To communicate, spacecraft and satellites rely on microwave devices, which at present are based on relatively inefficient thermionic electron sources that require heating and cannot be switched on instantaneously. Here we describe a microwave diode that uses a cold-cathode electron source consisting of carbon nanotubes and that operates at high frequency and at high current densities. Because it weighs little, responds instantaneously and has no need of heating, this miniaturized electron source should prove valuable for microwave devices used in telecommunications.

When a carbon nanotube emitter is operated at high currents (typically above 1 microA per emitter), a small voltage drop ( approximately few volts) along its length or at its contact generates a reverse/canceling electric field that causes a saturation-like deviation from the classical Fowler-Nordheim behavior with respect to the applied electric field. We present a correction to the Fowler-Nordheim equation to account for this effect, which is experimentally verified using field emission and contact electrical measurements on individual carbon nanotube emitters. By using rapid thermal annealing to improve both the crystallinity of the carbon nanotubes and their electrical contact to the substrate, it is possible to reduce this voltage drop, allowing very high currents of up to 100 microA to be achieved per emitter with no significant deviation from the classical Fowler-Nordheim behavior.

Most long-range telecommunication systems are based upon microwave links. The transmitters use microwave amplifiers which in the very near future will be required to work at up to 30-100 GHz with output power in the region of a few tens of watts. Carbon nanotubes ͑CNTs͒, which exhibit extraordinary field emission properties because of their high electrical conductivity, ideal high aspect ratio whisker-like shape for geometrical field enhancement, and remarkable thermal stability, can be used as the emitter in such applications. This article will describe the plasma enhanced chemical vapor deposition growth of vertically aligned carbon nanotubes, and how well controlled arrays of such structures can be grown. We will also describe how high current densities of ϳ1 A/cm 2 , under direct current and 1.5 GHz direct modulation, can be obtained from CNT cathodes. These CNT cold cathodes offer considerable weight and size savings over conventional hot cathodes used in microwave applications ͑e.g., SATCOM, radar͒.

We present a fully elaborated process to grow arrays of metallic nanowires with controlled geometry and density, based on electrochemical filling of nanopores in track-etched templates. Nanowire growth is performed at room temperature, atmospheric pressure and is compatible with low cost fabrication and large surfaces. This technique offers an excellent control of the orientation, shape and nanowires density. It is applied to fabricate field emission arrays with a good control of the emission site density. We have prepared Co, Ni, Cu and Rh nanowires with a height of 3 µm, a diameter of 80 nm and a density of ∼10(7) cm(-2). The electron field emission measurements and total energy distributions show that the as-grown nanowires exhibit a complex behaviour, first with emission activation under high field, followed by unstable emission. A model taking into account the effect of an oxide layer covering the nanowire surface is developed to explain this particular field emission behaviour. Finally, we present an in situ cleaning procedure by ion bombardment that collectively removes this oxide layer, leading to a stable and reproducible emission behaviour. After treatment, the emission current density is ∼1 mA cm(-2) for a 30 V µm(-1) applied electric field.