Taguchi Gas Sensors (TGS) At the surface of some metal oxides, oxygen adsorbs electrons to form oxygen ions. −−⇒+ OeOk2212 Î The surface is depleted of electrons, change of conductivity nne∆=∆µσ This process competes with combustible gases (e.g. H2) reacting with the −O ions, restoring electrons. −−+⇒+ eHOHk0222 Total Change of Conductance for a given Oxygen pressure: rHkkG ][212∝∆ With 9.05.0 << r Kinetic ConstantTGS Characteristics TGS can be optimized for various gases: Disadvantage: For high sensitivity, TGS need heating Îlarge power consumption (several hundreds mW)ADFET Gas Sensor (Adsorbtion Field Effect Transistor) MOSFET-based structure with extremely thin oxide (5nm). Adsorbed molecules in the oxide modify the electric field, and therefore the current Advantage: built-in gain Disadvantage: Lack of selectivity, NoisePalladium-Based Hydrogen Sensors Pd-Gate MOS (Lundström, 1975): -Palladium gate and thin tin oxide (about 10nm) Hydrogen atoms diffuse to the Pd/Oxide interface and act like a dipoles, thus modifiying the Pd work-function. Î [H2] modifies the threshold voltage. oxBsABoxssmsTHCqNCQVψεεψϕ042 ++−= Basic Measurement Circuit: 2)(TgsoxdVVLWCI −=µÎWCLIVVgoxdTHµ+= Measured Voltage Shift: 2222/1/POPHCPOPHCVVMAXg+∆=∆Pd-MOS Response Curves This kind of device can reach a sensitivity of 1ppm in air.ISFET (Ion-Sensitive FET) Adsorbed Species modify the effective Gate-Voltage. ISFET pH sensors can reach sensitivities of 0.001pH unit.Selective ISFETS Ion-selective membranes can be added above the gate to filter specific ions. The membrane can include biological molecules (e.g. enzymes) to detect biological compounds