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TT17.08 - 10MHz Cross-Coupled LC Oscillators Based on Plasma-Enhanced ALD Zinc Oxide Thin-Film Transistors 
April 10, 2015   3:30pm - 3:45pm

In this work, we present cross-coupled plasma-enhanced ALD (PEALD) ZnO thin-film transistor oscillators that operate at frequencies of 10MHz. To be compatible with flexible substrates used in large-area electronic systems, thin-film transistor (TFT) circuits are processed below 250oC. However, such TFTs typically exhibit reduced cutoff frequencies ft compared to their high-temperature counterparts. Resonant TFT circuits can enable oscillation frequencies above ft by negating the impact of parasitic capacitances within the resonant network. We find numerous applications for these circuits at such frequencies, including efficient wireless transmission of power and signals [1,2]. Previously, we demonstrated a-Si cross-coupled oscillators with 5MHz oscillations at an overdrive voltage VOV of 12V [3]. By changing the TFT technology in our oscillators from a-Si to higher-mobility PEALD ZnO, we doubled oscillation frequency, reduced overdrive voltage by 4X, and maintained process temperatures below 200oC. Our cross-coupled LC oscillators consist of two PEALD ZnO TFTs (?>10cm2/Vs, Vth~3V, subthreshold slope~200mV/decade [4]) where the gate of TFT 1 is connected to the drain of TFT 2, and vice versa. Planar coil Cu inductors patterned on freestanding polyimide connect both drain terminals to the supply voltage Vdd, and both source terminals are grounded. Several challenges must be addressed in order to increase the oscillation frequency of these circuits. To begin oscillating, the circuit must meet a positive feedback condition: (gm/Cpar)*(L/(Rind+Rg))>1, where gm is the TFT transconductance, Cpar is dominated by the TFT gate-to-drain capacitance, L is the inductance of the Cu inductor, and Rind and Rg are the resistances of the inductor and the TFT gate [1]. A large inductor allows this condition to be met more easily, but reduces the oscillation frequency, 1/(2??(LC)). A large overdrive voltage increases gm, enabling smaller-valued inductors and thus higher frequencies, but also elevates power consumption and endangers the gate dielectric (and thus the robustness of the circuit). Our ZnO circuits were able to meet the oscillation condition at low enough L to oscillate at 10MHz at a Vdd of just 6V (3V VOV). This was accomplished by (1) substantially reducing gate resistance from 100?/sq to 10MHz. [1] Hu et al., CICC 2012 [2] Huang et al, ISSCC 2013 [3] Rieutort-Louis et al., IEEE JPV Jan. 2014 [4] Mourey et al., IEEE TED, Feb. 2010.

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