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Principles of Metallic Field Effect Transistor (METFET)

S.V. Rotkin and K. Hess
University of Illinois at Urbana-Champaign, Beckman Institute for Advanced Science and Technology, US

Keywords: metal FET, nanotube, molecular devices, theory, symmetry breaking

Abstract:
Field effect transistors in current use are semiconductor devices. The scaling trend to nanometer dimensions calls for ever higher doping and channel conductance of these devices. Ultimately one desires a conductance close to that of a metal if one wants to scale devices to the smallest possible size. However, a metallic conductance also prevents the penetration of the electric field except for extremely short distances, too short to achieve device function. We propose a new approach to control electronic transport in metallic one dimensional (1D) systems by use of the inhomogeneous electric field induced by a highly localized gate such as an STM tip (Figure 1), nanotube tip [2] or nano-interconnect. The highly localized gate induces a high electric field in a narrow region. The weak screening of electric potentials in 1D channels enhances any possible field effects. Most importantly, inhomogeneous electric fields lead to the opening of a band gap in carbon nanotubes due to the symmetry breaking. Because of the possibility to open a band gap by use of inhomogeneous electric fields, metallic carbon nanotubes are suitable material to design metallic field effect transistors (METFET's). The band-structure of the metallic nanotube has been studied by tightbinding techniques [1]. We propose two possible mechanisms for the local band gap opening: (i) The non-linear Stark effect and (ii) band-structure modulation by a very non-uniform (multipole) electric field. The gated region of the channel (with the semiconductor band gap) represents a barrier for charge carriers, that can be modulated by the gate voltage. We calculate the reflection coefficient taking into account tunneling and (at non zero temperature) thermionic emission for a typical tube of 1.4 nm diameter. The reflection at the barrier is low unless the gap induced by the gate field is very large. Low enough reflection prevents the formation of a Coulomb blockaded island in the gated region [3]. However, at high gate voltage the reflection grows and the Coulomb blockade at the interface may add to the impedance of the channel in OFF state. We present currentvoltage characteristics of METFET's (shown in Figure 2) for a [10,10] metallic (1.4 nm) nanotube channel as well as the OFF/ON current ratio. We also have studied the temperature dependence of METFET's as well as the scaling properties with respect to the local gate width. The new transistor type is promising for a variety of applications, particularly because of the metallic conductance in the ON state. 1. http://www.itg.uiuc.edu/exhibits/iotw/2003-09-09/ 2. Y. Li, SV. Rotkin, U. Ravaioli, Nano Letters 3 (2), 183-187, 2003. 3. K.A. Matveev, Phys. Rev. B 51, 1743-1751, 1995.

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