Polycrystalline silicon thin-film transistors (TFTs) with different front- and back-gate lengths are investigated. In addition, the laser annealing process yields high-quality directional grains that enable us to orient TFT channels parallel or perpendicular to the grain boundaries. It is demonstrated that the turn-on voltage is not dependent on grain orientation, unlike the subthreshold swing and the maximum transconductance. Moreover, it is shown that double-gate TFTs are fully depleted and therefore back interface properties exert critical influence on the overall TFT electrical performance. From electrical measurements the back interface state density was estimated to reach values >6× 1011 cm-2 eV-1 and it was shown that the electrical performance of the double-gate devices is highly dependent on the back-to-front gate-length ratio.