Effect of charge storage engineering on the NO₂ gas sensing properties of a WO₃ FET-type gas sensor with a horizontal floating-gate

Abstract

In this paper, we investigate the effects of charge storage engineering (CSE) on the NO₂ gas sensing properties such as response, recovery, and sensitivity of a FET-type gas sensor with a horizontal floating-gate (FG) having tungsten trioxide (WO₃) as a sensing layer. When the FET transducer is set at an erase state (ΔV_th = -2 V), the holes injected into the FG by Fowler-Nordheim (F-N) tunneling increase the electron concentration at the WO₃-passivation layer interface. Accordingly, an oxidizing gas, NO₂, can take more electrons from WO₃, which increases the change in the FG voltage (ΔV_FG) by a factor of 2.4. Also, the recovery speed of the sensor in the erase state can be improved by applying pre-bias (V_pre) which is larger than the read bias (V_read). As the carriers in the WO₃ film that can interact with NO₂ increase by the excess holes stored in the FG by the erase operation, the sensitivity of the sensor also increases 3.2 times. The effects of CSE on various sensing performances are explained using energy band diagrams.