Biology Reference
In-Depth Information
Figure 6.1.
Structure of a metal-oxide-semiconductor field-effect tran-
sistor (MOSFET) and an ion-sensitive field-effect transistor (ISFET).
(a) Cross section of an n-type MOSFET. (b) An ISFET is created by
replacing the metal gate of the MOSFET by an electrolyte and a reference
electrode.
can be used as potentiometric chemical and biological sensors.
The basic structure is the metal-oxide-semiconductor field-effect
transistor (MOSFET).
A single crystal silicon based n-channel enhancement mode
MOSFET is shown in Fig. 6.1a. It consists of a p-type single crystal
silicon semiconductor substrate with two heavily doped n-type
regions (named source and drain), a gate dielectric, and a metal
gate electrode on top of the gate dielectric [5]. When the voltage
V
G
applied to the metal gate is lower than the threshold voltage
V
T
,the
p-n junction between the drain and the substrate is reverse biased
and no current flows between source and drain. For
V
G
>
V
T
,the
electric field induced by the gate voltage is large enough to convert
thelightlydopedp-typesiliconsubstrateinton-type(inversion):an
n-type channel is created at the insulator-semiconductor interface
and current can flow between source and drain. Due to the
presence of the insulating layer, no current flows from the gate into
the semiconductor. The amplitude of the current flowing through
source and drain is modulated by the electric field set up by gate
voltage, which is determined by the charge on the metal gate
electrode.
By its working principle, the MOSFET amplifies the input signal
V
G
with an intrinsic gain given by the transconductance
g
m
.In
the linear region where
V
G
is small and in the saturation region
