The electronic properties of charge carriers in carbon nanotubes exhibit striking similarities to relativistic elementary particles. A remarkable prediction of relativistic quantum mechanics is the ability of particle-antiparticle pairs to be created and annihilated when interacting with other high-energy particles. In this thesis, I discuss optoelectronic experiments that probe the extremely efficient generation of electron-hole pairs and the (ultra-) fast transit of these electrons and holes through nanotube PN junction photodiodes. Spatially, spectrally, and temporally resolved photocurrent measurements suggest that that the generation of multiple electron-hole pairs from a single high energy carrier is extremely efficient, and that electrons and holes may undergo electron-hole pair annihilation as they transit the junction on sub-picosecond time scales. These processes, analogous to relativistic particle-antiparticle creation and annihilation, set new boundaries for the performance limits of nanoscale optoelectronic devices.