Biology Reference
In-Depth Information
where
V
G
issu
cientlylarge,
g
m
isgivenbythefollowingequations,
respectively:
V
D
=
const
=
∂
I
D
∂
V
G
W
L
μ
CV
D
g
m
=
(6.1)
V
D
=
const
=
∂
I
D
∂
V
G
W
L
μ
C
(
V
G
−
V
T
)
g
m
=
(6.2)
where
I
D
is the drain current,
μ
the carrier mobility of the substrate
material,
C
the gate capacitance per unit area,
W
and
L
the width
andlengthoftheconductingchannel,respectively.Hencetheampli-
fication power of a MOSFET device is closely related to the mobility
ofthesemiconductormaterialandcanbetunedbythedesignofthe
transistor. The sensitivity of the drain current to the charge on the
gate electrode can hence beexplored forsensor applications.
If the metal gate of a MOSFET is removed from the field-effect
transistor and the gate dielectric placed in contact with a liquid
solution, as shown in Fig. 6.1b, ions can adsorb on the surface of the
gate dielectric, which generates an electric field similar to applying
a voltage at the metal gate [6, 7]. When an external gate voltage is
applied through a reference electrode in the solution, the electrical
field introduced by the adsorbed ions leads to a shift on the device
characteristic. As the shift is quantitatively linked to the type and
densityoftheadsorbedions,thisnewdeviceishencenamedanion-
sensitive field-effect transistor. Selectivity of ISFETs can be induced
by the appropriate incorporation of certain pH-sensitive insulators
or ion-selectivemembrane.
Successful application of ISFETs in pH meters has generated
greatinterestregardingthepossibilityofusingthewell-understood
FET technology to produce amplifying devices that would respond
to larger and more complex molecules in solution or gas phase,
such as DNA, enzymes, antibodies, or antigens, or even whole
tissue layers [4, 7-11]. Numerous biosensors have been developed
based on similar principles, with a large variety of targets, gate
materials, and device structures. More recently, FETs with a metal
gatefunctionalizedwithabiologicalrecognitionlayerhavealsobeen
developed [4, 7, 11].
