This article investigates the stability of hydrogenated amorphous silicon (a-Si:H) p-i-n junctions employed as temperature sensors in lab-on-chip (LoC) applications. The devices have been tested under forward current injection and different temperatures (from room temperature up to 90 °C) in order to reproduce the practical operating conditions. Two sets of devices with different diborane concentrations in the p-doped layer have been investigated as a case study. For both sets, a temperature-drift error of 0.05 °C/h and a sensitivity around 3 mV/°C in the range between 30 °C and 90 °C have been achieved. These results demonstrate the device suitability as a thin-film temperature sensor integrated into LoC systems that implement thermal treatment of the biological samples as, for example, DNA amplification.