Elsevier

Organic Electronics

Volume 99, December 2021, 106343
Organic Electronics

Impact of injection limitations on the contact resistance and the carrier mobility of organic field effect transistors

https://doi.org/10.1016/j.orgel.2021.106343Get rights and content

Highlights

  • Injection limited IV characteristics are frequent in current OFET technologies.

  • Criteria are given to identify injection and bulk limitations in IV curves of OFETs.

  • Contact resistance and mobility should be extracted from bulk limited IV curves.

  • Contact parameters are extracted from TLM line intersection and not interception.

  • Architecture-dependent injection optimization is derived from current crowding model.

Abstract

The contact resistance as well as the mobility have developed to key performance indicators for benchmarking organic field-effect transistors. Typically, conventional methods for silicon transistors are employed for their extraction thereby ignoring the peculiarities of organic transistors. This work outlines the required conditions for using conventional extraction techniques for the contact resistance and the mobility based on TCAD simulations and experimental data. Our experimental data contain both staggered and coplanar structures fabricated by exploiting different optimization techniques like SAM treated electrodes, different shearing speeds, PS blending and silicon oxide functionalization. In addition, the work clarifies how injection limited current–voltage characteristics can affect high-performance organic field-effect transistors. Finally, we introduce a semi-physical model for the contact resistance to accurately interpret extracted benchmark parameters by means of the transfer length method (TLM). Guidelines to use conventional extraction techniques with special emphasis on TLM are also provided.

Introduction

Since the contact resistance and the mobility are routinely used for benchmarking OFET technologies, the feasibility of typically employed extraction techniques for these parameters must be thoroughly evaluated for different material combinations and device architectures. Contact and channel engineering techniques [1], [2], [3], [4], [5], [6], [7], [8] are commonly employed to further boost the device performance by e.g. reducing the Schottky barrier height or improving the semiconductor morphology. Injection limited device characteristics are thereby either the trigger or the (unwanted) result of the various optimization methods [9], [10], [11]. However, the presence of injection limitations makes a benchmark of the materials and methods based on contact resistance and mobility values error-prone [9], [12], [13], [14]. Non-ideal interface properties spatially varying across the contacts further complicate the interpretation of measured current–voltage characteristics of organic semiconductors [15].

The results discussed here cover coplanar and staggered device architectures (see Fig. 1) employing small molecule semiconductors in combination with PS blending, SAM functionalization of the electrodes and the silicon oxide surface and optimized solution shearing. Despite the limited number of material combinations tested, the results of this analysis have been generalized into guidelines applicable to other experiments.

In order to determine the contact resistance, various techniques have been discussed in the literature. The recent review in [16] gives an overview including common strategies to reduce the contact resistance. A widely spread and commonly accepted method is the transfer length method (TLM) [17]. We limit our analysis here to this method. Our conclusions are – where required – verified with the also commonly employed Y-function based method (YFM) [18]. We comment on our preference of the TLM for OFETs and the relation between TLM and YFM in the Supplementary Materials. Since mobility extraction techniques are described in detail elsewhere [9], [12], [19], [20], [21], we limit our discussion to the impact of injection limitation on the extracted mobility values.

The paper is organized in five sections. Section 2 analyzes the signatures of injection limited behavior identified in the experimental data and in TCAD simulation results. Section 3 provide a guidance which conditions must be fulfilled to get reasonable results by using conventional mobility extraction methods. Section 4 presents a phenomenological interpretation and semi-physical modeling of the contact resistance. Section 5 concludes the present paper and gives an outlook.

Section snippets

Injection limited current–voltage characteristics

Injection limited current–voltage (IV) characteristics are mainly indicated by a deviation from the ideal, i.e., long-channel, MOSFET electrical characteristics. For these devices, the square root of the IV characteristics changes linearly with the gate voltage overdrive Vgt=VgsVth. The related power law exponent β=(logID)/(logVgt) is bias-independent and equals 2 in saturation. The bias region where the IV characteristics fulfill these criteria is called bulk limited region (BLR). A value β=

The mobility extraction

Typically, organic semiconductor materials are benchmarked against mobility values extracted from experimental current voltage characteristics. In this work, we limit the discussion to mobility data extracted in OFET saturation by μapp=2LWCoxIDVgt2.Here L is the distance between the source and the drain contact, W the gate width and Cox the geometrical gate oxide capacitance [17]. Since Eq. (1) is applied to the raw data without de-embedding the impact of the contact resistance, we call it

Phenomenological interpretation of TLM parameters

One of the most common methods to extract a contact resistance is the transfer length method (TLM) [17], which requires a set of transistors with different channel lengths L. Plotting the measured total resistance Rtot=ΔVds/ΔID at low drain–source voltages Vds over L (also known as TLM plot) typically reveals a linear channel length dependence of the total resistance. The slope of the Rtot(L) lines gives a gate bias-dependent parameter Rsh,ch. The intersection of the Rtot(L) lines obtained for

Conclusion

In our experiments, we have seen that the current–voltage characteristics of OFETs can deviate significantly from the bulk limited behavior. The origin of the apparent deviations could be traced to the limited injection of charge carriers into the channel. Interestingly, injection limited current–voltage characteristics can also be provoked by increasing the current carrying capability of the channel since any injection limitation becomes more constraining for decreasing channel resistance.

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 work was founded by the German Research Foundation (Deutsche Forschungsgemeinschaft-DFG CL384/5 and MA 3342/2-1 SPP Fflexcom) as well as SCHR 695/6, FAPDF 0193.001363/2016 which are gratefully acknowledged.

References (41)

  • MinagawaM. et al.

    Characteristics of 9, 10-diphenylanthracene field-effect transistors obtained by exposing the silver electrodes to oxidative conditions

    Japan. J. Appl. Phys.

    (2019)
  • HouJ.-L. et al.

    Reduced contact resistance in top-contact organic field-effect transistors by interface contact doping

    Appl. Phys. Lett.

    (2016)
  • C. LiuY.-Y.N.

    Contact engineering in organic field-effect transistors

    Mater. Today

    (2015)
  • NagaseT. et al.

    Influence of substrate modification with dipole monolayers on the electrical characteristics of short-channel polymer field-effect transistors

    Appl. Sci.

    (2018)
  • BittleE.G. et al.

    Mobility overestimation due to gated contacts in organic field-effect transistors

    Nature Commun.

    (2016)
  • FratiniS. et al.

    Current saturation and Coulomb interactions in organic single-crystal transistors

    New J. Phys.

    (2008)
  • SirringhausH.

    25th anniversary article: Organic field-effect transistors: The path beyond amorphous silicon

    Adv. Mater.

    (2014)
  • XuY. et al.

    Essential effects on the mobility extraction reliability for organic transistors

    Adv. Funct. Mater.

    (2018)
  • XuY. et al.

    Modified transmission-line method for contact resistance extraction in organic field-effect transistors

    Appl. Phys. Lett.

    (2010)
  • PereiraV.S. et al.

    Drift-diffusion simulation of leakage currents in unintentionally doped organic semiconductors with non-uniform interfaces

    J. Comput. Electron.

    (2018)
  • Cited by (0)

    View full text