Taguchi Gas Sensors (TGS)

Taguchi Gas Sensors (TGS)
At the surface of some metal oxides, oxygen adsorbs
electrons to form oxygen ions.
− k1
−
+ 2e
⇒2O
O2
The surface is depleted of electrons, change of
conductivity
∆ σ = µ
n e∆ n
This process competes with combustible gases (e.g. H 2 )
−
reacting with the O ions, restoring electrons.
k 2
−
−
+ O ⇒ H
2
0 + e
H
2
Total Change of Conductance for a given Oxygen pressure:
k1
r
∆ G ∝ [ H
2]
k2
With 0 .5 < r < 0. 9 Kinetic Constant

TGS Characteristics
TGS can be optimized for various gases:
Disadvantage: For high sensitivity, TGS need heating
large 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, Noise

Palladium-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 workfunction.
[H 2 ] modifies the threshold voltage.
Q
4qN
ss
Aε
0ε
sψ
B
V = ϕ − + 2 ψ +
TH
ms
C
ox
B
C
Basic Measurement Circuit:
ox
I
d
C
W
Vg = V
ox
2
= µ d
( Vgs
−VT
)
L
TH
µ CoxW
+
I
L
Measured Voltage Shift:
∆V
g
=
∆V
MAX
1+
C
C
PH
PH
2
2
/
/ PO
PO
2
2

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
”BioSensors”