The first resonant tunnelling diodes to be built in amorphous carbon have been demonstrated by researchers at the University of Surrey. The result should lead to low-cost devices that can switch at gigahertz frequencies and be made over large areas at room temperature.

Resonant tunnelling diodes – in which negative resistance can be used to create high speed oscillators – have traditionally been made using high-quality crystalline materials such as GaAs.

These materials offer very good charge transport due to minimal disorder in the lattice. However, in amorphous materials such as carbon the mean free path for a carrier is very short, which limits conductivity.

"Normally the amorphous materials have low electron mobility, which limits the switching speed or oscillation frequency to low values," said Professor Jeremy Allam, deputy director of the Advanced Technology Institute at the University of Surrey. "However, the properties of our carbon tunnelling devices are consistent with fast electron tunnelling."

Quantum mechanical tunnelling is an effect in which a charge carrier such as an electron encounters an energetic barrier that it doesn’t have enough energy to climb. However, according to an alternative solution to the wave equation describing the system, it can tunnel through the barrier, its energy decaying exponentially with the barrier’s width.

By inserting a valley, or well, between two barriers it is possible to create a resonator for electrons, in which the width of the well controls the energy at which electrons can tunnel through the combined structure. When a bias is applied there is a limited range of voltages at which the electron can progress.

"What that gets you is negative resistance, because as you increase the voltage on the device electrons can tunnel through this confined state, this resonance," said Allam. "Then you go too far, and they can no longer go through this resonant value and the conductivity of the device decreases."

The key achievement was to control the proportions of diamond-like and graphite-like carbon in the layers used to build up the resonant tunnelling diodes. The Surrey scientists used laser ablation of a carbon block to generate layers of differing composition, then layered them to create the structures they needed.

www.ati.surrey.ac.uk