How does a Ion-sensitive Field Effect Transistor work (accumulation)
φ
Ox
Charge
φ 
C0
n-Si
DNA, Protein, etc.
Qbio = QMOS = C0φ
I =QMOSυ =Qbio µVD
L
The response is proportional to the total charge of the biomolecule
Alam, Principles of Nanobiosensors, 2013 |
6 |
ISFET in electrolyte ( accumulation)
φ
electrolyte
Metal |
Ox |
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CDL C0
n-Si
Qbio =QL +QR
=CDLφ +C0φ
φ= QQbiobio (pH)
CDLDL (+saClt0) +C0
QMOS =C0 C QbiobioC
0 DLDL + 00
II =QMOSυ
Electrolyte reduces sensitivity
Alam, Principles of Nanobiosensors, 2013 |
7 |
Calculating CDL: screening in 1D
2ψ = |
d dψ |
= − |
ρ |
DL |
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κwε0 |
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=− zq (n+ −n− )
κwε0
=2zqI0 Nav sinh(zqψ kBT )
κwε0
≈ − |
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(if ψ ~ k T |
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B L |
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ψ(x) =ψ0 ×e−x
LD
ψ(0) =ψ0 , ψ(∞) = 0
n− = I0 Nave+zqψ 
kBTL
n+ = I0 Nave−zqψ 
kBTL
L2 |
≡ |
κwε0kBTL |
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D |
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2z2 I0 Nav q2 |
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Alam, Principles of Nanobiosensors, 2013 |
8 |
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Centroid of DL charge & capacitance
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∞∫xρDL (x)dx |
= L |
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∞ |
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D |
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∫ρDL (x)dx |
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0 |
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ρDL (x) = − |
zq |
(n+ −n− ) |
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κwε |
0 |
LD |
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zq |
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zq |
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≈ − |
κ |
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k |
×ψ |
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w |
ε |
0 |
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T |
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B L |
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z2q2 |
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CDL = kwε0 |
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×ψ(0)e−LD |
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κwε0kBTL |
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LD |
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9 |