Source/Drain Overlap Length Dependence of VT in Bottom Gated Thin Film Transistors on a-IGZO Channel Deposited by RF and DC Sputtering

We have investigated the threshold voltage (V-T) variation of the bottom gated a-IGZO TFTs according to the various source/drain overlap length (3um similar to -3um). As the source/drain overlap length decreases to negative value forming the offset, V-T of a-IGZO TFTs is increased and subthreshold slop and field-effect mobility is deteriorated due to the increased series resistance of the offset region. Besides, the V-T variation increases sharply, which is aggravated as the channel length decreases. In this paper, experimental V-T variation has been compared with devices of different channel deposition methods by RF and DC sputtering. When the channel layer is of superior quality, the V-T variation can be suppressed but still remains. This means that the source/drain overlap length is very important control parameter for the uniform device characteristics of a-IGZO TFTs.