Semiconductor Carbon Nanotube Diode Design Simulation Gary Pennington and Neil Goldsman,Department of Electrical and Computer Engineering Motivation: Band edge of junction CNT Y-junction array Recently techniques for the production of junctions between different carbon nanotubes (CNT) have been developed. The experimental observation that these contacts exhibit nonlinear IV characteristics leads to their potential application as molecular nanoscale device components. Objective: Develop tube-diameter dependent models for the effective mass, doping, band offset, and dielectric constant of a CNT and then, using standard heterojunction physics, relate the physical properties of a y-junction array of CNTs (diameters) to its electrical properties (IV). Unit cell Side 1 Side 2 •Electron orbital hybridization gives acceptor density: β N,β constants •Average bond energy method gives band offset: ΔEv a c c 1 d2 1 d1 •Linear response approximation gives dielectric constant : d NA ε * m X (CNT number in array) Effective mass averaged over indices (n,m) * ac c d 1. 3m e Effective mass with varying dr (gcd) m e :Electron mass IV plot for a y-junction array -V N A N ac c d m Laboratory •Treating semiconducting CNT y-junction as a thermionic emitter of holes, obtain the forward bias current: eV a 1 d 1 d c c 2 1 I d d V I o d1 , d 2 exp 1 2 k T 18 d d 2 β 2 1 2 b 2 β 2 β 3 2 β 1 2 18 d1 d2 d ac c 2 1 2 β Io d 2 d 2 1 a c c:C bond length Average effective mass d :diameter Upper valence bands of CNT 2 eN2 a c c π kb T 10 •Comparison with experiments of: [ C. Papadopoulos et al., Phy. Rev. Lett. 85, 3476 (2001). ] β 2. 74 N2 3X10 28 3 cm Conclusion: • Theory for CNT diameter dependence of effective mass, doping, band offset, and dielectric constant useful in predicting electric properties of rectifying junctions. • Comparison with limited data available gives a donor level of about one in every 60 Carbon atoms in the CNT.