University of Houston have created an elastic semiconductor with 8.57cm2/Vs charge carrier mobility, and then made transistors, logic and a sensor array from it. Carrier mobility is largely retained when stretched by 50%.

UofHouston-rubber-transistor-robotic-hand
The semiconductor concerned is poly(3-hexylthiophene) – ‘P3HT’.
This is not the first time the team has made stretchy transistors from this material, but it is the first time that such a high carrier mobility has been achieved, and retained during stretching – the result of a new fabrication technique where the initial P3HT is film cast on the surface of water.
Casting involves dissolving the polymer in toluene and carefully pouring the solution onto the water. The solution floats, and neither of its constituents dissolve in water.
As the toluene evaporates, the P3HT molecules self-assemble into an ordered film – the hydrophobic nature of the polymer encouraging its chains to pack together as they minimise water contact, leading to strong π-π stacking, according to the scientists.
Once set, with care, the film can be transferred to a substrate – in this case a thin flexible sheet of polydimethylsiloxane (PDMS).
Transistors were created by modifying the PDMS substrate prior to covering it with the P3HT film – pattering a mixture of gold nano-particles and silver nano-wires into the P3HT to create drain and source contacts.
Once all the water had dried from the P3HT-on-PDMS structure, a layer of non-conductive gel was applied to form a gate insulator.
The finished p-channel transistors worked with gate voltages from 0 to −4V offering 2×105 on-off current ratio.
Because only p-channel devices are available, logic gates were constructed with a gate-to-source-connected active load transistor at the bottom and series or parallel-connected top transistors depending on the logic function required.
The sensor, a 5 x 5 pressure sensing array, was created using a planar process that introduced a small force-sensitive resistive patch in series with each transistor drain, allowing the resistances to be read using five word lines and five bit lines.
As part of an experimental robot hand (pictured) aimed at providing tactile feedback for telemedicine, these resistive arrays were reinforced to form sensing e-skin.
This work is described in ‘Air/water interfacial assembled rubbery semiconducting nanofilm for fully rubbery integrated electronics’, a clear well-written paper published in Science Advances.
More information on the transistors structure, including gate materials, is available from the early Science Advances paper ‘Fully rubbery integrated electronics from high effective mobility intrinsically stretchable semiconductors‘.