We have performed a comparative study of rubrene single-crystal field-effect transistors fabricated using different materials as gate insulator. For all materials, highly reproducible device characteristics are obtained. The achieved reproducibility permits one to observe that the mobility of the charge carriers systematically decreases with increasing the dielectric constant of the gate insulator, the decrease being proportional to −1 . This finding demonstrates that the mobility of carriers in organic single-crystal field-effect transistors is an intrinsic property of the crystal/ dielectric interface and that it does not only depend on the specific molecule used. The quality of organic single-crystal field-effect transistors (FETs) opens new opportunities for investigations of both fundamental and applied character. In particular, the use of single-crystalline devices permits one to study the intrinsic-not limited by disorder-transport properties of organic semiconductors as a function of carrier density, as recently demonstrated by the observation of an anisotropic mobility in rubrene FETs exhibiting a "metallic-like" temperature dependence. 7,8 In addition, the reproducibility of single-crystal FETs permits one to investigate in detail how different aspects of the devices influence transistor operation, which is necessary to individuate the ultimate performance limits of organic transistors.In this letter, we report a comparative experimental study of the electrical characteristics of rubrene single-crystal FETs fabricated using Ta 2 O 5 , Al 2 O 3 , SiO 2 , and Parylene C as gate insulator. For the different dielectrics, field-effect transistors exhibiting stable and hysteresis-free electrical behavior can be reproducibly realized. In all cases, the hole mobility extracted from room-temperature measurement of the transistor characteristics is remarkably gate-voltage independent. From these measurements, we find that the mobility decreases from 10 cm 2 /V s (Parylene C, = 3.15) to 1.5 cm 2 /V s ͑Ta 2 O 5 , =25͒ with increasing the relative dielectric constant. By comparing our data to those recently reported for transistors fabricated using Parylene N ͑ =15 cm 2 /V s; = 2.65͒ 2,9 and polydimethylsiloxane (PDMS) air-gap stamps ͑ =20 cm 2 /V s; =1͒, 7 we conclude that a decrease in mobility with increasing the dielectric constant of the gate dielectric occurs systematically in rubrene singlecrystal FETs. This result demonstrates that the mobility measured in organic transistors is not only a property of the specific organic molecule used, but that it intrinsically depends on the organic/dielectric interface.The devices used in our investigations have been fabricated by means of two different, recently developed techniques. Transistors based on Parylene C have been built following the processing described in Ref. 1, using aqueous colloidal graphite or silver epoxy for the source, drain, and gate electrodes (with colloidal graphite resulting in better performances as compared to epoxy). For these devices, rathe...

Low-noise heterodyne mixing at 1 THz is demonstrated in a quasioptical mixer incorporating Nb superconductor–insulator–superconductor tunnel junctions and a NbTiN/SiO2/Al tuning circuit. Receiver noise temperatures as low as 250 K at 850 GHz, 315 K at 980 GHz, and 405 K at 1015 GHz are measured—a factor of 2 improvement in sensitivity versus state-of-the-art 1 THz receivers, which incorporate normal metal tuning circuits. An analysis of the receiver sensitivity at 980 GHz demonstrates that NbTiN is low loss up to ∼1 THz.

We investigate the effect of a small leakage current through the gate insulator on the stability of organic single-crystal field-effect transistors ͑FETs͒. We find that, irrespective of the specific organic molecule and dielectric used, leakage current flowing through the gate insulator results in an irreversible degradation of the single-crystal FET performance. This degradation occurs even when the leakage current is several orders of magnitude smaller than the source-drain current. The experimental data indicate that a stable operation requires the leakage current to be smaller than 10 −9 A/cm 2 . Our results also suggest that gate leakage currents may determine the lifetime of thin-film transistors used in applications. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1852089͔The study of organic semiconductor transistors aims at the development of organic electronics, for its advantages of being flexible, cheap and suitable for large-area production.1,2 So far, considerable research effort has been focused on the optimization of the organic layer to improve the performance of thin-film transistors.3-5 Much less attention has been devoted to other important device aspects, such as, for instance, the choice of the gate insulator.Recent work has demonstrated that the gate insulator plays an important role in determining the device performance. 6,7 In particular, it has been shown that in polymer as well as in single-crystal organic transistors the mobility of charge carriers is systematically larger the lower the dielectric constant of the gate insulator. This implies that the use of low-⑀ dielectrics will result in a higher device switching speed. In view of this result, it appears useful to investigate systematically how different properties of the gate insulator affect the behavior of organic transistors.In this letter we use organic single-crystal field-effect transistors ͑FETs͒ 8 to investigate how a small leakage current through the gate insulator affects the stability of the device operation. Specifically, we have investigated the behavior of organic single-crystal FETs of different molecules ͑tetracene, rubrene, perylene͒ in combination with different dielectrics ͑Ta 2 O 5 , ZrO 2 , and SiO 2 ͒. We find that, irrespective of the specific molecule and dielectric used, leakage current flowing through the gate insulator results in an irreversible degradation of the single-crystal FET operation. The degradation is not due to the electrical breakdown of the insulating layer and it also occurs when the leakage current is several orders of magnitude smaller than the source-drain current. From the experimental data, we conclude that a stable operation of organic single-crystal FETs requires the current leaking to the FET channel to be smaller than 10 −9 A/cm 2 . The fabrication of the single-crystal FETs used in this work is based on electrostatic bonding of an organic single crystal to a dielectric surface, with prefabricated source, drain and gate contacts. The electrical properties of all the different...

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We have investigated the dielectric properties of thin layers of five oxides of transition metals (Ta 2 O 5 , HfO 2 , ZrO 2 , (ZrO 2 ) 0.91 (Y 2 O 3 ) 0.09 , and Sn 0.2 Zr 0.2 Ti 0.6 O 2 ) sputtered from ceramic targets at different pressures. We find that layers deposited at low pressure behave as expected from literature, whereas layers deposited at high pressure all exhibit an anomalous dielectric response similar to that reported for the so-called ''colossal'' dielectric constant materials. The characterization of the thickness, frequency, and temperature dependence of the capacitance, as well as the comparison of film properties before and after annealing show that the anomalous dielectric response is due to quenched-in vacancies that act as dopants and cause the insulating layers to behave as semiconductors. An increase in quenched-in vacancies concentration with sputtering pressure results in a transition from normal to anomalous dielectric response and gradual increase in layer conductivity. In contrast, the refractive index does not depend on sputtering pressure. This observation indicates the possible application of these materials as transparent coatings with a tunable electrical conductivity.