We propose a thermal sensor employing a ring oscillator composed of poly-Si thin-film transistors (TFTs). First, temperature dependences of transistor characteristics are compared between n-type TFTs with self-aligned and offset gate structures. It is confirmed that the temperature dependence of the offset TFT is larger. Next, a ring oscillator is composed using the n-type offset TFTs. It is clarified that the temperature can be detected by measuring the oscillation frequency. We think that this kind of thermal sensor is available as a digital device. (C) 2012 Elsevier Ltd. All rights reserved.