Fabrication and pH Sensing Characteristics Measurement of Back Gate ZnO Thin Film Planar FET

Abstract

Here in we demonstrate the design of a low-cost zinc oxide (ZnO) thin-film planar transistor-based pH sensor controlled by the bottom gate fabricated by a fairly simple fabrication approach. The performance of the fabricated device is evaluated by electrical as well as surface characterization. The surface morphology is analyzed by scanning electron microscope (SEM) and atomic force microscopy (AFM) and it shows surface properties that are essential required device to function as a pH sensor. The fabricated thin-film FET comprises Zinc Oxide (ZnO) as a channel layer of length 6 μ m and thickness 200 nm, Silicon Nitride (Si3N4) as a passivation layer, and Aluminum (Al) as a contact layer. The effect on pH sensitivity for varied channel lengths (6 μ m, 12 μ m, and 15 μ m) is also examined and optimum results have been achieved at channel length = 6 μ m. The change in threshold voltage (△Vth) & change in current ( $\triangle I_{\max \limits }$ ) are used as a sensing metrics to analyze the sensing performance of the device. The device shows excellent pH sensitivity in terms of average current and average voltage sensitivity 120.97 mA/pH and 97.85 mv/pH respectively at pH ranging from 3.2 to 11.1 with best pH stability (linearity) for pH value 4 to 10. The voltage sensitivity is higher than the Nernstian value (59 mv/pH) at room temperature.