Influence of the gate leakage current on the stability of organic single-crystal field-effect transistors

Abstract

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...

Colloquium: Electronic transport in single-crystal organic transistors

“…͑Color online͒ Transfer characteristics I SD ͑V G ͒ of a rubrene single-crystal OFET with Ta 2 O 5 gate dielectric. At a high density of charge carriers, 5 ϫ 10 13 cm −2 , a pronounced deviation from the expected linear regime is clearly visible for V G Ͻ −60 V. Similar behavior is observed in most devices upon slowly increasing the gate voltage ͑de Boer, Iosad, et al, 2005͒. are among the many phenomena that can occur at the semiconductor/insulator interface and affect the electrical characteristics of single-crystal OFETs. For example, localized electronic states within the HOMO-LUMO gap impair the performance of field-effect transistors by increasing the field-effect threshold voltage and reducing the effective mobility of charge carriers ͓see, e.g., Schmechel and von Seggern ͑2004͔͒.…”

“…Defects in molecular materials acting as traps can also be induced during the device operation. Figure 21 shows I SD ͑V G ͒ in a tetracene single-crystal transistor with a Ta 2 O 5 gate dielectric ͑de Boer, Iosad, et al, 2005͒. The source drain current exhibits a nonmonotonic dependence on V G and is completely suppressed at high values of the gate voltage.…”

“…The distribution of localized states in the bulk of organic crystals can be probed by photoconductivity ͑Silinsh and Č ápek, 1994; Pope and Swenberg, 1999;Lang et al, 2004͒ and space charge limited current ͑SCLC͒ measurements ͑Kao and Hwang, 1981; de Boer, Jochemsen, et al, 2004;Jurchescu et al, 2004͒. Figure 19 shows photocurrent spectra obtained at room temperature for pentacene crystals in recent experiments by Lang et al ͑2004͒.…”

“…Second, for robust crystals such as rubrene, PDMS stamps provide a unique opportunity to reestablish the contact many times without noticeable degradation of the crystal surface. However, the density of field-induced charges achievable is typically greater in Si-based stamps, especially if these stamps utilize the high-gate insulators ͑Stassen et de Boer, Iosad, et al, 2005͒; this is important for exploring the regime of high carrier densities in which novel electronic phases might emerge.…”

“…18͒ ͑de Boer, Iosad, et al, 2005͒. With a breakdown field of E BD = 6.6 MV/ cm and a dielectric constant of = 25, this material should allow for reaching an even higher density of charge carriers, ϳ10 14 cm −2 .…”