Effect of Biaxial Bending Strains on the Electrical Characteristics of Flexible Low-Temperature Polysilicon Thin-Film Transistors

The change of electrical properties of flexible low-temperature polysilicon (LTPS) thin-film transistors (TFTs) under biaxial bending strains was studied. Biaxial strains were applied using a ring-on-ring bending test rig. The magnitude of the strain was determined by theoretical calculations and finite-element simulations. With an increase in strain, the transfer curve shifted to a negative bias. At a biaxial strain level of 12%, the threshold voltage decreased by 27%, the field-effect mobility decreased by 13%, and the subthreshold slope increased by 25%. It had a more significant effect on the property changes than the uniaxial strain; this difference may be attributed to additional trap states generated by the biaxial strain. The analysis showed that the trap density increased when the strain level increased. The contributions of the increases in the grain-boundary trap state density and the interface trap state density to the degradation in the electrical properties were derived to be 10% and 90%, respectively. After removal of the biaxial bending strains, the electrical performance showed recovery behavior, but the performance could not return fully to its original state.