SmartKem has recently published research of the fabrication of a truFLEX based all-organic 5-stage OTFT ring oscillator using solution coating that operates with an oscillation frequency above 500 kHz and stage delay of less than 200 ns. According to the research this is the highest frequency reported to date for a unipolar ring oscillator fabricated using a solution processed organic semiconductor and gate dielectric. It is also expected that with trivial optimisation of the circuit design, oscillator frequencies well above 1MHz can be easily attained.
Integrated circuits based on organic thin film transistors (OTFTs) have been in development for over 15 years with development scientists working on both the speed and complexity of integrated circuits for a range of electronics applications. However, one of the main barriers to the growth of printed electronic circuits was the ability to manufacture circuits using high-throughput, solution-coating techniques with the necessary semiconductor performance to allow fast logic circuits to be realised in standard digital circuits such as ring oscillators.
Figure: Smartkem - Ring oscillator circuit
According to Smartkem this result demonstrates that the historic barrier to realising fast, solution printed electronics no longer exists, opening the door to a wide range of exciting application possibilities.
For instance with the emergence of the internet of things, one market requirement that has not been met is for low cost, thin, flexible and disposable RFID and NFC tags for the transmission of data from devices, sensors or objects.
To achieve the necessary bit rate of 53kbits/s for a RFID (Radio Frequency Identification) or NFC (Near Field Communication) device operating at the standardised frequency of 13.6MHz, a stage gate delay in the logic of less than 400ns is needed.
SmartKems said that their ring oscillator can operates below this requirement at 200ns, making it a potential option contender for fully printed, low cost, flexible electronics.
However, Smartken believes that there is a more immediate and pressing application - TFT backplane platform for the manufacture of conformal LCD and flexible AMOLED displays. Narrow bezels can now be achieved by the use of 'integrated gate drivers' fabricated on the display backplane. Whilst it was once thought that this could only be achieved using inorganic materials such as LTPS, this new result shows that SmartKem’s tru-FLEX would allow the manufacture of integrated gate drivers that would not only drive 4K2K displays but, would offer the unique capability of true physical flexibility.
Steve Kelly, CEO and Founder of SmartKem, said, "This is a great result and a testament to SmartKem’s world beating end-to-end capability in material and transistor expertise. This emphasises the enabling nature of materials innovation in driving forward the potential of flexible electronics, not just for application to smart, flexible electronics for the Internet of things for instance, but also the unique position it holds in enabling the growth of the flexible display industry."
Organic ring oscillators with sub-200ns stage delay based on a solution-processed p-type semiconductor blend
Watson, C. P. | Brown, B. A. | Carter, J. | Morgan, J. | Taylor, D. M.
First published: 8 January 2016 | DOI: 10.1002/aelm.201500322
High-frequency ring oscillators with sub-microsecond stage delay fabricated from spin-coated films of a specially formulated small-molecule/host-polymer blend are reported. Contacts and interconnects are patterned by photolithography with plasma etching used for creating vias and removing excess material to reduce parasitic effects. The characteristics of transistors with 4.6 μm channel length scale linearly with channel width over the range 60–2160 μm. Model device parameters extracted using Silvaco's Universal Organic Thin Film Transistor (UOTFT) Model yield values of hole mobility increasing from 1.9 to 2.6 cm2 Vs−1 as gate voltage increased. Simulated and fabricated Vgs = 0 inverters predict that the technology is capable of fabricating 5-stage ring oscillators operating above 100 kHz. Initial designs operated mainly at frequencies in the range 250–300 kHz, due to smaller parasitic gate overlap capacitances and higher supply voltages than assumed in the simulations. A design incorporating graded inverter sizes operates at frequencies above 400 kHz with the best reaching 529 kHz. The corresponding stage delay of 189 ns is the shortest reported to date for a solution-processed p-type semiconductor and compares favorably with similar circuits based on evaporated small molecules. Significant further improvements are identified which could lead to the fabrication of digital circuits that operate at much higher bit rates than previously reported.