Elsevier

Microelectronic Engineering

Volume 160, 1 July 2016, Pages 39-48
Microelectronic Engineering

Characterization and modeling of organic thin-film transistors based π-conjugated small molecule tetraphenyldibenzoperiflanthene: Effects of channel length

https://doi.org/10.1016/j.mee.2016.03.002Get rights and content

Highlights

  • Bottom-gate, bottom-contact DBP-TFTs with various channel lengths were studied.

  • Electrical parameters of DBP-TFTs were degraded by the channel length variation.

  • Rc is higher than Rch only for the channel lengths lower than 5 μm.

  • The obtained model results are in good agreement with the experimental data.

Abstract

P-type organic thin film transistors (OTFTs) with different channel lengths have been fabricated and characterized by thermal evaporation using the small tetraphenyldibenzoperiflanthene (DBP) as an active material on Si/SiO2 substrate. The influence of the channel length on the electrical performance of DBP based organic thin film transistors (DBP-TFTs) prepared with bottom gate-bottom contact in the linear and saturation regimes was systematically examined in this work. All devices showed a significant increase in the output and transfer drain current as the channel lengths were decreased in the linear and saturation regimes. We have reported the variation of the electrical parameters such as transconductance (gm), field effect mobility (μlin and μsat), contacts and total resistances (RC and RT), threshold voltage (Vth), total trap density (Ntrap), subthreshold slope (SS), the interface trap density (Dit), turn-on voltage (Von) and the ratio current (Ion/Ioff) by channel length variation which are extracted from the experimental electrical data current–voltage of DBP-TFTs. We found that the field effect mobility is extremely dependent on the channel length dimensions. We also show that for smaller channel length, it results in a good mobility and a good ratio current of the DBP-TFTs with a short channel length (good saturation mobility and current ratio μsat.max = 3 × 10 2 cm2 V 1 s 1, 1.6 × 104, respectively, for L = 2.5 μm). The developed model shows a good agreement with the measured data for all values of channel lengths (L).

Introduction

During these last years, organic semiconductors have attracted comprehensive interest among researchers all over the world, in particular the use of organic compounds in electronic applications has known an awesome evolution mostly in the organic thin film transistors (OTFTs), which have drawn a great attention by the research area because they are one of the most important devices in electronics and they lie at the heart of modern computing due to their many advantages over conventional inorganic electronics such as light-weight, low cost processing, large area capability, structural flexibility and low temperature process [1], [2], [3]. The fabrication technology of OTFTs has improved considerably in recent years. Presently, the electrical performance of the OTFTs made from the small-molecule organic semiconductors (pentacene, N, N′-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI-C13H27), fullerene (C60), 1, 4, 5, 8-naphthalene tetracarboxylic dianhydride (NTCDA)) is similar to hydrogenated amorphous silicon (a-Si: H) TFTs. Accordingly, the organic small molecules based OTFTs are more widely popular and they have already demonstrated their potentials toward organic electronic applications such as gas sensors, smart cards, active-matrix displays, radio-frequency identification tags (RFID), HD-TVs, image sensors, iPods and flexible microelectronics [4], [5], [6], [7], [8], [9]. The effects of extrinsic factors such as illumination, temperature and humidity [10], [11], [12], [13], [14], [15] and the nature of the gate dielectric surface properties [16], [17], [18] that directly affect the performance and drift of OTFTs have been widely studied. The understanding of material transport properties and the characterization of the injecting properties of the metal-semiconductor interface are a crucial interest in the fabrication of efficient devices including especially the effects that result from the device size miniaturization. Therefore, the miniaturization of device dimensions such as the channel length effects on electrical performances of organic thin film transistors has been previously reported [19], [20], [21], [22], [23]. It was found that decreasing channel length resulted in significant degradation of transistor electrical performance. Another most important factor which drops mobility under high gate voltages and which degrades the performances of the thin film transistors is the interface quality between the organic semiconductors and the metal which constitutes the source–drain electrodes. Generally speaking, the main origin of the contact resistance in the p-type OTFTs is the mismatch between the work function of the source–drain electrodes and the energy levels of the organic semiconductors [24], [25], [26]. The resistance effects are directly related to the increase of the charge carrier density in the channel of the transistor. Several methods have been developed for the extraction of the contact resistance in OTFTs [27], [28].

The main aim of this present work is to study the channel length variation effects on the electrical stability of the tetraphenyldibenzoperiflanthene based OTFTs with SiO2 as a gate insulator in the linear and the saturation regimes. Accordingly, we have extracted the various electrical parameters of DBP-TFTs with different channel lengths from experimental data. Finally, we have developed an analytical model in order to reproduce the experimental characteristic current–voltage (output and transfer) of the studied DBP-TFTs for several channel lengths ranging from 2.5 μm to 20 μm.

Section snippets

Experimental details

The chemical molecular structure of the tetraphenyldibenzoperiflanthene (DBP) organic semiconductor with a molecular formula of C64H36 and a molecular weight of 804.97 g/mol and with purity 98% is shown in Fig. 1(a). The material was commercially available from Sigma Aldrich chemical and it was used without any further purification process. The schematic diagram of bottom gate bottom contact (BGBC) type of the fabricated DBP-TFTs is shown in Fig. 1(b). An n-doped crystalline silicon wafer was

Output characteristics of the DBP-TFTs with different channel lengths

Fig. 3(a)–(d) shows the output characteristics (ID vs. VD) curves of the DBP-TFTs with channel lengths ranging from 2.5 μm to 20 μm (L = 2.5 μm, 5 μm, 10 μm and 20 μm) at a fixed channel width (W) of 2000 μm. Devices were measured under vacuum by varying the drain voltage (VD) from 0 to − 80 V with − 0.8 V increments for different gate voltages (VG) from 0 to − 60 V with − 10 V increments. As seen in Fig. 3(a)–(d), the drain current (ID) increases linearly at low negative drain voltages (VD) and thereafter ID

Modeling of current–voltage characteristics of DBP-TFTs

Significant progress has been made towards improved understanding of the electrical properties in various types of thin film transistors (TFTs) including a-Si, poly-Si and organic TFTs. Several analytical models have been proposed to describe electrical behaviors and to reproduce the experimental electrical characteristic current–voltage of various organic field effect transistors [30], [47], [48], [49], [50]. Moreover, many methods are used for extracting electrical parameters of these kinds

Conclusions

The p-small molecule DBP based organic thin film transistors with bottom-gate bottom-contact structure were successfully fabricated and were characterized. The exploitation of experimental curves obtained on the DBP based thin film transistors (TFTs) for each channel length have confirmed the effects of channel length variation on the electrical performance of OTFTs in the linear and saturation regimes and enabled us to determine the electrical parameters of the fabricated devices. It is found

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