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We report an unconventional chemical p- and n- doping scheme utilizing novel materials and a charge transfer mechanism to obtain self-aligned, unipolar, stable carbon nanotube field effect transistor (CNTFET). This scheme introduces the tunability of the threshold voltage Vth, increases the drive current 2-3 orders of magnitude, transforms CNTFETs from ambipolar to unipolar, suppresses minority carrier injection and yields an excellent Ion/Ioff ratio of 106. We utilize the ambipolar conduction in CNTFETs for optoelectronic applications, and demonstrate spatially-resolved electron-hole recombination in long channel transistors. This allows us to probe the carrier transport processes under varying bias conditions in nanotubes, determine the recombination lengths and recombination times, observe defects, etc.