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As the conventional silicon metal-oxide-semiconductor field-effect transistor (MOSFET) approaches its scaling limits; many novel device structures are being extensively explored. Among them, the silicon nanowire transistor (SNWT) has attracted broad attention. To understand device physics in depth and to assess the performance limits of SNWTs, simulation is becoming increasingly important. The objectives of this work are: 1) to theoretically explore the essential physics of SNWTs (e.g., electrostatics, transport and band structure) by performing computer-based simulations, and 2) to assess the performance limits and scaling potentials of SNWTs and to address the SNWT design issues. The computer based simulations carried out are essentially based on DFT using NEGF formalism. A silicon nanowire has been modeled as PN diode (Zener Diode), PIN diode, PIP & NIN diode configurations by selectively doping the nanowire and simulated by biasing one end of the nanowire to ground and sweeping the other end of the nanowire from -1 V to 1 V to obtain the electrical characteristics of the respective diodes. In order to determine the effectiveness of the modeled diodes in silicon nanowire, the same diodes have been modeled using a germanium nanowire by selective doping and simulated in the same manner to obtain the electrical characteristics of the germanium nanowire based diodes which has been used as a reference to analyze the characteristics obtained using silicon nanowire. The modeled diodes are extremely small in dimension when compared to the conventional bulk silicon and germanium based diodes. This work is followed by modeling and simulation of a gate all around nanowire field effect transistor using two different gate dielectrics, followed by temperature dependence of the nanowire FET characteristics and the off state current and conductance variation using the two dielectrics.