Organic Single-crystal Transistors Research Articles

Large-domain Organic Crystalline Films for Field-effect Transistors

AbstractWe report a method to fabricate thin films of large-domain organic semiconductor single crystals dispersed over the whole surface of centimeter-scale substrates for field-effect transistors. Growing less than 500-nm thick film-like organic crystals of sub-millimeter sizes densely in a furnace independently of substrates by physical vapor transport, the collection of the single crystals is mechanically attached to the surface of gate dielectric layers. The organic transistors made of large-domain benzo-annulated pentathienoacene crystals exhibited pronounced transistor performances with mobility values of ∼ 0.2-2 cm2/Vs, which is as high as devices of one-piece crystals. The result demonstrates that the above technique provides a method to apply high performance of organic single crystal transistors to real circuitry devices on large-area substrates.

Organic Single Crystal Transistors Gated by Electric Double Layers in Ionic Liquid

AbstractGating organic transistors with electric double layers (EDL) of electrolytes is advantageous in injecting high-density carriers with the application of minimum gate voltage. The drawback of such devices, however, has been that commonly used polymer electrolytes suffer relatively slow ionic diffusion before forming the EDLs. In this report, we disclose a new class of EDL devices incorporating low-viscosity room temperature ionic liquid as the electrolyte layer, so that the rapid ionic diffusion allows MHz operation for the transistor performance. We fabricate a well structure using an elastomeric rubber stamp of poly-dimethylsiloxane to hold the ionic liquid 1-ethyl 3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide, known for high ionic conductivity. The transistor performs without hysteresis with the carrier mobility of 5 cm2V−1s−1, realizing the highest sheet transconductance ever achieved.

Gate dielectric materials for high-mobility organic transistors of molecular semiconductor crystals

High-mobility organic single-crystal transistors are constructed with silicon dioxide, a polymeric insulator (polyvinylphenol), and an organic insulating single-crystal (diphenylanthracene). All the single-crystal devices show high-mobility μ exceeding 10 cm 2/V s with relatively low-density carriers of ∼10 11 cm −2, though μ is significantly reduced at one-order higher carrier density. Among the three, the diphenylanthracene device holds relatively high-mobility even at carrier density of ∼10 12 cm −2, so that it realizes the maximum conductivity ever achieved for organic transistors. Employing simultaneous measurements of Hall-effect and conductivity of the field-effect carriers, the result is attributed to different extent of randomness in the surface potential on the gate insulators.

Scaling effect on the operation stability of short-channel organic single-crystal transistors

Organic single-crystal transistors allowed the authors to investigate the essential features of short-channel devices. Rubrene single-crystal transistors with channel lengths of 500 and 100nm exhibited good field-effect characteristics under extremely low operation voltages, although space charge limited current degrades the subthreshold properties of 100nm devices. Furthermore, bias-stress measurements revealed the remarkable stability of organic single-crystal transistors regardless of device size. The bias-stress effect was explained by the trapping of gate-induced charges into localized density of states in the single-crystal channel.

High-mobility double-gate organic single-crystal transistors with organic crystal gate insulators

High-mobility organic transistors are fabricated on both surfaces of approximately 1-μm-thick rubrene crystals, molecularly flat over an area of 10×10μm2. A thin platelet of 9,10-diphenylanthracene single crystal and surface-passivated SiO2 are used for the gate insulators. Because of the minimized densities of hole-trapping levels at the interfaces and in the rubrene crystal, the field-induced carriers do not necessarily reside near the interface but are distributed in the bulk of the semiconductor by adjusting the two gate voltages. Making use of the highly mobile carriers in the inner crystal, the mobility is maximized to ∼43cm2∕Vs.

Very high-mobility organic single-crystal transistors with in-crystal conduction channels

Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.

Organic single-crystal complementary inverter

The authors demonstrate the operation of an organic single-crystal complementary circuit in the form of a simple inverter. The device is constructed from a high mobility p-type organic single-crystal transistor of tetramethylpentacene (TMPC) and a n-type single-crystal transistor of N,N′-di[2,4-difluorophenyl]-3,4,9,10-perylenetetracarboxylic diimide (PTCDI). Field-effect mobilities of up to 1.0cm2∕Vs are reported for TMPC devices, while a mobility of 0.006cm2∕Vs is reported for a n-type PTCDI single-crystal device. Considering that organic single-crystal inverters have not yet been explored, they are representative of potential candidates for use in high-performance complementary circuits.

Organic Single Crystal Transistors: Interface Modification with SAMs and Double Gate Structure

Effects of Interface modification on the organic single crystal transistors have been investigated using self-assembled monolayers and double gate configurations. Systematic carrier density control associated with the shift of Vth was achieved by the polarized self-assembled monolayers fabricated in between the gate oxide and organic single crystals, and by the second gate electrode, which was fabricated on the other side of the primary gate electrode.

High-density electrostatic carrier doping in organic single-crystal transistors with polymer gel electrolyte

High-density carrier accumulation in organic semiconductors is demonstrated in Au∕polymer gel electrolyte/rubrene crystal∕SiO2∕doped Si dual-gate transistors, forming electric double layers in the polymer gel. Application of only 1.2V across the polymer gel electrolyte drastically enhances the conductance of the rubrene single crystal with the field-induced carrier density up to ∼5×1013cm−2. Directly comparing the transfer characteristics of the same device channel in the dual-gate transistors revealed that the achieved doping level is beyond the maximum of the SiO2-based transistor on the opposite side of the organic crystal.

Shedding light on organic transistors: insights could lead to brighter and cheaper displays

The first single-crystal organic transistor that can be switched on and off by light is giving physicists a unique peek into the way photons interact with organic semiconductors. For the makers of organic light-emitting diode (OLED) displays, which are used in everything from cameras to mobile phones, poor understanding of light's interactions with material properties becomes a problem. Today's amorphous or polycrystalline organic transistors are just not up to the job because bathing them in light generates unwanted photo-induced currents. In this paper, the author explains why the mobility drops as the temperature falls. In silicon transistors, the mobility increases as the material is cooled because electrons move more easily at low temperatures.

TOWARD BETTER, COLORFUL DEVICES

TWO NEW REPORTS IN THE hot field of organic electronics highlight advances in the fabrication of flexible transistors and colorful displays. A group led byjohn A. Rogers, a materials science professor at the University of Illinois, Urbana- Champaign, and Michael E. Gershenson, a physics professor at Rutgers University, has made single-crystal organic transistors using an unusual fabrication method that may allow them to gain a deeper understanding into the basic operation of these devices [ Science , 303, 1644(2004)}. Organic transistors usually are built by depositing components such as electrodes and dielectrics onto an organic material. The fabrication process, though, often damages the organic material's fragile surface. The team, which also includes scientists from Lucent Technologies' Bell Laboratories, circumvents this difficulty by placing the components on a silicone rubber support and then covering the support with the organic material—a highquality rubrene crystal This lamination process can ...