Hayoung Jeon

Low-voltage pentacene field-effect transistors with ultrathin polymer gate dielectrics

Organic field-effect transistors (OFETs) for low-voltage operation have been realized with conventional polymer gate dielectrics such as polyimides and cross-linked poly-4-vinyl phenols (PVPs) by fabricating ultrathin films. These ultrathin polymers (thickness ∼10nm) have shown good insulating properties, including high breakdown fields (>2.5MV∕cm). With ultrathin dielectrics, high capacitances (>250nF∕cm2) have been achieved, allowing operation of OFETs within −3V. Pentacene OFETs with ultrathin PVP dielectrics exhibit a mobility of 0.5cm2∕Vs, an on-off ratio of 105, and a small subthreshold swing of 174mV∕decade when devices are operated at −3V.

Low-operating-voltage pentacene field-effect transistor with a high-dielectric-constant polymeric gate dielectric

Low-operating-voltage organic field-effect transistor has been realized by using the cross-linked cyanoethylated poly(vinyl alcohol) (CR-V) as a gate dielectric. The cross-linked CR-V dielectric was found to have a high dielectric constant of 12.6 and good insulating properties, resulting in a high capacitance (92.9nF∕cm2 at 20Hz) for a dielectric thickness of 120nm. A pentacene field-effect transistor fabricated with the cross-linked CR-V dielectric was found to exhibit a high carrier mobility (0.62cm2∕Vs), a small subthreshold swing (185mV∕decade), and little hysteresis at low operating voltages (⩽−3V).

Hysteresis-free pentacene field-effect transistors and inverters containing poly(4-vinyl phenol-co-methyl methacrylate) gate dielectrics

The influence of hydroxyl groups on the hysteresis of pentacene field-effect transistors (FETs) and metal-insulator-semiconductor diodes containing poly(4-vinyl phenol) and poly(4-vinyl phenol-co-methyl methacrylate) (PVP-PMMA) gate dielectrics was investigated. The electrical characteristics and Fourier transform infrared spectroscopy measurements show that hysteresis is intimately related to the presence of free OH groups in the polymer gate dielectrics. The methyl methacryl moieties in PVP-PMMA minimize residual water in the polymer and form hydrogen bonds with the hydroxyl groups, thus reducing the number of free OH species. Therefore, pentacene FETs and inverters using PVP-PMMA gate dielectrics exhibit high, hysteresis-free performances.

Effects of polymer gate dielectrics roughness on pentacene field-effect transistors

The effects of the surface roughness of the polymer gate dielectrics on pentacene field-effect transistors were investigated. Using a poly(methylmethacrylate)/anodized Al2O3 dual-layer gate dielectric, the root-mean-square roughness of the gate dielectrics varied from 0.45 to 1.51nm, independently of other gate dielectric properties such as the capacitance and surface energy. This range of root-mean-square roughnesses had little effect on the carrier mobility. X-ray diffraction analyses further revealed that the roughness of poly(methylmethacrylate) neither decreased the degree of crystallinity nor distorted the crystalline structure of pentacene.

Thin-film passivation by atomic layer deposition for organic field-effect transistors

The thin-film passivation of organic field-effect transistors (OFETs) using AlOx films grown by atomic layer deposition was investigated. A high-quality AlOx passivation layer was deposited on OFETs at 90°C using trimethylaluminum and water. Despite the low deposition temperature, the 50-nm-thick AlOx passivation layers exhibited a low water-vapor-transmission-rate value of 0.0434g∕m2∕day. In addition, the mobility of the AlOx-passivated OFETs was only slightly below that of the unpassivated devices (i.e., within 9%). Unlike unpassivated devices, the electric performance of the passivated OFETs remained almost unchanged after 2months as a result of the excellent barrier properties of the passivation layer.

Low-voltage organic transistors on a polymer substrate with an aluminum foil gate fabricated by a laminating and electropolishing process

The authors report flexible and low-voltage pentacene organic field-effect transistors (OFETs) constructed with an aluminum foil gate electrode, which was fabricated by the simple and low-cost roll-to-roll lamination process. Electropolishing the surface of laminated aluminum foil and spin coating it with an additional thin polymer film resulted in a gate dielectric surface with a root-mean-square roughness of about 0.85nm. These pentacene OFETs with a poly(α-methylstyrene)/anodized Al2O3 dual-layered gate dielectric exhibit a mobility of 0.52cm2∕Vs, an on-off ratio of 105, a subthreshold swing of 317mV/decade, and little hysteresis when operating at −5V.

Oxygen plasma treatment and postaging of pentacene field-effect transistors for improved mobility

The authors fabricated pentacene transistors with high mobilities by controlling the morphology of pentacene film through adjustments to the surface energy of the gate dielectrics with oxygen plasma treatment, and then by improving the interfacial properties through postaging. The increased surface energy of poly(methylmethacrylate) dielectric that results from the oxygen plasma treatment improves the interconnections between grains and enlarges the grain size. The postaging of transistors is presumed to rearrange the interface functional groups and as a result decrease the polar functionality without changing pentacene film morphology, which reduces the number of trap states and increases the mobility to 0.73cm2∕Vs.