(Invited) Multi-Gate Contact-Controlled Transistors

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Contact effects are a central focus in the development of high performance thin-film transistors (TFTs) and circuits, motivated by the increased speed and current density achievable by reducing contact energy barriers. Conversely, the rectifying properties of contacts have been utilised to create devices in which the current is, for the most part, controlled by the contact properties and bias conditions rather than the semiconductor channel [1]. These transistors operate at lower speeds but bring operational advantages such as geometrical and bias stress tolerance and have been extensively studied in their operation [2] and potential applications [3, 4] in varied material systems [5-7].We have recently introduced the Multimodal Transistor (MMT), in which channel conductance and charge injection at the source are controlled independently by individual gates [8], offering important benefits to the functionality of analog and mixed signal circuits. These devices build on earlier innovations which modulate contact barriers [1, 5, 6, 7, 9-11] and take advantage of the recent acceptance into the mainstream of multi-gate TFTs [12, 13] to enable constant transconductance, high-speed switching, low output conductance, low-voltage saturation, and robustness against geometrical variability.Here, the structure and operation of MMTs (Figure 1) is briefly reviewed and challenges and opportunities for this device are discussed, focusing on the specifics of solution-processed and organic implementations.[1] J. M. Shannon et al., IEEE Trans. Electron Devices, 60, 2444–2449 (2013).[2] A. Valletta et al., J. Appl. Phys. 114 064501 (2013).[3] R. A. Sporea et al., Scientific Reports 4 4295 (2014).[4] Bestelink, E. et al., IEEE Sens. J., 20 (24), 14903 (2020).[5] Zhang, J., et al, PNAS 116, 4843 (2019).[6] Lee, S. and A. Nathan, Science 354, 302 (2016).[7] Jiang, C. et al, Science 363, 719 (2019).[8] E. Bestelink et al., Advanced Intelligent Systems, 2000199 (2020).[9] Torricelli, F. et al, Nat. Commun. 7, 1 (2016).[10] Yoo H. et al., Sci. Rep. 7, 1 (2017).[11] Uemura T. et al., Adv. Mater. 26, 2983 (2014).[12] Myny, K. et al., IEEE JSSC 46 (5), 1223, (2011).[13] Lee S. et al., JSID 27, 507, (2019).Figure 1. A) Schematic cross-sections of the multimodal transistor (MMT) showing its principle of operation: conduction state is set by the channel gate CG2, while current magnitude is controlled by source gate CG1. B) output characteristics of a nanocrystalline silicon MMT indicating the early saturation and constant transconductance, gm . C and D) are transfer characteristics obtained by varying CG1 and CG2 voltages, respectively. Figure 1