Patent ID: 7122103

Claim:
An air/fuel ratio detection apparatus for a gas, comprising: a heater portion that has an elongate cylindrical shape and heats by applying electricity to the heater portion from an outside; a solid electrolyte layer formed on and entirely surrounding periphery of the heater portion, the solid electrolyte layer being activated by heat from the heater portion to conduct oxygen ions through the solid electrolyte layer; a first electrode that is positioned between the heater portion and the solid electrolyte layer and is in contact with an inner surface of the solid electrolyte layer; second and third electrodes that are in contact with an outer surface of the solid electrolyte layer and are away from each other such that the solid electrolyte layer is interposed between the first and second electrodes and between the first and third electrodes; a first voltage applying device for applying a first voltage between the first and second electrodes; a second voltage applying device for applying a second voltage between the first and third electrodes, the second voltage being higher than the first voltage; and a diffusion layer made of a porous material, the diffusion layer covering the second electrode to adjust transmission of the gas to the second electrode.