This paper presents the fabrication and characterization of electrolyte-gate (EG) hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). In these devices, the metal gate is replaced by a Pt electrode immersed in an electrolyte. The source-drain current of these devices is modulated by the voltage applied through the Pt electrode. Device characteristics are compared with structurally equivalent top-gate a-Si:H TFTs. The EG devices show higher mobility and smaller subthreshold slope than their counterparts with metal gate and work in a narrower voltage range. EG-TFTs show chemical sensitivity, illustrated by a voltage shift in the transfer curve as a consequence of pH variation. The sensitivity of the devices to p
H is different depending on whether the top layer in contact with the electrolyte is SiO2 or SiNx.