One limitation of films is that they must undergo post-exposure processing, whereas semiconductor FET-based devices offer the user real time measurements of the dosage. On the other hand semiconductor FETs are known to have a limited lifetime, however it turns out this could be overcome with the novel application of carbon-nanotubes in an FET arrangement.

In their experiment, the Technische Univestität Dresden researchers use a di-electrophoresis method to deposit carbon nanotubes between metallic contacts on oxidized boron doped Si substrates. The oxide layer forms the insulating part of the gate. After an initial conditioning of the device through a “burn-out” process, whereby metallic nanotubes were removed by a high through current, the electrical properties of the remaining semiconducting-type wires are found to be highly sensitive to the gate voltage and, as a result, x-ray exposure. A suggested reason for this is charge build-up in the interface region between the wire and substrate-oxide caused by electron-hole pair creation in the oxide layer and their subsequent separation by the applied voltage drop across the oxide. By varying the gate voltage the sensitivity of the device could be controlled and, if necessary, the device can be reset into its starting state. This facilitates the exposure limit-free use of the device. Moreover, like a semiconductor FET the device was able to store the previous exposure as long as no reset was applied.

A high sensitivity to the x-ray exposure was obtained, which was comparable to that of semiconductor FETs. Although further refinement of the device needs to be undertaken, these promising results hint at significant improvements on the current state of the art.

More information about this research can be found in the journal Nanotechnology 27 454003.