The transport properties of a-Si:H based thin film transistors have been characterized by the ambipolar diffusion length (Lamb) which was measured by the steady-state photocarrier grating technique (SSPG). The results show that with increasing positive gate voltage (Vg) the photoconductivity increases, but Lamb decreases. The above results can be consistently explained in terms of the shift of the Fermi level towards the conduction band edge with increasing V g. On the other hand, from the analysis of the SSPG results it is found that the factor γS, which is related to the interface recombination velocity, increases with V g.This indicates that when narrowing the conductive channel the recombination of the carriers at the interface increases.
