Four-Terminal Polycrystalline-Silicon Vertical Thin-Film Transistors on Glass Substrates for pH Sensor Applications

We developed polycrystalline silicon (poly-Si) vertical thin-film transistors with a four-terminal structure. The TFT has two gates, called the top gate (TG) and bottom gate (BG), at opposite sites of the vertical poly-Si channel. The TG and BG are connected to the sensor and detection circuit, respectively. The variations in the pH of the solution were successfully detected by varying the output voltage of the outer circuit.