Researchers in Stuttgart have made a 6bit DAC using organic semiconductors, claiming that it is "1,000 times faster and 30 times smaller than the currently fastest 6-bit DAC in organics".
The team, from the Institute for Microelectronics Stuttgart (IMS Chips) and the Max Planck Institute (MPI) revealed the current steering DAC at the IEEE International Solid-State Circuits Conference in San Francisco this week.
Key to the speed and low voltage operation are small feature sizes, down to 1µm, made possible by using micromachined silicon as a stencil mask.
"Most international competitors design OTFTs [organic thin-film transistors] for operation above 50V, where OTFTs switch comparably faster," said IMS Chips. "Besides supply voltage, the transistor speed is to a large extent determined by the length of the transistor's channel that can be controlled by the gate contact. A shorter channel leads to a faster transistor."
Until recently Dr Hagen Klauk's group at MPI used evaporation of materials through a plastic shadow mask, patterned by laser cutting, where the minimum feasible channel is 20μm.
Dr Florian Letzkus at IMS Chips can fabricate silicon membrane stencil masks with features down to 100nm, depending on the thickness of the membrane.
His organisation made a set of stencils for MPI with circuits and test transistors having channel lengths down to 0.8μm, allowing it "to demonstrate transistors with a more than 20-fold shorter channel than before", said MPI. "Those transistors switched more than 100 times faster than the OTFTs built by using the plastic shadow masks."
The silicon masks are also stiff and stable, providing consistent results across the circuit.
"In contrast, the plastic shadow masks tend to wrinkle and do not attach entirely to the substrate. This causes a rather large spread of device characteristics, which severely limit's the choice of circuit topologies that can be realised," said IMS.