Suppressing off-state current via molecular orientation in submicrometer polymer field-effect transistors

Field-effect transistors based on conjugated polymers have reached mobilities comparable to those of polysilicon rivals and recently demonstrated self-healing capability, indicating their great potential for use in future plastic electronics. For practical applications, the operating speed of polymer transistors must be increased, which requires not only high mobility but also short channel length. Decreasing channel length is an important engineering issue for high-frequency operation and high-density integration. However, shortening channel length has proved challenging because of the poor compatibility of conjugated polymer transistors with conventional lithography; i.e., fragile polymer semiconductor films may be damaged during lithographic processes. The underlying device physics, particularly of those with short channels, remain poorly understood. Here, we fabricated polymer transistors with channel lengths below 1000 nm. A dry self-patterning process lowered the gate leakage current to less than 10–12 A, enabling quantitative analysis of devices with very short channels. We found that a high Schottky barrier with a sizable space-charge region (>1 μm) was responsible for the high off-state current because the space-charge zones at the source and drain joined when the channel was short, lowering the energy barrier to carrier diffusion in the off state. Such diffusion was suppressed simply by controlling the molecular alignment so that the deep traps distributed at the domain boundaries limited the carrier diffusion and thus suppressed the off-state current.