The University of Texas, Dallas, has made flexible CMOS by combining amorphous silicon and polymer semiconductors.
Both materials are compatible with flexible substrates, but individually cannot produce CMOS as viable p-channel transistors are impossible in amorphous silicon (a-Si:H), and n-channel transistors are proving difficult in polymers.
As a result, flexible electronics is almost exclusively limited to n-mos active-load logic, and other power hungry circuits.
For example, said the University, an n-mos a-Si:H flexible display column driver consumes around 150µA static current per column - equating to a total column drive power of 700mW in an n-mos-only 15Hz QVGA display, compared with 18mW for the backplane and 660µW for the row drivers.
To make power-efficient CMOS, the researchers first fabricated n-channel transistors in a-Si:H silicon on a polyethylene napthalate (PEN) substrate, then made p-channel devices on top using pentacene.
The result is thin-film n-mosfets with a saturation mobility of 0.8cm[super2]/Vs and an on/off ration above 108, and p-mosfets with a saturation mobility of 0.08cm2/Vs and an on/off ratio better than 104.
Combining amorphous silicon and pentacene transistors allows CMOS logic to be deposited on flexible substrates.
A CMOS inverter with a p-channel pentacene transistor at the top, and an amorphous silicon n-channel device below.
An equivalent column driver in hybrid CMOS, said the University, consumed under 1µA, and dissipated only dynamic power - a total of 2.3mW for all column drivers in the 15Hz QVGA display.
The University has focussed on column drivers because all column drivers are active all the time in a display, while only one row driver at a time on.
Part of the project is an attempt to move driver electronics on to the active backplanes of electrophoretic displays - the displays favoured in e-books.
To be larger than 150mm, these displays have to be on flexible substrates as glass is too brittle.
Moving the drivers to the display substrate cuts the number of connections to the substrate dramatically as data can then be shifted onto the display as a serial stream.
The poor mobility of both a-Si:H and pentacene transistors, leading to slow operation, precluded the use of a single bit stream and one long shift register to deliver column data.
Instead, a set of parallel shift registers was implemented, reducing the number of connections to the display up to seven times compared with off-substrate drivers.
Two bits per column were implemented, giving the QVGA display four grey levels.
The characteristics of both a-Si:H and polymer transistors are known to drift substantially with electrical stress.
Test devices running at 20V showed 15 and 35% drop in mobility over three days for a-Si:H and pentacene transistors respectively, and a inverter made using the hybrid CMOS failed after 19 hours of switching, probably due to the failure of the polymer transistor's parylene gate dielectric.
The parametric drift of a-Si:H is well known and it compensated for in commercial applications.
The University is now working on predictive tools for pentacene transistors.
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