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Photoresponse of Carbon Nanotube Field-Effect Transistors

Photoresponse of carbon nanotube field-effect transistors (FETs) is investigated using microscopic measurements. The nanotube FETs, with an isolated single-walled carbon nanotube (SWNT) for the channel, were fabricated by means of the position-controlled nanotube growth technique. An increase in the off-state current and the threshold-voltage shift of the FET were caused by laser illumination. The increase in the off-state current is attributed to photocurrent due to carriers excited in the SWNT channel. The excitation spectrum of the photocurrent had a peak corresponding to optical absorption by the third interband gap of the van Hove singularity of the semiconducting SWNT with a diameter of ∼2 nm. The photocurrent increased in proportion to incident laser power with a dynamic range over four orders of magnitude. The external quantum efficiency was 2×10-7. An inverter action to optical-signal input was observed near the threshold voltage of the FET. The responsivity was as high as 2×10-3 A/W for a single SWNT channel. This high responsivity is explained by the field-effect amplification phenomenon.