Biomedical Engineering Reference
In-Depth Information
a
b
Ge
Si
Si
Si
Ge
E c
E F
E v
Fig. 2.13 ( a ) Cross section of the radial Ge/Si heterostructure and ( b ) the corresponding energy
band diagram
NWs
D
S
S
dielectric
gate - doped Si
Fig. 2.14
The NW FET with millions of NWs in parallel as channel
For biomolecule sensing applications, the NW FET must be functionalized. In
general, only the NW is functionalized, while the source and drain contacts are
protected against unwanted adsorption of biomolecules or chemical reactions by
polymer or oxide layer coatings. In the noncovalent functionalization case, when
the detection method is based only on electrostatic interactions with electrically
charged biomolecules, which attach to the NW and alter the channel conductivity of
NW FET biosensors, the sensitivity of the sensor is defined as
S Dj G G 0 j =G 0 D G=G 0 ;
(2.24)
where G and G 0 are the NW conductance values in the presence and absence of
immobilized biomolecules, respectively. In NW FET sensors with longer channels
than the mean-free path, a semiclassical model is adequate to describe the conduc-
tance of the biosensor ( Nair and Alam 2007 ) which in the absence of biomolecules is
G 0 D eN D d 2 =4L:
(2.25)
In ( 2.25 ), is the mobility of charge carriers, d is the diameter and L the length
of the NW, and N D denotes the doping density. The semiclassical approach is valid
 
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