Organic Thin-Film Transistors with a Bottom Bilayer Gate Dielectric Having a Low Operating Voltage and High Operational Stability

This study reports on p-channel bottom-gate organic thin-film transistors (OTFTs) that achieve low-voltage operation with a thin bilayer gate dielectric of CYTOP/HfO2 that also leads to high operational stability. A bottom-gate geometry allows the thickness of the gate dielectric to be reduced to 17 nm, thereby increasing the gate dielectric capacitance density to a value of up to 258 nF/cm(2). All OTFTs yielded threshold voltages (V-TH) lower than -1 V, thereby providing a minimum operating voltage (gate to source voltage similar to V-TH) of -1 V with field-effect mobility values in the range 0.3-0.8 cm(2)/(V s) and subthreshold swing values below 100 mV/dec. Operational stability during DC bias stress was found to be similar to that measured in devices with a top-gate geometry, with about 1% of drain current change after 6 h. Using a double-stretched exponential function, the operational stability characteristics were modeled. It was found that, by increasing the metal oxide layer thickness, the threshold voltage can shift toward positive values under DC stress tests. For CYTOP layer thicknesses above 15 nm, a thicker CYTOP layer was found to shift the threshold voltage in the opposite direction toward negative values. These p-channel bottom-gate organic transistors with a bilayer gate dielectric of CYTOP/HfO2 combine high mobility, high operational stability, and lower voltage operation of -1 V compared to top-gate OTFTs.