Field effect transistors (TFTs) with different channel lengths L and widths W have been prepared to study the influence of the geometry on the characteristics of the transistors. The mobility μ and the threshold voltage Vth are determined from the √Ids (Uds) plot. Effective values of L and W can be determined by analyzing their dependence on geometry. It can be shown that parts of the amorphous silicon which are outside the area between the source and drain contacts contribute to the current. The reason for this is that the drain-source field spreads out into these regions. Taking this effect into account we obtain the values for the mobility μ. The part of the overlap contributing to the transistor current is smaller than 2 μm.