Transcript Lecture7.2(ChemFets).ppt

Lecture 7.2
ChemFET Sensors
Bipolar Transistor

Combination of
two back-to-back
p-n junctions

P-N-P
 or
 N-P-N
Bipolar Transistor
Field Effect Transistor (FET)
Electron Tunneling

Electron Transmission, T, through
thickness, δ.
T  e 2
8 me U  E 

2
h
2

U=Potential Energy of Barrier
 E=Total Energy of Electron
Voltage Controlled Resistor
Inversion Zone - Poisson’s Eq.
 2U
= -/( o )
–Metal on
• N Zone
P Zone
–n= - e Nd
-p=+ e Na
– Boundary Conditions
• U=Uo at x=0
• U=0 V at x=
Inversion Layer
Different Metals have different
Fermi Energies

Fermi Energy is
related to the Work
Function for the
Metal
 Work Function is
changed by gas
adsorption or
intercalation
Metal
Ef(eV)
Na
3.22
Cu
7.00
Ag
5.46
Au
5.49
Mg
7.05
Zn
9.38
Al
11.58
Sn
9.99
Gate Material

Metal
– Pd
• H2  2 H
(intercalated in Pd)
• Alters Pd work
function and Ef
– Hydrogen Sensor

Semi-Conducting
Oxide
– SnO2_
• Reducing Gas
• Alters Oxygen
Vacancy
• Alters Ef and
Conductivity
– Alcohol Sensor
– Formaldehyde Sensor
– CO sensor
Non-Stoichiometric Dielectrics

Metal Excess

Metal with Multiple valence

Metal Deficiency
x
TiO2 
 TiO2 x  O2 ( g )
2
+4
+3 '
1
2TiTi  OO 
 2TiTi  VO..  O2 ( g )
2
x
ZnO  O2 ( g ) 
 Zn1 x O
2
+2
+3
x
NiO  O2 ( g ) 
 Ni1 x O
2
Density Change with Po2
SrTi1-xO3
Non-Stoichiometric Dielectrics
Ki=[h+][e-]
K”F=[O”i][V”O]
Conductivity
=f(Po2 )
Density =f(Po2 )
Non-Stoichiometric Dielectrics
Excess
M1+x O
Deficient
M1-x O
Dielectric Conduction due to Non-stoichiometry

N-type
P-type
Dielectric Conduction due to Non-stoichiometry

N-type
P-type
+h
+h
Excess
Zn1+xO
Deficient
Cu2-xO
Extrinsic Conductivity
Donor Doping
 n-type

Ed = -m*e e4/(8 (o)2 h2)
Ef=Eg-Ed/2
Acceptor Doping
p-type
Ef=Eg+Ea/2
Inversion Layer