Ion sensitive field effect transistor with amorphous tungsten trioxide gate for pH sensing

Abstract

In this study, the pH sensitive properties of the amorphous tungsten trioxide (a-WO 3) thin films by rf sputtering system from a-WO 3 target have been investigated. The a-WO 3 thin films with 600–4750 Å thickness were deposited on the electrolyte–insulator–semiconductor (EIS) structure maintained at room temperature and a total pressure of 30 mTorr in Ar mixed O 2 gas for 0.5–2 h, and we could obtain the electrical resistivity of the a-WO 3 films, was about 7.8×10 5–4.5×10 9 Ω-cm. The EIS structure with a-WO 3 thin films can be used to detect the ion sensitivity and can be explained by C– V curve in the different acidic buffer solutions (pH=1–7) using the C– V measurement. In addition, the a-WO 3 thin films were also deposited on the double layer structure of a-WO 3/SiO 2 gate ion sensitive field effect transistor (ISFET), and these devices were packaged with epoxy. Then, we can obtain the shift of the linear region threshold voltage (Δ V T) of the ISFET devices in the acidic solutions (pH=1–7). The a-WO 3 materials exhibited a fairly high response, and the sensitivity was about 50 mV/pH.