Impact of bending on flexible metal oxide TFTs and oscillator circuits

Thin-film circuits on plastic capable of high-frequency signal generation have important applications in large-area, flexible hybrid systems, enabling efficient wireless transmission of power and information. We explore oscillator circuits using zinc-oxide thin-film transistors (ZnO TFTs) deposited by the conformal, layer-by-layer growth technique of plasma-enhanced atomic layer deposition. TFTs on three substrates—glass, 50-µm-thick freestanding polyimide, and 3.5-µm-thick spin-cast polyimide—are evaluated to identify the best candidate for high-frequency flexible oscillators. We find that TFTs on ultrathin plastic can endure bending to smaller radii than TFTs on commercial 50-µm-thick freestanding polyimide, and their superior dimensional stability furthermore allows for smaller gate resistances and device capacitances. Oscillators on ultrathin plastic with minimized parasitics achieve oscillation frequencies as high as 17 MHz, well above the cutoff frequency fT. Lastly, we observe a bending radius dependence of oscillation frequency for flexible TFT oscillators and examine how mitigating device parasitics benefits the oscillator frequency versus power consumption tradeoff.