1. An integrated three-dimensional semiconductor system comprising: a base level comprising a substrate; at least one level above the substrate comprising an insulating layer and at least one nanotube field effect transistor on the insulating layer, each nanotube field effect transistor comprising: a nanotube fabric having a plurality of semiconducting nanotubes, the nanotube fabric having a source region and a drain region, wherein the source region and drain region are in a spaced relation relative to one another and wherein the spaced relation defines a channel region in the nanotube fabric; and a gate element electrically coupled to the channel region, wherein the gate element modulates the conductivity of the channel region such that a conductive pathway is formed or unformed between the source and drain in response to electrical stimulus.