Low-Voltage Double-Gate ZnO Thin-Film Transistor Circuits

In this letter, we report double-gate ZnO thin-film transistor (TFT) circuits deposited by plasma-enhanced atomic layer deposition that are suitable for low-voltage operation. Compared to bottom-gate-only ZnO TFTs, double-gate ZnO TFTs have improved mobility, subthreshold slope, and bias stability. In this letter, the TFT top gate is used to adjust the bottom-gate turn-on and threshold voltage. This allows the logic transition point for circuits to be adjusted for operation at a low voltage. Using this approach, high-gain inverters (gain > 100) and low-voltage ring oscillators using double-gate TFTs have been demonstrated. Double-gate inverters with a beta ratio of 5 have a gain larger than 100. Fifteen-stage double-gate ZnO TFT ring oscillators operate with V-DD = 1.5 V, I-D = 28 mu A, and propagation delay of 2 mu s per stage.