Nano Science (Lec 10 Nanoelectronics Beyond Si)

Nano Science (Lec 10 Nanoelectronics Beyond Si) - MAE...

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MAE 287/EE 257 1
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1822 Mechanical Computer, UK 200 AD “Saun - pan” China 1943, ENIAC, US 3000 BC Sand abacus Babylonia 1948 Transistor, US ? 2
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Resource: International Technology Roadmap for Semiconductors http://public.itrs.net/Home.htm 3
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CNT transistors are currently the focus of considerable attention due to their many remarkable properties: Small diameter. Some of the semiconducting CNTs have bandgaps. Stable properties (large current density, high mobility) 4
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CNT transistor is a 1D device due to the small diameters of CNTs . The 1D nature of CNT transistor draws performance comparisons to Si nanowire transistors, which have exhibited superior scalability over even double-gate FETs. CNTs offer several advantages over Si nanowires. They are about 5× smaller in diameter than the smallest Si nanowires (no short-channel effect). Si carrier mobility degrades, especially for holes, as the Si becomes small. CNTs also have atomically smooth surfaces which is good for ballistic transport and higher on-currents than Si nanowires (low defect density). 5
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CNT has superior characters compared with Si MOS transistor CNT Si MOS 6
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Integration of CNT into large scale circuits Inhomogeneous properties of metal/semiconductor and single/multi-wall CNTs While CNTs are very attractive for use in extremely scaled ICs, methods must be developed to precisely and consistently control the structure (e.g., desired positions, diameters, and lengths), and thus the electrical properties (metal or semiconductor), of CNTs during synthesis. 7
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Unlike silicon and CNT, which has a bandgap, graphene does not have a gap, so it can’t be ordinarily turned it on and off like a transistor. But a bandgap can be opened by confining the electrons in a very graphene narrow ribbon (<20 nm in width), which can result in a room-temperature graphene transistor. Graphene ribbon 8
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High-speed graphene transistors were fabricated with a self-aligned nanowire gate. A Co2Si–Al2O3 core–shell nanowire is used as the gate, with the source and drain electrodes defined through a self-alignment process and the channel length defined by the nanowire diameter. The graphene transistors with a channel length as low as 140 nm have been fabricated. Measurements demonstrate that the self-aligned devices have a high intrinsic cut- off (transit) frequency up to 300 GHz. The reported intrinsic frequency of the graphene transistors is comparable to that of the very best high-electron-mobility transistors with similar gate lengths. Lei Liao, et al, Nature 467, 305 (2010).
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10
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Graphene has one significant disadvantage compared with the silicon-it has low energy bandgap. Although graphene can be switched between different states of electrical conductivity—but the on/off ratio isn't very high (~30). That means that unlike silicon, which can be switched off, graphene continues to conduct a lot of electrons even in its "off" state. A chip made of billions of such transistors would waste an enormous amount of energy and therefore be impractical.
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Nano Science (Lec 10 Nanoelectronics Beyond Si) - MAE...

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