High voltage metal oxide thin film transistors to drive arrays of dielectric elastomer actuators

This thesis advances the field of high-voltage thin film transistors (HVTFTs) and dielectric elastomer actuators (DEAs) by demonstrating a strategy for low-voltage addressing of an array of high voltage soft actuators suspended on a flexible substrate.

First, I present the first HVTFTs operating at 1 kV drain-source voltage, switching with an on-off ratio of 20 at 80 V gate-source voltage. The HVTFTs can operate at high voltage thanks to geometrical features increasing the breakdown voltage: a thick gate dielectric composed of a bilayer of alumina (100 nm) and Parylene-C (1 um), a long semiconducting channel (500 um), and a 150 &mlong non-gated region between the drain and the gate electrode called the offset gate. The use of an amorphous oxide semiconductor (AOS), zinc tin oxide (ZTO), enables a high on-currents of 0.1 mA. The ZTO was synthesized by a sol-gel process after spin-coating on a flexible polyimide substrate, previously passivated with alumina. I optimized the HVTFT switching properties by doping the ZTO layer with yttrium (5%). It improved the on-off ratio up to 1000 at 500 V operation voltage by decreasing the leakage current down to 100 nA.

Then, I show the first integration of HVTFTs with DEAs. My ZTO HVTFTs switch DEAs on and off with only 30 V gate voltage under a bias voltage of 1.4 kV. The system time response in 50 ms. The demonstrator is a 4x4 array of diaphragm DEAs. A layer of 4x4 DEAs is suspended over a layer of 4x4 HVTFTs built on flexible polyimide. The DEAs and the HVTFTs were interconnected thanks to a flexible PCB in a resistive load inverter circuit architecture. A flexible 3D printed chamber was constantly biasing the DEA diaphragms with a back-pressure. The DEAs were made of PDMS and the active region is defined by overlapping carbon-PDMS electrodes. The device operates down to a 5mm radius of curvature.

Finally, I demonstrate latching of the HVTFT and the DEA by using triboelectric sensors. Under a constant 500 V circuit bias, the control of the HVTFT gate with triboelectric generators enabled 4s latching of the inverter output voltage at 470 V for the off-state and at 120 V for the on-state. The latching of the DEAs with the HVTFT circuit finally proves that this approach can lead to a bistable control of DEAs.

This PhD thesis results show that my HVTFTs are versatile components usable not only to address DEAs but also to interface low voltage sensors with high voltage actuators.