Browsing by Subject "Thin-film transistor"
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Item Carbon nanotube thin film transistor on flexible substrate and its applications as switches in a phase shifter for a flexible phased-array antenna(2010-12) Pham, Daniel Thanh Khac; Chen, Ray T.; Akinwande, Deji; Bank, Seth; Chen, Maggie; Ho, Paul; Subbaraman, HarishIn this dissertation, a carbon nanotube thin-film transistor is fabricated on a flexible substrate. Combined printing and stamping techniques are used for the fabrication. An ink-jet printing technique is used to form the gate, source, and drain electrodes as well as the dielectric layer. A self aligned carbon nanotube (CNT) thin film is formed by using a new modified dip coat technique before being transferred to the device substrate. This novel modified dip-coat technique utilizes the capillary effect of a liquid solution rising between gaps to coat CNT solution on a large area of the substrate while consuming minimal CNT solution. Several key solutions are addressed to solve the fabrication problems. (1) The source/drain contact with the CNT channel is developed by using droplets of silver ink printed on the source/drain areas prior to applying CNT thin. The wet silver ink droplets allow the silver to "wet" the CNT thin-film area and enable good contact with the source and drain contact after annealing. (2) A passivation layer to protect the device channel is developed by bonding a thin Kapton film on top of the device channel. This thin Kapton film is also used as the media for transferring the aligned CNT thin-film on the device substrate. Using this technique, printing the passivation layer can be avoided, and it prevents the inter-diffusion of the liquid dielectric into the CNT porous thin-film. (3) A simple and cost effective technique to form multilayer metal interconnections on flexible substrate is developed and demonstrated. Contact vias are formed on the second substrate prior bonding on the first substrate. Ink-jet printing is used to fill the silver ink into the via structure. The printed silver ink penetrates through the vias to contact with the contact pads on the on the bottom layer, followed by an anneal process. High drain current of 0.476mA was obtained when V[subscript G]= -3V and source-drain voltage (V[subscript DS]) was -1.5V. A bending test was performed on the CNT TFT showing less than a 10% variation in performance. A bending test was also performed on via structures, which yielded less than a 5% change in resistance. The developed CNT TFT is used to form a switch in a phase shifter for a flexible phased-array antenna (PAA). Four element 1-dimensional and 2-dimensional phased-array antennae are fabricated and characterized. Multilayer metal interconnects were used to make a complete PAA system. For a 2-bit 1x4 PAA system, by controlling the ON/OFF states of the transistors, beam steering of a 5.3GHz signal from 0° to -27° has been demonstrated. The antenna system also shows good stability and tolerance under different bending radii of curvature. A 2-bit 2x2 PAA system was also fabricated and demonstrated. Two dimensional beam steering of a 5.2GHz signal at an angle of [theta]=20.7° and [phi]=45° has been demonstrated. The total efficiency of the 1-dimensional and 2-dimensional PAA systems are 42% and 46%, respectively.Item Thin-film transistor circuits based on inkjet printed single-walled carbon nanotubes and zinc tin oxide(2015-12) Kim, Bongjun; Dodabalapur, Ananth, 1963-; Akinwande, Deji; Chen, Ray; Lee, Jack; Viswanathan, T. R.; Yu, GuihuaRecently, various novel functional materials and low-cost device fabrication techniques have emerged in the field of thin-film electronics. Active semiconductors in the form of thin-films are one of the critical components in thin-film transistors (TFTs) to achieve high-performance large-area electronics. Semiconducting single-walled carbon nanotubes (SWCNTs) and amorphous zinc tin oxide (ZTO) are considered to be some of the most promising semiconductors for TFTs due to their advantages such as high electrical performance, air-stability, and optical transparency. In this dissertation, SWCNTs and ZTO are employed as p-channel/ambipolar and n-channel semiconductors in TFTs, respectively, and integrated into various circuits through use of the cost-effective inkjet printing technique. High-performance p-channel TFTs are demonstrated by using single-pass inkjet printing of SWCNTs. Dense uniform networks of SWCNTs are formed in the channel of TFTs with single-pass printing after application of UV O3 treatment on the dielectric surface for suitable surface energy modification. By employing these SWCNT TFTs for p-TFTs along with ZTO n-TFTs, high-speed complementary circuits are demonstrated with low power consumption. The material combination of high-performance inkjet printed n- and p-channel semiconductors results in the fastest ring oscillators (ROSCs) among previously reported ROSCs where printed semiconductors were utilized. Furthermore, adding additional top-gate dielectric and top-gate electrode layers on top of the ROSCs can impart new functionalities that can be used to control the oscillation frequency of the ROSCs linearly with applied top-gate bias. Various basic circuits are also demonstrated by using inkjet printed ambipolar semiconductors. SWCNTs and ZTO, employed as p- and n-channel semiconductors for individual TFTs, turn into an ambipolar semiconductor when they are printed in a bilayer heterostructure. The bilayer ambipolar TFTs show high electron and hole mobilities in air, and ROSCs based on the ambipolar TFTs show the fastest oscillation frequency among the best reported ambipolar TFT-based ROSCs. Ambipolar SWCNT circuits are also demonstrated by encapsulating SWCNTs with aluminum oxide (Al2O3) layer deposited by atomic layer deposition (ALD). These ambipolar circuits are realized on flexible plastic substrates with inkjet printed electrodes, and show high operational and environmental stability.