The University of Durham has produced an all-organic memory-transistor.
"It is an evolution of silicon flash memory," Professor Mike Petty told Electronics Weekly. "Our step has been making both the semiconductor and the insulator organic."
In the Durham device, the equivalent to a flash transistor's floating gate is a layer of gold nano-particles.
These particles are buried in the interface between the mosfet's polymethylemethacrylate (PMMA) gate insulator and a pentacene channel.
Each speck of gold is insulated from the surrounding pentacene by a mono-molecular coating of polymer, known as a 'capping layer', applied before the device was assembled.
According to Petty, a negative potential on the gate (the device is a p-channel mosfet) not only turns on the device, but causes holes to tunnel into the gold particles, deplete the particles of electrons and giving them a positive charge.
This charge remains trapped in the gold and can represent a stored bit as it continues to bias the channel when the gate potential is removed.
A positive gate bias tunnels electrons into the gold particles, giving them a negative bias which also remains when the gate is released.
The geometry of the device is such that drain and source potentials cannot apply sufficient electric field to the particles to cause tunnelling.
With a 10V read bias on the gate, drain currents of 50nA and 20pA represent the two stored states. These levels are fairly stable, drifting to 20nA and 1pA over a few days.
Clear hysteresis which can reliably be read for date storage is exhibited by the experimental memory transistors, said Petty, although: "If you are going to move to practical devices, you need to make the hysteresis larger by storing more charge on the particles," he said. For this "you need to look at the density of particles and the capping layer".
At the moment, several tens of volts is required on the gate of the transistor to programme it - inconvenient for practical circuits.
Petty points out that many organisations have reduced organic transistor operating voltages to below 10V. "It was easier for us to make tens-of-volts-devices because they have less stringent requirements on the transistor insulator.
Although there is much research and development aimed at organic devices, they are still far from as durable, or as fast, as silicon semiconductors. And it is highly unlikely they will ever catch up, but they will be cheap to make.
Petty sees organic semiconductors making their way into flexible applications - smart textiles for example, and in low-cost applications such as ID tags and smartcards.
Built on a glass substrate, the memory-transistor is a bottom gate mosfet with an aluminium gate layer deposited on directly onto the glass. Next to be deposited is a polymethylemethacrylate (PMMA) gate insulator, followed by the layer of gold particles which self-assembles from solution. On top of this comes the channel, formed from the polymer semiconductor pentacene, then the drain and source electrodes.
The research at Durham was funded by the Engineering and Physical Sciences Research Council (EPSRC).