ZnO Thin-Film Transistor Ring Oscillators with 31-ns Propagation Delay

We have fabricated ring oscillators (ROs) using ZnO thin films deposited by using a spatial atomic layer deposition process at atmospheric pressure and low temperature (200degC). Bottom-gate thin-film transistors with aluminum source and drain contacts were fabricated with a field-effect mobility of > 15 cm
²
/V ldr s. Seven-stage ROs operated at a frequency as high as 2.3 MHz for a supply voltage of 25 V, corresponding to a propagation delay of 31 ns/stage. These circuits also had propagation delays of ~100 ns/stage at a supply voltage of 15 V. To the best of our knowledge, these are the fastest ZnO circuits reported to date.