We fabricated the first bottom-gate amorphous silicon (a-Si:H) thin-film transistors (TFTs) on a clear plastic substrate with source and drain self-aligned to the gate. The top source and drain are self-aligned to the bottom gate by backside exposure photolithography through the plastic substrate and the TFT tri-layer. The a-Si:H channel in the tri-layer is made only 30 nm thick to ensure high optical transparency at the exposure wavelength of 405 nm. The TFTs have a threshold voltage of ∼3 V, subthreshold slope of ∼0.5 V/dec, linear mobility of ∼1 cm2V-1 s-1, saturation mobility of ∼0.8 cm2V-1s-1, and on/off current ratio of >106. These results show that self-alignment by backside exposure provides a solution to the fundamental challenge of making electronics on plastics: overlay misalignment.