This is the first paper to discuss the ON-state drain-current of a special thin-film transistor structure with a wide channel width and a narrow source/drain width in the linear region. The experimental results indicate that when the channel width is wider than the source/drain width, the side-channel current effect is generated. This effect increases the ON-state drain-current due to the additional current-flow paths existing in the side-channel regions and low channel resistance. As the side-channel width increases, the ON-state drain-current initially increases and then gradually becomes independent of the side-channel width when the side-channel width is larger than the effective side-channel width, which depends on the channel width and is largely independent of the source/drain width. This paper also demonstrates that the ON-state drain-current gain is directly proportional to the channel length and the ratio of the channel length to the source/drain width and dependent on the side-channel width.