Basic Logic Gates and Digital Circuits Based on Flexible IGZO Enhancement-Mode nMOS TFTs

Abstract

An increasing number of new applications beyond displays have been reported for amorphous oxide semiconductors, especially indium-gallium–zinc oxide (IGZO), based on flexible thin-film transistors (TFTs). The atomic composition of the IGZO target was In:Ga:Zn = 2:2:2. From sensors to microprocessors, the flexible IGZO TFTs aim to diversify the functionality of silicon legacy nodes. In this work, IGZO enhancement-mode nMOS TFTs, with a minimum channel length of $5 \, \mu $ m, were fabricated on flexible polyimide (PI) substrates at temperatures below 150 °C. We observed a small dependency of threshold voltage ( $2.23 \, \pm \, 0.06$ V) and saturation mobility ( $3.74 \, \pm \, 0.57$ cm2/V $\cdot $ s) with the channel width-to-length ratio. The IGZO TFTs were interconnected in the saturation load configuration for the successful demonstration of basic logic gates (inverter, NAND, and NOR gates), a seven-stage ring oscillator, an xor gate, and a half-adder. Due to this configuration, their output voltage was limited to VDD–VTH. Their electrical response was evaluated with direct and alternating voltages at a maximum input frequency of 500 Hz and a power supply of 5 V. Under ac analysis, their fall time/rise time were in the 3-43/148-212- $\mu $ s range. The inverter, as the fundamental building block for digital circuits, was also tested at different bending radii from 10 to 2.5 mm. Our results are oriented to a better understanding of the flexible IGZO TFT technology to reach reliable integrated circuits (ICs).