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Recent Advances in Ambipolar Transistors for Functional Applications REVIEW Ambipolar Transistors www.afm-journal.de Recent Advances in Ambipolar Transistors for Functional Applications Yi Ren, Xiaoyang Yang, Li Zhou, Jing-Yu Mao, Su-Ting Han,* and Ye Zhou* communities. For the sake of the suc- Ambipolar transistors represent a class of transistors where positive (holes) cessful implementation of their intricate and negative (electrons) charge carriers both can transport concurrently functionalities, diverse fundamental elec- within the semiconducting channel. The basic switching states of ambipolar tronic constituents are imperative to act transistors are comprised of common off-state and separated on-state mainly as functional system blocks. As one of the most significant components, transistors impelled by holes or electrons. During the past years, diverse materials are are the foundation and key ingredients of synthesized and utilized for implementing ambipolar charge transport and modern electronic devices and products.[1] their further emerging applications comprising ambipolar memory, synaptic, Since the 1950s, transistors have gradually logic, and light-emitting transistors on account of their special bidirectional replaced vacuum tubes and finally realized carrier-transporting characteristic. Within this review, recent developments mass production of integrated circuits of ambipolar transistor field involving fundamental principles, interface and microprocessors. Transistors pos- sessing advantages of low cost, flexibility modifications, selected semiconducting material systems, device structures, and reliability have revolutionized elec- ambipolar characteristics, and promising applications are highlighted. The tronics, making electronic devices smaller, existed challenges and prospective for researching ambipolar transistors in cheaper, and much more efficient.[2–9] electronics and optoelectronics are also discussed. It is expected that the In accordance with switching charac- review and outlook are well timed and instrumental for the rapid progress of teristic and predominant charge carriers within semiconductors, the polarity of academic sector of ambipolar transistors in lighting, display, memory, as well transistors can be categorized as unipolar as neuromorphic computing for artificial intelligence. (hole-dominated p-type or electron-dom- inated n-type) or ambipolar (comparable hole and electron contributions). Appar- 1. Introduction ently, ambipolar transistors are capable of integrating p- and n-type electrical performance into single device and thereby With the rapid advances of network and information tech- have aroused plenty of concern and discussion of researchers niques, the emerging artificial intelligence (AI) which refers from various fields such as organic chemistry and device sci- to the intelligence exhibited by an artificially manufactured ence.[10] Nonetheless, up to now vast majority of transistors dis- electronic system and the Internet of things (IoT) which con- play unipolar dominated charge transport in spite of the theory nects common physical objects with cyber space systems both that each semiconductor is capable of migrating holes and have provoked multitudes of focus in industrial and academic electrons according to semiconductor physics. For purpose of fabricating high-performance ambipolar transistors with large Y. Ren, J.-Y. Mao, Prof. Y. Zhou carrier mobility and high on/off ratio as well as low threshold Institute for Advanced Study voltage, appropriate semiconducting materials with relatively Shenzhen University small bandgap, smooth and flawless interface characteristics Shenzhen 518060, P. R. China E-mail: [email protected] together with electrode contacts with eligible charge injection Dr. X. Yang, Dr. L. Zhou, Prof. S.-T. Han barrier are both pivotal. During past years, miscellaneous semi- Institute of Microscale Optoelectronics and Key Laboratory conducting materials comprising organic small molecules,[11] of Optoelectronic Devices and Systems of Ministry of Education conjugated polymers,[12] 2D materials,[13] and organic–inor- and Guangdong Province ganic hybrid materials[14] have been utilized for realizing ambi- College of Optoelectronic Engineering Shenzhen University polar charge transport and promising functional applications. Shenzhen 518060, P. R. China Ambipolar transistors which transport both types of charge E-mail: [email protected] carriers synchronously make the fabrication of complemen- Prof. S.-T. Han tary metal-oxide-semiconductor (CMOS) inverters simpler due Department of Electrical Engineering and Computer Science to controllable separated unipolar mode of them, meanwhile The University of Michigan the manufacture of ambipolar light-emitting transistors which Ann Arbor, MI 48109, USA integrate switching characteristic (property of transistors) and The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201902105. light emission capacity (nature of light-emitting diodes) into an unitary device will be more efficient.[10,15] Moreover, by adding DOI: 10.1002/adfm.201902105 another two layers (floating gate and tunneling dielectric), Adv. Funct. Mater. 2019, 29, 1902105 1902105 (1 of 65) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de ambipolar transistors can be utilized as well-known flash memory that can exhibit much larger memory window due to Yi Ren received his physics bidirectional threshold voltage shifts on the basis of trapping/ bachelor’s degree from detrapping mechanism.[16] Inspired by the similarity between Institute for Advanced Study the fluxion of neurotransmitters in synaptic cleft and the at Shenzhen University, actional transport and trapping operation of charge carriers, Guangdong, China, in 2018. ambipolar transistors also possess great potential for artificial He is now a postgraduate synaptic emulation and hence neuromorphic computing in student at the Institute for artificial intelligence.[17] Advanced Study in Shenzhen The overview structure of this review is described as fol- University. His research lows. The basic structure, energy band alignment and funda- interests include organic/inor- mental principles of ambipolar transistors are first introduced. ganic materials for nanoscale Particularly, functionalities and influences of semiconducting devices, such as data storage materials, interface characteristics and electrode contacts and artificial synapse devices. on ambipolar performance are elucidated. Then, the recent selected diverse semiconducting materials for instance organic and 2D materials as well as various device architectures of ambi- Su-Ting Han is an asso- polar transistors are contrasted and the merits and demerits of ciate professor at Shenzhen them are also analyzed. Within stated various material systems, University and a visiting in spite of existed challenges, basic mechanisms of fabricated associate professor at The transistors are well comprehended and hence some realistic University of Michigan. She applications begin to appear. Thus, the promising applications received her M.Sc. degree of ambipolar transistors comprising ambipolar flash memories, in analytical chemistry from artificial synaptic transistors, logic devices and light-emitting Hong Kong Baptist University transistors are then focused (Figure 1). Finally, the existing and her Ph.D. degree in challenges and outlook of the future developments and appli- physics and materials science cations of ambipolar transistors in printed electronics are dis- from City University of Hong cussed. Hopefully, this review can provide instructive roadmaps Kong. Her research interests for the evolution of ambipolar transistor area. include functional electronic devices and flexible, stretch- able, and wearable electronics. 2. Structures and Fundamental Principles Ye Zhou is an IAS Fellow of Ambipolar Transistors and group leader in the Ambipolar transistors represent transistors that allow synchro- Institute for Advanced Study, nous transport of electrons and holes and their accumulation Shenzhen University. His within semiconductors.[18,19] An ambipolar transistor is usu- research interests include ally constructed from gate electrode, dielectric, semiconducting flexible and printed elec- layer, and two source–drain electrodes (Figure 2a). The first three tronics, organic/inorganic parts make up a metal–insulator–semiconductor (MIS) architec- semiconductors, surface and ture which is capable of propelling holes or electrons to accu- interface physics, nano- mulate at the interface between dielectric and semiconducting structured materials, and layer via exerting a gate bias with specific voltage polarity. The nanoscale devices for techno- additional source–drain electrodes coming into contact with the logical applications, such as interface of active layer dominate the injection of charge car- logic circuits, data storage, energy harvesting, photonics, riers which are necessary for the subsequent accumulation pro- and sensors. cess. The injected holes and electrons can transfer in the light of various biases applied at these three terminals. Thus, both the amount and polarity of charge carriers within accumulation as defects and vacancies within semiconducting layer and the zone are different under gate biases with diverse amplitudes and interface between dielectric and semiconducting layers can cap- polarities (positive gate bias for electron accumulation whereas ture minority carriers and hence impede the accumulation
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