DOI: 10.25549/usctheses-c40-346723
title: Carbon nanotube radio-frequency electronics and macroelectronics

description: In this dissertation, I present the solutions for achieving high‐performance radio frequency (RF) transistors based on carbon nanotubes (CNTs), and the explorations of CNT macroelectronics for future flexible, portable and wearable electronics. ❧ Chapter 1 gives a brief introduction of CNTs, including their physical and electronic properties, as the foundation of the whole dissertation. Chapter 2 to 6 report the achievements I have made in CNT research areas. Chapter 7 summarizes my work, and discusses the future directions of CNT‐based electronics. ❧ Chapter 2 reports the effects of semiconducting purity on the RF performance of CNT transistors. Polyfluorene‐separated ultra‐high purity (&gt;99.99%) semiconducting CNTs are utilized to fabricate RF transistors with a self‐aligned T‐shaped gate. Because of the ultra‐high semiconducting tube purity and self‐aligned T‐shaped gate structure, these transistors showed an excellent direct current (DC) and RF performance. In regard to the DC characteristics, these transistors showed a transconductance up to 40 μS/μm and an excellent current saturation behavior with an output resistance greater than 200 kΩ•μm. In terms of the RF characteristics, an extrinsic maximum oscillation frequency (fmax) of 19 GHz was achieved, and an extrinsic current gain cut‐off frequency (ft) of 22 GHz was achieved, which is the highest among transistors based on CNT networks. The results take the RF performance of carbon nanotube transistors to a new level, and can further accelerate the application of carbon nanotubes for future RF electronics. ❧ Chapter 3 presents the high‐performance RF transistors based on the CNTs with a refined average diameter of 1.6 nm. These diameter‐separated CNT transistors show excellent transconductance of 55 μS/μm, and desirable drain current saturation with an output resistance of 100 kΩ•μm. An exceptional RF performance is also achieved with ft and fmax of 23 GHz and 20 GHz (extrinsic) and 65 GHz and 35 GHz (intrinsic), respectively. These radio-frequency metrics are among the highest reported for the CNT thin‐film transistors. This study provides demonstration of RF transistors based on CNTs with tailored diameter distributions, which will guide the future application of carbon nanotubes in radio‐frequency electronics. ❧ In Chapter 4, I report record RF performance of CNT transistors based on combined use of a self‐aligned T‐shaped gate structure, and well‐aligned, high‐semiconducting‐purity, high‐density polyfluorene‐sorted semiconducting CNTs, which were deposited using dose‐controlled, floating evaporative self-assembly method. These transistors show outstanding direct current performance with on‐current density of 350 μA/μm, transconductance as high as 310 μS/μm, and superior current saturation with normalized output resistance greater than 100 kΩ•μm. These transistors create a record as CNT RF transistors that demonstrate both ft and fmax greater than 70 GHz. Furthermore, these transistors exhibit good linearity performance with 1 dB gain compression point (P1dB) of 14 dBm and input third‐order intercept point (IIP₃) of 22 dBm. This study advances state‐of‐the‐art of CNT RF electronics, which have the potential to be made flexible and may find broad applications for signal amplification, wireless communication, and wearable/flexible electronics. ❧ In Chapter 5, a method to achieve the record maximum bending radius of CNT transistors and circuits is presented. Flexible thin‐film transistors based on semiconducting CNTs are promising for flexible digital circuits, artificial skins, radio frequency devices, active‐matrix‐based displays, and sensors due to the outstanding electrical properties and intrinsic mechanical strength of CNTs. Nevertheless, previous research effort only led to nanotube thin‐film transistors with the smallest bending radius down to 1 mm. We have realized the full potential of carbon nanotubes by making ultraflexible and imperceptible p‐type transistors and circuits with a bending radius down to 40 μm on very thin flexible substrates (~1.5 μm). In addition, the resulted transistors show mobility up to 12.04 cm² V⁻¹s⁻¹, high on‐off ratio (∼10⁶), ultralight weight (&lt;3 g/m²), and good mechanical robustness (accommodating severe crumpling and 67% compressive strain). Furthermore, the nanotube circuits can operate properly with 33% compressive strain. On the basis of the aforementioned features, our ultraflexible p‐type nanotube transistors and circuits have great potential to work as indispensable components for ultraflexible complementary electronics. ❧ In Chapter 6, I report a hybrid integration scheme to achieve complementary flexible macroelectronics. CNTs and metal oxide semiconductors have emerged as important materials for p‐type and n‐type thin‐film transistors, respectively; however, realizing sophisticated macroelectronics operating in complementary mode has been challenging due to the difficulty in making n‐type carbon nanotube transistors and p‐type metal oxide transistors. I have developed a hybrid integration of p‐type carbon nanotube and n‐type indium‐gallium‐zinc‐oxide thin‐film transistors to achieve large‐scale (&gt;1,000 transistors for 501‐stage ring oscillators) complementary macroelectronic circuits on both rigid and flexible substrates. This approach of hybrid integration combines the strength of p‐type carbon nanotube and n‐type indium‐gallium‐zinc‐oxide thin‐film transistors, and offers high device yield and low device variation. Based on this approach, I report the successful demonstration of various logic gates (inverter, NAND and NOR gates), ring oscillators (from 51 stages to 501 stages) and dynamic logic circuits (dynamic inverter, NAND and NOR gates). ❧ Chapter 7 summarizes my achievements presented in Chapter 2 to 6. In addition, it also presents future directions for CNT‐based RF electronics, including optimization of CNT self‐aligned T‐shape gate RF transistors, design of transceivers based on CNT self‐aligned T‐shape gate RF transistors, three‐dimensional (3D) integration of CNT‐based analog electronics with Si‐based digital electronics, and demonstration of flexible CNT self‐aligned T‐shape gate RF transistors. ❧ In summary, this dissertation presents my contributions to CNT‐based RF electronics and macroelectronics. CNT‐based electronics show the great potential for next‐generation high‐performance RF electronics and macroelectronics.