Field effect transistor ͑FET͒-like structures with a trench as "gate" were fabricated on GaAs substrates. The bottom of the trench as well as the "source" and "drain" regions were metallized. Bundles of nanotubes were then suspended over the trench. At a certain threshold, these trenched FET-like structures display an S-shaped negative resistance, which breaks into parallel branches when the voltage is increased in equal steps. Several such steps were observed at room temperature and under

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... l pressure. The steps were reversibly enabled and disabled via changes in the applied voltage. A maximum stable gain with a quasilinear behavior was observed between 0.5 and 3.25 GHz. Carbon nanotube ͑CNT͒ transistors are extensively studied since it is believed that they will replace the existing Si transistors in numerous applications due to their superior performances. For instance, the carrier transport in CNTs is ballistic at room temperature, the mean free path being around 0.7 m, and the carrier mobility is higher than 10 000 cm 2 V -1 s -1 . The CNT field-effect transistors ͑FETs͒ display, up to now, a maximum cutoff frequency greater than 30 GHz while fabricated on many substrates ranging from doped Si to flexible plastics. 1 The configuration where CNTs are suspended over a trench between two electrodes, denoted drain ͑D͒ and source ͑S͒, with another electrode-the gate ͑G͒-located at the bottom of the trench, was studied in the following contexts: this architecture occurring in FET-like structures or in double-clamped CNTs, electrostatically actuated by the gate electrode. In this way, CNT nanoelectronics, as well as nanoelectromechanical systems ͑NEMS͒ consisting of doubleclamped nanotubes, benefit from the external control of a buried gate that monitors the electron transport or the mechanical movement of the device. The net advantage of a suspended CNT is the absence of Van der Waals forces between the nanotube and the substrate, and the tunability of the devices based on them. Double-clamped CNT NEMS resonators measure very small forces in a FET-like configuration acting as a highfrequency mixer, 2 the mechanical resonant frequency of such structures being tuned by the gate voltage, which changes the state of the double-clamped CNT from a regime without strain to a vibrating string. 3 Addressable nanoelectronic devices employing CNTs suspended over trenches were also demonstrated. 4 In addition, the electrical transport of a single CNT suspended over a trench was found to display an N-shape negative differential resistance ͑NDR͒ with low onoff ratio at low applied voltages. 5 Trenched configurations were also used in the conversion of ambipolar to unipolar transport in CNT transistors, with the purpose of achieving high-performance devices compatible with CMOS logic gates. 6 Very recently, a NEMS switch using the trench configuration was reported. 7 Tunable NDR was predicted for a single semiconducting CNT suspended over the trench, 8 the tunability being achieved through changes in both gate and drain voltages. Because of the lack of screening in CNTs, the potential profile controls the carrier transport in suspended CNTs.