Highly Sensitive NO_x Gas Sensor with Heterojunction Structure on the n/p⁺-Si Substrate Operating at Roon Temperature

Abstract

The novel semiconductor NO_x 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_??_200nm 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 NO_x. Clear response was obtained at the NO_x gas concentration as low as 1ppm at room temperature, while the almost no response was observed for the n-Si and p-Si substrate. The generation of large change in current for the sensor with heterojunction structure was considered that 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.