Elsevier

Synthetic Metals

Volume 273, March 2021, 116670
Synthetic Metals

Research paper
Contact resistance corrected-electrical characteristics with channel length effects in π-conjugated small-molecule benzanthracene organic thin film transistors

https://doi.org/10.1016/j.synthmet.2020.116670Get rights and content

Highlights

  • Multiple small molecule benzanthracene based OTFTs were fabricated, characterized, and modeled.

  • Electrical properties of benzanthracene OTFTs were affected by channel length (L) and contact resistance (Rc).

  • The effect of Rc is more dominant in short-channel device.

  • Contact resistance corrected-electrical characteristics in linear regime.

  • Corrected output drain current values with including contact resistance is in good agreement with the measured data.

Abstract

In this work, bottom-contact p-type organic thin-film transistors based on small-molecule benzanthracene with various channel lengths were fabricated, characterized, and theoretically investigated. The fabricated devices showed a pronounced shift for threshold voltage as the function of channel length. Electrical parameters characterizing fabricated devices have been systematically evaluated as a function of channel length with and without contact resistance in the calculation. Contact resistance (Rc) and channel resistance (Rch) are extracted individually using the transfer line method (TLM) for output characteristic curves that were done only at low drain voltage (linear regime). The effect of contact resistance on the values of various OTFTs parameters is more pronounced in short channel length devices. The obtained results revealed that the linear regime's corrected mobilities of short channel devices are about 15% smaller than uncorrected mobility. Comparatively, the corrected threshold voltage values were found to be nearly close to uncorrected values for all channel lengths. The analytical model was also applied to reproduce output characteristics in the linear regime of different channel lengths devices with or without contact resistance. It was found that the calculated results were in good agreement with the measured results (when considering the effect of contact resistance), which confirms that the used model can correctly describe the charge transport in these kinds of devices.

Introduction

Organic semiconductors (OSCs) such as π-conjugated small molecules and polymers remain a topic of considerable interest for basic and applied research due to its noble advantages such as easily modified molecular structures, light-weight, mechanical flexibility, low-cost manufacturing, low-temperature processing, high sensitivity, biocompatibility, compatibility with non-planar geometries and large areas applications [1], [2], [3], [4], [5], [6], [7], [8]. This has led to the realization of completely plastic flexible devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and made them very promising candidates in several areas of modern industrial technologies. Such areas include chemical and biological sensors, smart cards, active-matrix displays, various organic microelectronic devices, such as organic solar cell, sensors, organic memories, organic light-emitting diodes (OLEDs), and organic thin-film transistors [11], [12], [13], [14], [15], [16], [17], [18]. Among various organic devices, organic thin-film transistors (OTFTs) are one of the key elements of organic microelectronic devices and functional large-area electronic systems [2], [7], [15], [16], [17], [18], [19], [20]. From a practical perspective, OTFTs technologies must know a remarkable boost in development and have emerged as one of the revolutionary and indispensable flexible electronic technology. The OTFTs are important as vital elements in numerous applications such as displays of mobile devices and smart televisions, sensors, organic displays, smart cards, E-paper applications and other emerging and innovative applications where these OTFTs operate as the main or control element [21], [22], [23], [24]. On the other hand, OTFTs offer multiple sensing key matrices such as subthreshold swing (SS), turn-on voltage (Von), field-effect mobility (µFET), and threshold voltage (Vth) [23]. Likewise, due of their ability to control charge carrier concentration electro-statically rather than chemically, OTFTs provide a useful scientific tool and reliable means to study charge transport and injection behavior in organic semiconductors as well as their structural property [25]. In OTFTs, besides the influences of the applied materials, the quality of resulting interfaces between various layers of the device and many other external and internal factors e.g., the channel dimension also significantly influences their electrical parameters. Basically, the device dimension miniaturization is of fundamental interest and importance to improving the device performance. To the best of our knowledge, the real OTFTs can present deformation in the linear region of the output characteristics, which are manifested by a non-linear behavior at low drain voltage, often attributed to the contact resistance effect. Channel dimensions are also critical in determining the contact resistance effect [25], [26]. In this regard, divers’ applications like high-resolution, flexible displays and integrated circuits necessitate higher integration densities and switching speed transistors [25], [26]. A mutual way to improve these properties is to reduce the device channel dimensions, most importantly, the channel length. However, as the channel length is reduced, the effect of contact resistance becomes an important parameter, limiting OTFTs electrical properties [25]. This problem is more existing in OTFTs as compared to its inorganic-TFTs counterpart. This account is worthy of investigating the effects of the channel length variation in correlation with contact resistance effects on organic thin-film transistors (OTFTs).

In the present work, bottom-contact p-type small-molecule benzanthracene based organic thin-film transistors with various channel lengths were fabricated, characterized, and modeled. We have investigated the effect of the channel length variation in relation with the contact resistance effect on electrical properties and parameters of the devices. Many electrical parameters in terms of output and transfer drain current, transconductance, field-effect mobility, and threshold voltage, have been systematically evaluated as a function of channel length and contact resistance. Contact resistance (Rc) and channel resistance (Rch) are extracted individually by using the transfer line method (TLM). Afterwards, we have suggested a method for extracting linear mobility and threshold voltage values, including contact resistance for the linear regime of OTFTs. Furthermore, the output characteristics were also calculated along with the contact resistance and gate voltage for the devices with various channel lengths by the analytical model. The calculated results agree with measured results by considering the effects of contact resistance and the mobility dependence with the gate voltage concurrently.

Section snippets

Experimental details

Fig. 1(a) presents a schematic view of the chemical structure of benzanthracene. Benzanthracene was purchased from Aldrich chemical (98% purity) and used without any further purification. In this experiment, all fabricated benzanthracene-based organic thin-film transistors were constructed on a substrate of n-doped silicon wafer 150 mm, which has also served as the bottom-gate electrode (BG) (doping at wafer surface n~3 ×1017 cm−3). Silicon wafer substrates were ultrasonically cleaned with

Results and discussion

The measured output characteristic curves of the fabricated benzanthracene-OTFTs with different channel lengths ranging from L = 2.5–20 µm were investigated. The output characteristics with various gate voltages (VG) ranging between 0 V to − 50 V are plotted (Fig. 2(a)–(d)). As can be seen, the drain current (ID) in benzanthracene-OTFTs increases quasi-linearly at low negative drain voltages (linear regime). Afterwards, at high negative drain voltages, the electric field becomes zero at the

Conclusions

In summary, a set of bottom-contact p-type small-molecule benzanthracene based organic thin-film transistors with different channel lengths and a fixed channel width were fabricated and characterized. Many device characteristics key metrics in terms of drain current, transconductance, field-effect mobility, resistance and threshold voltage have been systematically evaluated as a function of channel length. The obtained results demonstrate that OTFTs employing short channel length (L = 2.5 µm

CRediT authorship contribution statement

Ahmed Al-Ghamdi: Conceptualization, Funding acquisition, Project administration, Supervision, Resources. W. Boukhili: Conceptualization, Data curation, Investigation, Methodology, Modeling, Writing. S. Wageh: Conceptualization, Data curation, Investigation, Project administration, Methodology, Supervision, Writing. All authors have read and agreed to the published version of the manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, grant no. (G: 529-130-1441). The authors thank DSR for technical and financial support.

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