Highly sensitive semiconductor NOx gas sensor operating at room temperature

The novel semiconductor NOx gas sensor based on a heterojunction structure workable at room temperature has been investigated. The sensor consists of Pt/Tin-dioxide(SnO2)/n-Si/p+-Si/Al in which vertical direction current between Pt and Al electrode was measured with applying reverse direction bias voltage on Pt electrode. All the films including SnO2 with the thickness of 50–200 nm were deposited on an epitaxial layer of n-Si over p+-Si substrate by RF sputtering method. As a result, the current in the sensor decreased when the gas flow was switched from dry air to mixed gas of air and NOx. Clear response was obtained at the NOx gas concentration as low as 1 ppm at room temperature, while the almost no response was observed for the n-Si, p-Si, and p-Si/n+-Si substrate. The generation of large change in current for the sensor was considered that the barrier height change or conductivity change of SnO2 gas sensitive layer may cause the modulation of the depletion layer at the n/p+-junction of Si substrate.