This is the accepted version of the article: Nafria, Montserrat; Crespo Yepes, Albert; Rodriguez Martinez, Rosana; [et al.]. «DC characterization and fast small-signal parameter extraction of organic thin film transistors with different geometries». IEEE Electron Device Letters, Vol. 41, Issue 10 (October 2020), p. 1512-1515. DOI 10.1109/LED.2020.3021236 This version is available at https://ddd.uab.cat/record/248843 under the terms of the license DC characterization and small signal modelling of organic thin film transistors with different geometries A. Arnal1-3, A. Crespo-Yepes2, E. Ramon1*, L. Terés1, R. Rodríguez2 and M. Nafría2. 1. Institut de Microelectrònica de Barcelona, IMB-CNM (CSIC), 08193, Bellaterra, Catalonia, Spain. 2. Department of Electronic Engineering, Autonomous University of Barcelona (UAB), Edifici Q, 08193, Bellaterra, Spain. 3. PhD in Electrical and Telecommunication Engineering, Universitat Autònoma de Barcelona (UAB). * Corresponding author E. Ramon (Eloi.Ramon@imb-cnm.csic.es). Abstract— Organic Devices offer low-cost manufacturing and this work takes advantage of this dependence in a Common- better flexibility, sustainability and solution-processability than Source (CS) amplifier to experimentally study the their Si-based MOS counterparts, which make them suitable for performance of Organic Thin Film Transistors (OTFT) with new applications where those characteristics are an advantage. However, organic device performance is still far from that different architectures and areas fabricated with an already provided by CMOS technology and many issues are still unclear. patented and commercial technology [11]. Device level In this work, a performance comparison of Interdigitated and measurements are also carried out, for comparison. Special Corbino geometries of Organic Thin Film Transistors (OTFT), emphasis is done on their frequency response related with different areas but fabricated with identical stack materials parameters, i.e., the role of the dielectric stack capacitance was and techniques, is done. With this purpose, I-V characteristics analyzed in detail, for its inclusion in a small signal model. and C-V curves of the OTFTs were measured and a Common- Source circuit was proposed and implemented for extracting This novel methodology allows a fast extraction of device relevant electrical parameters of the devices, through a standard performance parameters, through the modelling of the small signal analysis. The parameter extraction methodology in frequency response of the test circuit. The method has been the frequency domain proposed allows rapid testing of the applied to the characterization of OTFTs, which operates far device/circuit performance of this technology, which is for the afield from CMOS devices. first time applied to organic devices. Results show that organic transistors exhibit similar channel dimensions dependencies as II. SAMPLES AND EXPERIMENTAL SETUP MOS devices, despite the large voltages, and can be also described Two different architectures of Organic Thin Film by the same small signal model. Transistors have been studied: 1) Interdigitated fingers for the Organic Thin Film Transistor (OTFT), Corbino Thin Film source and drain terminals (OTFT), and 2) Corbino [12] OTFT Transistor (CTFT), Organic Electronics, Capacitance-Voltage, DC (CTFT) with a cylindrical shape (see Figure 1A and B, characterization, small signal model. respectively). The devices are fabricated by NeuDrive Ltd. I. INTRODUCTION (UK) [13], using gold contacts, CytopTM as a dielectric and proprietary organic p-type semiconductor FlexOSTM [14]. The Organic Electronics (OE) is one of the most promising fabrication process combines sputtering, spin coating and technologies for the fabrication of new smart and flexible photolithographic steps. The cross section of the OTFT (Fig. electronic systems [1]. Thanks to the diverse fabrication 1A) and CTFT (Fig. 1B) are presented together with the stack techniques that it comprises [2]–[5], this technology allows the of materials in Figure 1C. use of flexible substrates and novel solution-based materials, The Interdigitated OTFTs processed by NeuDrive present in new smart systems in different fields such as Radio the classical interdigitated geometry, where source and drain Frequency Identification (RFID) tags, disposable sensors, are interleaved behind the gate. This geometry allows easy flexible displays or functional circuits [6]–[8]. These systems scalability since different widths can be achieved by using can be instant made, fully additive, low temperature processed, multiple fingers. Two different interdigitated channel lengths lightweight, bendable, scalable and sustainable, all of this were used, 10µm and 4µm. For the 10µm length devices, two combined with low-cost manufacturing. These new features widths were studied: 980µm and 9800µm. For the 4µm enable wearable and disposable electronic systems, unfeasible channel length devices, five widths were studied: 160, 320, for the moment with the traditional silicon technology. 640, 1280 and 2560µm using two different finger widths. For However, more investigations are still needed to analyze the the Corbino OTFT, four different channels lengths (5, 10, 20 performance of different architectures and to develop suitable and 40µm), combining three different external radius R2 (105, models. To evaluate the properties of Organic Thin Film 100 and 120µm) with two internal radiuses R1 (100 and 80µm) Transistors (OTFT) some works analyze and model their DC were studied [15]. behavior through different parameters such as Threshold Voltage, Mobility or Driving Current, and also the Operating Voltages, which are much larger than for MOSFETs [9]. Other works analyze the frequency response of OTFTs through the measurement of S-parameters in isolated devices [10]. Moreover, some functional circuits could be used to study the OTFT performance and their response in the frequency domain, since the circuit frequency response (gain and cut-off frequency) mainly depends on the transistor parameters. Then, Fig. 1. Architecture of the (A) Interdigitated OTFT, and (B) Corbino OTFT. (C) OTFT Stack materials, common to both architectures. I-V and C-V curves of the devices were measured with the grounded (similar values to those measured on MIM structures Semiconductor Parameter Analyzer Keithley 4200, which were obtained for the capacitances per unit area, 5,94·10-5F/m) allows large current and capacitance resolution measurements CGD and CGS were also measured, with the drain or source with high voltage biasing (±30V). The frequency response of terminals grounded, respectively, while the other terminal is the Common-Source amplifiers built with the studied OTFTs kept floating. A strong dependence of the measured capacitance was determined by measuring the amplitude of the output with the device area is observed (Fig. 4) for the two OTFTs voltage, when the frequency of the input signal was swept from architectures. The inset in Figure 4A shows that, for negative 100Hz to 1MHz. General-purpose digital scope was used for polarities, the three capacitances (COX, CGS and CGD) are almost equal, indicating that the channel region acts as a bottom the output signal measurement. terminal of the stack when device is ON (accumulation of p- III. DEVICE CHARACTERIZATION type carriers). In the case of CTFT devices, capacitance To evaluate the device performance, I-V characteristics and increases with device area (i.e. channel length decreases) when C-V curves were measured on the devices with different applying positive bias (subthreshold region). This dependence architectures and dimensions. Figure 2 shows typical Drain is observed because, in that regime, the area of the capacitance Current – Gate Voltage (ID-VG) characteristics of all of them. depends only on the overlapping between the gate and drain- As can be seen, subthreshold region is located at voltages above source electrodes. However, at negative bias, CTFT 0V, implying that these devices (p-type transistors) are capacitance exhibits similar values for different areas, because operating in depletion mode [16]. Off currents are of the order the effective area under the gate terminal is the same due to the of pA in most of the cases, meaning a low consumption when effect of the channel (i.e. gate metal electrode does not scale the device is off (Fig. 2). Note that, when device is on, currents with device area). Therefore, their cylindrical architecture in the ~100µA-1mA range are achieved with mobilities implies a L-independent gate area when CTFT device is ON. between 1 to 1.75 cm2/V·s, under operating voltages around ±30V [17]. These voltages are much larger than in CMOS technology (~0.8V) [18], but usual in organic devices [19]. Fig. 4. C-V characteristic of the Gate stack for the interdigitated OTFT with (A) L=10µm, (B) with L=4µm, and for (C) Corbino OTFT. The inset in (A) shows the area dependence of the three measured capacitances. Fig. 2. Drain Current–Gate Voltage characteristics measured on IV. FREQUENCY RESPONSE AND SMALL SIGNAL MODEL Interdigitated OTFT with (A) L=10µm, (B) L=4µm and (C) Corbino OTFT. In order to evaluate the performance of the studied OTFT and For the interdigitated OTFTs, ID increases as expected, CTFT, a Common Source (CS) amplifier is used [21], which linearly with device width (Fig. 3A). Contrarily, for the Corbino allows easy correlation of device and circuit performances, architecture, an ID reduction is observed (Fig. 3A), because because the amplifier’s gain and cut-off frequency strongly device width and length, change with the radiuses as well depend on the transistor parameters [22]. This configuration is (L=R1-R2). However, when representing ID as a function of represented in Figure 5A and the corresponding small signal device aspect ratio, in both cases, Interdigitated and Corbino, it equivalent circuit in Figure 5B, where a MOSFET-like small- is proportional to W/L (Fig. 3B). To evaluate the dependence signal model is considered for the OTFT. Circuit analysis leads of Vth on the device area, as a criterium, Vth was defined as the to a transfer function given by equation 1, which shows that the gate voltage at which ID=10µA. Figure 3C shows that Vth of CS circuit gain depends on the transconductance (gm) and Interdigitated devices grows linearly with the device width, output resistance of the transistor (the last one included in ROUT, while in the Corbino it decreases with the channel length. Note which is defined as ROUT= r0//RD//ROSC). In addition, the cut-off that ID and Vth follow the same geometry scaling dependence frequency is determined by the parasitic Gate-Drain that MOS technologies [20]. capacitance (CGD), being independent of CGS. These parameters can be extracted from |H(s)| measurements. In our experiments, the value of RSS (the output resistance of our input source), RG1, RG2, CS, ROSC and COSC (being the last one the input impedance of our scope) and the DC voltage supply, VCC, were fixed. It must be emphasized that, in these experiments several parameters change from experiment to experiment (i.e. when a different device area or geometry are used, different voltages at device terminals are registered), thus direct comparisons are meaningless. Moreover, circuit values such as RD or Vx were selected in order to maximize the gain. Then, to account for the Fig. 3. ID as a function of (A) the device area for OTFT and CTFT and (B) as a function of the aspect ratio W/L. (C) Vth as a function of device area. differences in the OTFTs, RD valued could vary between 340kΩ and 1MΩ. VGS and VDS DC values were registered The parasitic capacitances associated to the stack were also before extracting the frequency response, to assure the measured (C-V curves) in the same devices. The gate transistor operating point. A frequency sweep of the input capacitance (Cox) was measured when source and drain were signal was performed from 100Hz to 1MHz. Only the mid and high frequency regimes were analysed, since low frequency V. CONCLUSIONS response is mainly given by the circuit parameters. Two different architectures of organic thin film transistors 𝑉𝑂𝑈𝑇 (𝑆) 𝑅𝑂𝑈𝑇 𝐶𝐺𝐷 𝑆−𝑔𝑚𝑅𝑂𝑈𝑇 (Interdigitated and Corbino) with different dimensions have 𝐻(𝑆) = 𝑉𝐼𝑁 (𝑆) = 𝑆(𝐶𝑂𝑆𝐶 +𝐶𝐺𝐷 )𝑅𝑂𝑈𝑇 +1 Eq. 1 been studied. Their performance has been evaluated through the frequency response of an OTFT-based Common-Source (CS) amplifier, since it strongly depends on the device properties. To extract the transistor parameters from the CS experimental data, a MOSFET-like small signal model of the OTFT has been assumed. The dependences observed for CGD in all the devices have been verified from the comparison of the CS-extracted values with those obtained from C-V data and analytical calculation. The results show that the extracted small signal model parameters (gm, r0, and CGD) depend on the device dimensions and materials properties. A linear dependence of r0, Fig. 5. (A) Common Source (CS) amplifier implemented to evaluate the gm and CGD with device width is observed in the Interdigitated OTFT performance in the frequency domain, and (B) small signal model for OTFT. In the case of the Corbino devices, CGD does not depend mid and high frequency regims. RG=RG1//RG2 and ROUT= r0//RD//ROSC . on the device widths or lengths (i.e. radiuses), since there is the Figure 6 shows the magnitude of the frequency responses of same effective area under the gate electrode for negative the CS amplifiers built with the different Organic transistors, biasing. gm and r0 present a non-linear area dependence, due to measured (lines) and their fitting to eq. 1 (circles). As observed, the relation between width and length in the CTFT. Therefore, the gain increases with the device width and in the best case it the circuit performance is strongly driven by device is ~6dB. The cut-off frequency also increases with the device dimensions, as for MOS devices, despite the large voltages width. In addition, the effect of the transmission zero (given by applied. CGD) in eq. 1 is clearly observed in the 100kHz-1MHz interval. Acknowledgments This work was supported by the TEC2016-75151-C3-1-R, RTI2018-102070-B-C21, and 2017SGR-1159. Projects funded by the Spanish AEI, ERDF and Generalitat de Catalunya. Fig. 6. (A) Experimental (lines) and fitted (circles) frequency response for the Interdigitated OTFT with L = 10 µm, ( B) Interdigitated OTFT with L = 4 µm and (C) Corbino OTFT. The device parameters obtained from the fitting of the experimental data in Figure 6 to eq. 1 are depicted in Figure 7. Figure 7A shows the parasitic capacitance between gate and drain terminals (CGD) as a function of the device aspect ratio obtained analytically (considering that for negative voltages CGD=COX, as shown in the inset in Fig. 4A) (squares), from the experimental C-V curve (circles) and from the fitting of the amplifier frequency response to the eq. 1 (triangles). Similar values of CGD are obtained for the different topologies and areas for the three extraction methods. The r0 and gm parameters are depicted in Figure 7B. A large dependence of these parameters with the channel dimensions is observed in both OTFT architectures, being less significant in the case of CTFT due to their complex device width and length radiuses-dependence. As observed, for the Interdigitated OTFT, gm increases and r0 decreases linearly with device area. Inversely, for the CTFT, gm decreases and r0 increases non-linearly with device area, what means larger separation between radiuses (i.e. L increase). Fig. 7. (A) Analytically calculated Gate-Drain capacitance (squares), measured from the C-V curves (circles) and obtained from the data fitting (triangles). (B) r0 (empty squares) and gm (circles) obtained from the fitting. REFERENCES [14] K. L. McCall et al., “Low voltage high mobility [1] X. Guo et al., “Current Status and Opportunities of organic semiconductors for flexible display Organic Thin-Film Transistor Technologies,” IEEE applications,” Proc. Int. Disp. Work., vol. 3, pp. 1766– Trans. Electron Devices, vol. 64, no. 5, pp. 1906– 1769, 2012. 1921, 2017, doi: 10.1109/TED.2017.2677086. [15] D. Munteanu, S. Cristoloveanu, and H. Hovel, [2] Y. Takeda et al., “Organic Complementary Inverter “Circular Pseudo-Metal Oxide Semiconductor Field Circuits Fabricated with Reverse Offset Printing,” Effect Transistor in Silicon-on-Insulator Analytical Adv. Electron. Mater., vol. 4, no. 1, p. 1700313, Jan. Model, Simulation, and Measurements,” Electrochem. 2018, doi: 10.1002/aelm.201700313. Solid-State Lett., vol. 2, no. 5, pp. 242–243, 1999, doi: [3] S. H. Ko, H. Pan, C. P. Grigoropoulos, C. K. 10.1149/1.1390798. Luscombe, J. M. J. Fréchet, and D. Poulikakos, “All- [16] S. D. Ogier et al., “Uniform, high performance, inkjet-printed flexible electronics fabrication on a solution processed organic thin-film transistors polymer substrate by low-temperature high-resolution integrated in 1 MHz frequency ring oscillators,” Org. selective laser sintering of metal nanoparticles,” Electron. physics, Mater. Appl., vol. 54, no. July 2017, Nanotechnology, vol. 18, no. 34, p. 345202, Aug. pp. 40–47, 2018, doi: 10.1016/j.orgel.2017.12.005. 2007, doi: 10.1088/0957-4484/18/34/345202. [17] B. Kumar, B. K. Kaushik, and Y. S. Negi, “Organic [4] H. Jeong, S. Baek, S. Han, H. Jang, S. H. Kim, and H. thin film transistors: Structures, models, materials, S. Lee, “Novel Eco-Friendly Starch Paper for Use in fabrication, and applications: A review,” Polym. Rev., Flexible, Transparent, and Disposable Organic vol. 54, no. 1, pp. 33–111, Jan. 2014, doi: Electronics,” Adv. Funct. Mater., vol. 28, no. 3, p. 10.1080/15583724.2013.848455. 1704433, Jan. 2018, doi: 10.1002/adfm.201704433. [18] C. Auth et al., “A 10nm high performance and low- [5] N. A. Azarova et al., “Fabrication of organic thin-film power CMOS technology featuring 3rd generation transistors by spray-deposition for low-cost, large-area FinFET transistors, Self-Aligned Quad Patterning, electronics,” Org. Electron. physics, Mater. Appl., vol. contact over active gate and cobalt local 11, no. 12, pp. 1960–1965, Dec. 2010, doi: interconnects,” in Technical Digest - International 10.1016/j.orgel.2010.09.008. Electron Devices Meeting, IEDM, 2018, pp. 29.1.1- [6] B. Kumar, B. K. Kaushik, and Y. S. Negi, “Organic 29.1.4, doi: 10.1109/IEDM.2017.8268472. thin film transistors: Structures, models, materials, [19] A. Arnal, C. Martínez-Domingo, S. Ogier, L. Terés, fabrication, and applications: A review,” Polym. Rev., and E. Ramon, “Monotype Organic Dual Threshold vol. 54, no. 1, pp. 33–111, 2014, doi: Voltage Using Different OTFT Geometries,” Crystals, 10.1080/15583724.2013.848455. vol. 9, no. 7, p. 333, 2019, doi: 10.3390/cryst9070333. [7] E. R. i Garcia, “Inkjet Printed Microelectronic Devices [20] T. Tsunomura, A. Nishida, and T. Hiramoto, “Analysis and Circuits,” no. September, 2014. of NMOS and PMOS difference in VT variation with [8] K. Myny, E. Van Veenendaal, G. H. Gelinck, J. Genoe, large-scale DMA-TEG,” IEEE Trans. Electron W. Dehaene, and P. Heremans, “An 8-bit, 40- Devices, vol. 56, no. 9, pp. 2073–2080, 2009, doi: instructions-per-second organic microprocessor on 10.1109/TED.2009.2026390. plastic foil,” IEEE J. Solid-State Circuits, vol. 47, no. [21] X. Cheng, S. Lee, and A. Nathan, “TFT Small Signal 1, pp. 284–291, 2012, doi: Model and Analysis,” IEEE Electron Device Lett., vol. 10.1109/JSSC.2011.2170635. 37, no. 7, pp. 890–893, 2016, doi: [9] J. S. Chang, A. F. Facchetti, R. Reuss, S. Member, A. 10.1109/LED.2016.2575924. F. Facchetti, and R. Reuss, “A Circuits and Systems [22] D. Lovelace, J. Costa, and N. Camilleri, “Extracting Perspective of Organic/Printed Electronics: Review, small-signal model parameters of silicon MOSFET Challenges, and Contemporary and Emerging Design transistors,” 1994 IEEE MTT-S Int. Microw. Symp. Approaches,” IEEE J. Emerg. Sel. Top. Circuits Syst., Dig. (Cat. No.94CH3389-4), pp. 865–868, 1994, doi: vol. 7, no. 1, pp. 7–26, 2017, doi: 10.1109/MWSYM.1994.335220. 10.1109/JETCAS.2017.2673863. [10] T. Zaki et al., “AC characterization of organic thin- film transistors with asymmetric gate-to-source and gate-to-drain overlaps,” Org. Electron., vol. 14, no. 5, pp. 1318–1322, 2013, doi: 10.1016/j.orgel.2013.02.014. [11] B. A. Brown, S. D. Ogier, M. Palumbo, and K. L. McCall, “Transistors and Methods for Making Them,” 2012. [12] M. Mativenga et al., “Infinite output resistance of corbino thin-film transistors with an amorphous-In ga znO active layer for large-area AMOLED displays,” IEEE Trans. Electron Devices, vol. 61, no. 9, pp. 3199–3205, 2014, doi: 10.1109/TED.2014.2338306. [13] “NeuDrive.” [Online]. Available: http://www.neudrive.com/. [Accessed: 10-Sep-2019].