Field Effect Transistors with Semiconducting Nanotubes

Figure 2 shows typical room-temperature current-voltage (I-V) characteristics for semiconducting nanotubes [12,13,14,15]. By sweeping the gate voltage from a positive value to negative, the I-V curve is changed from highly nonlinear insulating behavior with a large gap to linear metallic behavior, and the linear-response conductance is increased by many orders of magnitude (see inset). The I-V characteristics indicate that the nanotubes are hole-doped semiconductors and the devices behave as p-type Field-Effect Transistors (FETs). However, the exact doping and transport mechanism is unclear. Scanning tunneling spectroscopy measurements indicated that nanotubes are typically hole-doped by the underlying metal surface due to the high work function for the metal, and that the valence band edge of semiconducting tubes is pinned to the Fermi energy of the metal [16]. Based on this observation, Tans et al. explained the FET operation using a semiclassical bandbending model [12]. In addition, Martel et al. measured a large hole density that led them to suggest that the nanotubes are doped with acceptors, as a result of their processing [13]. They also suggested that transport is diffusive. Interestingly, it has been proposed that semiconducting nanotubes are more sensitive to disorder than their metallic counterparts [17,18]. By using a conducting tip in an AFM as a local gate, Tans et al. [19] and Bachtold et al. [20] found significant potential fluctuations along semiconducting nano-tubes. However, the microscopic origin of the disorder remains to be sorted

Fig. 2. Transport characteristics for a field-effect transistor employing semiconducting nanotubes (from [12])

Fig. 2. Transport characteristics for a field-effect transistor employing semiconducting nanotubes (from [12])

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