The carbon nanotubes and carbon nanobtube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale electronic devices. Single wall carbon nanotubes, as molecular wires, could be used as nanoscale interconnects, where as complex junctions of nanotubes of different chirality can be used as rectifying and transiting devices. It has been shown that pentagon-heptagon defect is responsible for the creation of simple hetero-junctions. Complex junctions, such as 3-termianl T-junctions and Y-junctions require entirely different arrangement of defects. These 3-terminal junctions form prototypes of nanoscale tunnel devices made entirely of carbon. Furthermore, either n-type or p-type doping of the semiconducting portion of these complex junctions should yield Schottky barrier type devices. We also investigate simple wires and junctions of boron and nitrogen (III-V elements) and compare with those of carbon. The structural and electronic properties on nanotubes and nanotube junctions (carbon as well as BN) are studied using a generalized tight-binding molecular dynamics (GTBMD) scheme.