Effect of Active Layer Thickness Variation on Overlap Length Scaling in a-IGZO Thin Film Transistors

Bandwidth improvement in amorphous oxide thin film transistors, demands lower overlap length between the contact and gate. But lowering overlap length can lead to lower efficiency of current transfer between the metal and the semiconductor due to reduced area for current injection. The influence of semiconductor thickness on this injection area is studied by fabricating three batches of TFTs; batch 1 with thickness of 5 nm, batch 2 with thickness of 10 nm and batch 3 with thickness of 30 nm. As the value of overlap length is scaled down the devices failed to operate with steadily increasing transconductance beyond a limiting value of overlap length. Batch 1 displayed a limiting overlap length of 5 µm and batch 2 provided a limiting overlap length of 10 µm. Batch 3 devices failed to display field effect operation even at an overlap length as high as 10 µm. It is found that lower thickness can lead to better immunity towards overlap length changes. The hump displayed by transconductance in thicker devices points to Schottky contact formation. Hence thickness of the semiconductor limits the extent to which overlap length can be scaled in thin film transistors.