Influence of Semiconductor Island Geometry on the AC Performance of Flexible InGaZnO TFTs

The AC performance of flexible TFTs sufferers from parasitics caused by tolerances needed for the fabrication on free-standing plastic foils. In this context, the semiconductor island can either be wider or narrower than the source/drain contacts. Traditionally, the second configuration is expected to result in faster TFTs as the total gate overlap area is smaller. However, here it is shown that $\mathrm {2.5~ \mu \text {m} }$ long flexible InGaZnO TFTs with wide semiconductor islands exhibit better frequency performance such as a ${f}_{T}$ of $\mathrm {26.1~ \text {M} \text {Hz} }$ (compared to ${f}_{T}$ of $\mathrm {13.8~ \text {M} \text {Hz} }$ of TFTs with narrow semiconductor islands). This effect is confirmed for flat and bend TFTs and is caused by current spreading in the semiconductor islands, as well as the frequency dependency of the gate capacitance.