Ring Oscillator Circuit Based on ZnO Thin-film Transistors Fabricated by RF Magnetron Sputtering

A 5-stage ring oscillator (RO) composed of zinc-oxide (ZnO) thin-film transistors (TFTs) with a bottom-gate configuration was fabricated on a glass substrate. For the formation of the channel and the gate dielectric layers of the ZnO TFTs, an undoped polycrystalline ZnO film and a silicon-nitride (SiNx) film were deposited by radio-frequency (RF) magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD), respectively. The ZnO TFTs used as drive transistors in the RO circuit exhibited a field effect mobility of 0.25 cm(2)V(-1)s(-1), an ON/OFF current ratio of 2.5 x 10(4), and a sub-threshold slope of 4 V/decade. The RO circuit using the ZnO TFTs successfully operated at an oscillation frequency of 3.52 kHz under a power supply voltage of 50 V which corresponds to a propagation delay of 28.4 mu s/stage. This is the first demonstration of a RO circuit that provides dynamic performance for TFTs having a RFsputtered ZnO channel layer.