Flexible electronics have considerably progressed in the last decade with the advent of novel applications, such as electronic paper-like displays, artificial skin and electronic tattoos that can be directly applied to the skin or even onto internal organs like the heart. However, most of these devices are made from organic materials like polymers, carbon-based nanowires and nanotubes. These materials are far from ideal in that they have much slower processing speeds than their silicon counterparts, which make up 90% of all electronics devices manufactured today. They are not very robust to higher temperatures either and cannot be formed as continuous films – unlike bulk silicon that can.

There are also other problems in that integration densities (the number of transistors that can be packed onto a single chip) for these organic materials are also much lower than those for the silicon-based transistors found in today's microprocessors. And although inorganic flexible electronics do exist, they rely on expensive substrates such as SOI, UTSOI and silicon (111).

Flexible and semi-transparent FinFETs

Earlier this year, a team led by Muhammad Hussain succeeded in using traditional state-of-the-art CMOS-compatible processes to transform silicon-on-insulator (SOI)-based FinFETs into flexible and semi-transparent silicon-on-polymer ones, while retaining silicon’s high performance and integration density.

Indeed, the FinFETs made had outstanding electrical and physical properties – for example, PMOS devices boasted high sub-threshold swings and high current on-off ratios. The devices were also semi-transparent.

Now, Hussain and colleagues have looked at how some basic FinFET electrical parameters such as leakage current (Igs), gate-induced drain leakage (GIDL) and low-field mobility (µo) of the flexible FinFETs they made change at operating temperatures of 125 °C.

The researchers found that although the mobility of charge carriers (electrons and holes) degrades at higher temperatures because of phonon scattering, the other characteristics do not appear to be affected. Phonons are vibrations of the crystal lattice.

Wide temperature range

The team obtained its results by carrying out extensive measurements in an off-the-shelf heated chamber probe station. “We measured the current-voltage characteristics of our FinFETs using a Keithley SCS-4200 semiconductor analyser on a ‘cascade probe bench’ and controlled the temperature with an ERS AirCool-Chuck System SP72 between 25 and 150 °C, over steps of 25°,” explains Hussain. “Our results clearly show that flexible FinFET devices can be used for a wide range of complex electronic devices, and over a wide range of temperatures,” he tells nanotechweb.org.

The researchers say that they are now busy studying how the properties of their FinFETs change when they are bent. “Let’s take an example – for instance, if we wear a patch containing the device on our elbow or inside our mouth in the jaw area, the device will be subject to a lot of bending. We want to understand how that bending impacts the device’s overall electrical performance.

Another example is when such a device is attached to your finger (which has a smaller bending radius). How do different curvatures affect the device’s electrical properties? Our future work will hopefully provide us with this information.”

The current work is detailed in Appl. Phys. Lett..