A thin film transistor serving as a switching device provided in respective pixels of an active matrix-type liquid crystal display, the thin film transistor including a source electrode overlapping the gate electrode, a first semiconductor layer and contacting one of the two parts of a second impurity semiconductor layer, and a drain electrode overlapping the gate electrode, the first semiconductor layer and contacting another one of the two parts of the second semiconductor layer, wherein the source and drain electrode define a channel region in the first semiconductor layer having a length between the first and second portions of the first semiconductor layer and a width same as a width of at least one of the two parts of the second semiconductor layer such that a ratio of the width to the length (W/L) of the channel region is in a range of 8 through 10.