[paper] Compact Modeling of Hysteresis in OTFTs

Compact modeling of hysteresis in organic thin-film transistors

A. Romero^a, J.A. Jiménez-Tejada^a, R. Picos^b, D. Lara^a, J.B. Roldán^a, M.J. Deen^c

Organic Electronics 129 (2024) 107048

DOI : 10.1016/j.orgel.2024.107048

a Departamento de Electrónica y Tecnología de Computadores, CITIC-UGR, Uni Granada, Spain
b Department of Industrial Engineering and Construction, Universitat de les Illes Balears, Spain
c Department of Electrical and Computer Engineering, McMaster University, Canada

Abstract: In this work, we propose a model that describes the temporal evolution of the threshold voltage and trapped charge density in Thin-Film Transistors (TFTs) under dynamic conditions, paving the way for the characterization and modeling of memory transistors. The model is expressed as a first-order differential equation for the trapped charge density, which is controlled by a time constant and an independent term proportional to the drain current. The time-dependent threshold voltage is introduced in a previously developed compact model for TFTs with special consideration to the contact effects. The combination of both models and the use of an evolutionary parameter extraction procedure allow for reproducing the experimental dynamic behavior of TFTs. The results of the model and the evolutionary procedure have been validated with published experimental data of pentacene-based transistors. The procedure is able to simultaneously reproduce three kinds of experiments with different initialization routines and constraints in each of them: output and transfer characteristics with hysteresis and current transients characteristics

FIG: a.) Modeling the contact regions and intrinsic channel of an OTFT structure (a bottom contact configuration); b.)  Comparison of experimental transfer characteristics

Acknowledgements : The authors acknowledge support from the project PID2022 139586NB-44 funded by MCIN/AEI/10.13039/501100011033 and FEDER, EU. Funding for open access charge: Universidad de Granada / CBUA.

Appendix: Supplementary material related to this article can be found online.

[paper] Coplanar OTFT

Blurred Electrode for Low Contact Resistance in Coplanar Organic Transistors

Xiaolin Ye, Xiaoli Zhao, Shuya Wang, Zhan Wei, Guangshuang Lv, Yahan Yang, Yanhong Tong, Qingxin Tang, and Yichun Liu

American Chemical Society; Nano; Dec.18, 2020

DOI: 10.1021/acsnano.0c08122

*Center for Advanced Optoelectronic Functional Materials Research, and Key Lab of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China

Abstract: Inefficient charge injection and transport across the electrode/semiconductor contact edge severely limits the device performance of coplanar organic thin-film transistors (OTFTs). To date, various approaches have been implemented to address the adverse contact problems of coplanar OTFTs. However, these approaches mainly focused on reducing the injection resistance and failed to effectively lower the access resistance. Here, we demonstrate a facile strategy by utilizing the blurring effect during the deposition of metal electrodes, to significantly reduce the access resistance. We find that the transition region formed by the blurring behavior can continuously tune the molecular packing and thin-film growth of organic semiconductors across the contact edge, as well as provide continuously distributed gap states for carrier tunnelling. Based on this versatile strategy, the fabricated dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) coplanar OTFT shows a high field-effect mobility of 6.08 cm2 V–1 s–1 and a low contact resistance of 2.32 kΩ cm, comparable to the staggered OTFTs fabricated simultaneously. Our work addresses the crucial impediments for further reducing the contact resistance in coplanar OTFTs, which represents a significant step of contact injection engineering in organic devices.

Fig: Coplanar Organic Transistors (oTFTs)

[paper] TFT for Mixed Signal and Analog Computation

Eva Bestelink, Olivier de Sagazan, Lea Motte, Max Bateson, Benedikt Schultes, S. Ravi P. Silva,

and Radu A. Sporea

Versatile Thin‐Film Transistor with Independent Control of Charge Injection and Transport

for Mixed Signal and Analog Computation

Adv. Intell. Syst.. (2020) pp.1-9, DOI:10.1002/aisy.202000199 

Abstract: New materials and optimized fabrication techniques have led to steady evolution in large area electronics, yet significant advances come only with new approaches to fundamental device design. The multimodal thin-film transistor introduced here offers broad functionality resulting from separate control of charge injection and transport, essentially using distinct regions of the active material layer for two complementary device functions, and is material agnostic. The initial implementation uses mature processes to focus on the device’s fundamental benefits. A tenfold increase in switching speed, linear input–output dependence, and tolerance to process variations enable low-distortion amplifiers and signal converters with reduced complexity. Floating gate designs eliminate deleterious drain voltage coupling for superior analog memory or computing. This versatile device introduces major new opportunities for thin-film technologies, including compact circuits for integrated processing at the edge and energy-efficient analog computation.

Figure: Outcomes of separating control for injection and conduction shown via TCAD simulation. a) MMT transient response is much faster than conventional contact-controlled TFTs

b) A MMT with multiple, appropriately sized CG1 gates can function as a digital-to-analog converter (DAC) with CG2 providing an enabling, sampleand-hold (S/H) function. 

Acknowledgements: E.B. and R.A.S. contributed equally to this work. This work was partly supported through EPSRC grants EP/R511791/1 and EP/R028559/1 and Research Fellowship 10216/110 from the Royal Academy of Engineering of Great Britain. Device fabrication had been performed on the NanoRennes platform. The authors thank Dr. Brice Le Borgne for initial liaison and process discussions, Prof. John M. Shannon for on-going advisory meetings, Prof. Craig Underwood for reviewing the manuscript, Dr. David Cox and Mr. Mateus Gallucci Masteghin for assistance with the SEM images.

[paper] Flexible MO TFT for Analog Applications

Giuseppe Cantarella¹, Júlio Costa², Tilo Meister³, Koichi Ishida³, Corrado Carta³, Frank Ellinger³, Paolo Lugli¹, Niko Münzenrieder^1,2 and Luisa Petti¹

Review of recent trends in flexible metal oxide thin-film transistors for analog applications

Flexible and Printed Electronics 2020, Vol. 5, No. 3

DOI: 10.1088/2058-8585/aba79a

¹Faculty of Science and Technology, Free University of Bozen-Bolzano, 39100, Bozen, Italy
²Flexible Electronics Laboratory, University of Sussex, Brighton, BN1 9QT, United Kingdom
³Chair of Circuit Design and Network Theory, TU Dresden, 01069 Dresden, Germany

Abstract: Thanks to the extraordinary advances flexible electronics have experienced over the last decades, applications such as conformable active-matrix displays, ubiquitously integrated disposable flexible sensor nodes, wearable or textile-integrated systems, as well as imperceptible and transient implants are now reachable. To enable these applications, specialized analog circuits able to transmit and receive data, condition sensors' parameters, drive actuators or control powering devices are required. High-performance sensor conditioning, driving and transceiver circuits on a wide range of flexible substrates are therefore extremely important to develop. However, the currently available materials and processes compatible with mechanically flexible substrates impose massive limitations in terms of large-area uniformity, device dimensions' shrinkability and circuit design, challenging the realization of flexible analog systems. Among state-of-the-art technologies employing low-temperature fabrication processes, thin-film transistors (TFTs) based on metal oxide semiconductors represent the potentially best compromise in terms of prize, performance, technology maturity and capacity to realize complex systems. This is why metal oxide TFTs are nowadays widely used for flexible, light-weight, transparent, stretchable and bio-degradable analog circuits and systems. Here, we review the current trends of flexible metal oxide TFTs for analog applications. First, an introduction is given, where current challenges and requirements related to the realization of flexible analog circuits and systems are analysed. Additionally, TFT performance parameters and configurations are briefly revised. Then, the recent advances in the field of flexible metal oxide TFTs for analog applications are summarized. In particular, all reported approaches to reduce the channel length and improve the AC performance are shown. Next, the current state of flexible metal oxide TFT-based analog circuits is shown, discussing n-type only and complementary circuit configurations. The last topic of the review covers systems based on flexible metal oxide analog circuits. Finally, a conclusion is drawn and an outlook over the field is provided.

Figure: Overview of published works on flexible metal oxide TFT based circuits, indicating the minimum channel length of the devices, the operation frequency of the circuits, the effective supply voltage used, as well as the total TFT count. Only integrated circuits are included.

Acknowledgments: This work was partially supported by the DFG FFlexCom Priority Programme, Germany, through projects WISDOM II and Coordination Funds, under Grants 271795180 and 270774198. This work was also partially funded with internal funding of the Faculty of Science and Technology of the Free University of Bolzano-Bozen (project ”EYRE” RTD Call 2019).

[paper] Compact Source-Gated Sensor

Eva Bestelink, Student Member, IEEE, Kham M. Niang, Georgios Bairaktaris, Luca Maiolo, Francesco Maita, Kalil Ali, Andrew J. Flewitt, S. Ravi P. Silva

and Radu A. Sporea, Senior Member, IEEE

Compact Source-Gated Transistor Analog Circuits for Ubiquitous Sensors

In IEEE Sensors. Jul 18, 2020

Abstract: Silicon-based digital electronics have evolved over decades through an aggressive scaling process following Moore’s law with increasingly complex device structures. Simultaneously, large-area electronics have continued to rely on the same field-effect transistor structure with minimal evolution. This limitation has resulted in less than ideal circuit designs, with increased complexity to account for shortcomings in material properties and process control. At present, this situation is holding back the development of novel systems required for printed and flexible electronic applications beyond the Internet of Things. In this work we demonstrate the opportunity offered by the source-gated transistor’s unique properties for low-cost, highly functional large-area applications in two extremely compact circuit blocks. Polysilicon common-source amplifiers show 49 dB gain, the highest reported for a twotransistor unipolar circuit. Current mirrors fabricated in polysilicon and InGaZnO have, in addition to excellent current copying performance, the ability to control the temperature dependence (degrees of positive, neutral or negative) of output current solely by choice of relative transistor geometry, giving further flexibility to the design engineer. Application examples are proposed, including local amplification of sensor output for improved signal integrity, as well as temperature-regulated delay stages and timing circuits for homeostatic operation in future wearables. Numerous applications will benefit from these highly competitive compact circuit designs with robust performance, improved energy efficiency and tolerance to geometrical variations: sensor front-ends, temperature sensors, pixel drivers, bias analog blocks and high-gain amplifiers.

FIG: a) Photomicrograph of a typical polysilicon SGT fabricated; b) Driver M1 output characteristics (black curves, VGmax = -15 V, step 0.5 V) and superimposed M2 load line (orange, VG = 0 V). VSAT1 occurs as a result from pinch-off at the source and VSAT2 represents channel pinch-off of the parasitic FET. 

Acknowledgment: R.A.S. acknowledges the Royal Academy of Engineering of Great Britain for the support through the Research Fellowship (Grant No. 10216/110), the Royal Society of Great Britain through project ARES IES\R3\170059 and EPSRC for grants EP/R028559/1 and EP/R025304/1. K.M.N. and A.J.F. acknowledge the support of the Engineering and Physical Sciences Research Council (EPSRC) through project EP/M013650/1. R.A.S. thanks Prof John Shannon for technical discussions, Dr Nigel Young and Dr Michael Trainor for assistance with polysilicon device design and fabrication.