A multi-scale model is presented that captures the experimentally observed behaviour of electroluminescence (EL) in carbon nanotube field-effect transistors (CNFETs) under ambipolar bias conditions, namely variations in mobile EL intensity, localized EL at a contact, and localized EL at a charge defect. A full, quantum mechanical approach is used to describe tunnelling and thermionic emission at the contacts, and the drift-diffusion equations, with a field-dependent mobility, are used for transport in the long devices (CN length > = 10 mu m). We find that contact-localized EL is only present when the height of the Schottky barrier at the ends of the CN favours the injection of one type of carrier. Charge defects on the CN surface also lead to localized EL, which is present only under certain bias conditions.
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