Physicists have long dreamt of a transistor that can be switched on and off by just one electron. To date, most prototype devices have only worked at very low temperatures, but a buckled carbon nanotube could be the answer, according to Cees Dekker and co-workers at Delft University of Technology in the Netherlands. Dekker's group has successfully controlled the current flow in a portion of such a tube at room temperature by loading individual electrons into it (H Postma et al 2001 Science 293 76).
Conventional transistors require millions of electrons to operate, so a single-electron version would enable electronic circuitry to occupy just a fraction of its present size. Transistors have three terminals: the source, drain and gate electrodes. The gate controls the electron density in the central region of the transistor, which is usually a semiconductor. If the electron density is high, current flows from the source to the drain. If it is low, the current is switched off.
Using an atomic force microscope, Dekker and colleagues made a kink at each end of a carbon nanotube, which was 25 nanometres long and one nanometre in diameter. Carbon nanotubes are rolled up sheets of graphite that can conduct electricity. When the team connected electrodes to the kinks, however, they found that the tube no longer conducted because the buckled regions blocked the current. At room temperature, the tube had a resistance of around 500 000 ohms.
But when a bias voltage was applied across the electrodes, the tube started to conduct again. Dekker and colleagues could also manipulate the conductance of the tube by applying a voltage directly to the section of the tube between the kinks. This region corresponds to the semiconductor portion of a conventional transistor. This ‘gate voltage’ was applied via the silicon substrate on which the nanotube rests.
Dekker and colleagues also found that the conductance of the device rose and fell repeatedly as the bias and gate voltages increased. This is due to the ‘quantised’ nature of the process – the current can flow when a single electrons tunnels through a kinked barrier into the central region. An electron needs a particular amount of energy to do this – known as the Coulomb energy – and this is provided by voltages that correspond to exact multiples of the charge on an electron.
“We’ve added yet another important piece to the toolbox for molecular electronics”, says Dekker. “The next step is to think about how to combine these elements into complex circuits”.
