A theory of the carrier transport, optical emission, and photoconductivity from optoelectronic devices based on ambipolar long-channel carbon-nanotube (CNT) field-effect transistors (FETs) is presented in this dissertation. In optical emitters based on ambipolar long-channel CNT FETs, an analytic diffusive-transport model for various recombination mechanisms is provided for the first time. The relationship and the scaling of emitted light-spot size and emitted optical power are clearly depicted for the first time as well. We also implement a numerical diffusive-transport approach for the light emission, in which the focus is on the effects of radiative and nonradiative recombination in the channel, with the movement of the spatial recombination profile in response to the gate and drain voltages. For the first time, we find that the emitted light-spot size and the emitted optical power depend sensitively on the operative nonradiative recombination mechanisms. We implement a numerical diffusive-transport approach including exciton photogeneration as well for photoconductors based on ambipolar long-channel CNT FETs with uniform and near-field photoexcitation. We show that the photocurrents are typically much smaller than the dark currents, and explain some possible reasons. Moreover, the exciton densities in CNTs are calculated and the effect of exciton diffusion is presented.