanowire multiple- gate field-effect transistors (I-MuGFETs or I-FinFETs), which have a multiple-gate/nanowire-channel architecture, were demonstrated. The superior gate-to-channel coupling reduces the breakdown voltage VBD for enhanced device performance. For the first time, an in situ doped source was incorporated with the impact-ionization MOS transistor. The in situ phosphorus-doped Si source with improved dopant activation and very abrupt junction profile reduces VBD and enhances the on-state current Ion. An additional improvement was also achieved by incorporating a strained Si1-yCy impact-ionization region (I-region) and an in situ doped Si1-yCy source, leading to reduction in Vbd and enhancement in Ion. This is due to strain-induced reduction of the impact-ionization threshold energy Eth. Furthermore, an excellent subthreshold swing of below 3 mV/decade at room temperature was achieved for all devices.