Impact of injection limitations on the contact resistance and the carrier mobility of organic field effect transistors
Graphical abstract
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 -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 . The related power law exponent is bias-independent and equals in saturation. The bias region where the IV characteristics fulfill these criteria is called bulk limited region (BLR). A value
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 Here is the distance between the source and the drain contact, the gate width and 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 . Plotting the measured total resistance at low drain–source voltages over (also known as TLM plot) typically reveals a linear channel length dependence of the total resistance. The slope of the lines gives a gate bias-dependent parameter . The intersection of the 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.
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