I. Introduction
Oxide semiconductor thin-film transistors (TFTs) provide high performance with low processing temperature. TFTs with amorphous InGaZnO (IGZO) layers deposited at room temperature by RF sputtering have demonstrated mobility [1], [2]. We have reported ZnO TFTs on both glass and flexible substrates with field-effect mobility fabricated using a weak oxidant plasma-enhanced atomic layer deposition (PEALD) process at 200°C [3], [4]. Much of the current interest in oxide semiconductors is for display backplanes, and many prototypes have been demonstrated using IGZO TFTs [5], [6]. However, the high mobility and low-temperature processing of oxide semiconductor TFTs allow broader applications such as sensors, flexible-substrate electronics, and medical electronics [7], [8]. Complementary logic is preferred for circuit applications because it provides simple circuit design, low power consumption for digital circuits, and high gain for amplifiers. Zinc-based oxide semiconductors are good candidates for -type channels, but only a few -type oxide TFTs deposited at low temperature have been reported [9]. Simple circuits based on all -type oxide TFTs have been reported by different groups [10]–[12]. Previously, we have demonstrated PEALD ZnO 15-stage ring oscillators with propagation delay per stage on both glass and flexible substrates [3], [4]. However, all-NMOS circuits typically use saturated loads, which results in low gain for inverters and amplifiers [10], [11]. Also, although relatively low voltage operation for oxide TFT circuits has been demonstrated [3], [12], even lower voltage operation is important for compatibility with Si low-voltage CMOS circuits.