5.3.2 Basic Transistor Working
Case I: When no bias voltage applied: the current between source and drain must be
zero.
Case II: When large positive bias voltage is applied: then a carrier region is formed
between two n-terminals (i.e. n-drain terminal and n-source terminal) allowing
current from source to drain. The source terminal supplied the carriers that flow
through the channel and drain terminal allows carriers to drain from the channel.
So, we can say that when V DS (voltage drain to source) is applied then charge
carriers start flowing from source to drain terminals. By convention, current is
flowing in opposite fashion i.e. from drain to source terminal. As used biasing gate
voltage is positive, it form electron inversion region, termed as enhancement
n-channel MOSFET or NMOS.
5.3.3 Ideal MOSFET V-I Characteristics
Threshold voltage (V TN) is nothing but applied “turn on” voltage for the transistor.
Case I: V GS < V TN : Drain current (I D ) = 0 and is said to be in “turn-off” state
because there is no inversion layer formation taking place.
Case II: V GS > V TN and small V DS is applied: It is a transistor “turn-on” state.
The I D increases with increase in V GS .
(a) V DS > V DS (sat) : I D becomes constant and independent of V DS (Drain to source
voltage) and the region is known as saturation region.
(b) V DS < V DS (sat) : The region is known as unsaturated state.
6 Designing of FET Based Sensors
In relevance to the above discussion, we can easily understand that, current flowing
through FETs is a function of biased gate voltage applied to it. Therefore, sensing of
organic and/or inorganic species is performed by adjusting gate material or by placing a
selective membrane or by using (bio/chemical)-recognition component onto it. The
resulting sensor is known as CHEMFET. When ion selective material is used then it is
known as ISFET. When organic sensing platform is used as sensing material then it is
known as OFET [37]. ISFET based sensors have numerous possible applications in
diversified fields of chemistry, sensor designing and development, microbiology,
flexible and wearable electronic devices and many more [38]. As of now, we are
assuming that, readers got the clear picture of how a MOSFET based devices works. In
the next section we will include the various nanomaterials reported in the literature
towards water pollutants.
Materials for Electrical Detection of Water Pollutants
115
Case I: When no bias voltage applied: the current between source and drain must be
zero.
Case II: When large positive bias voltage is applied: then a carrier region is formed
between two n-terminals (i.e. n-drain terminal and n-source terminal) allowing
current from source to drain. The source terminal supplied the carriers that flow
through the channel and drain terminal allows carriers to drain from the channel.
So, we can say that when V DS (voltage drain to source) is applied then charge
carriers start flowing from source to drain terminals. By convention, current is
flowing in opposite fashion i.e. from drain to source terminal. As used biasing gate
voltage is positive, it form electron inversion region, termed as enhancement
n-channel MOSFET or NMOS.
5.3.3 Ideal MOSFET V-I Characteristics
Threshold voltage (V TN) is nothing but applied “turn on” voltage for the transistor.
Case I: V GS < V TN : Drain current (I D ) = 0 and is said to be in “turn-off” state
because there is no inversion layer formation taking place.
Case II: V GS > V TN and small V DS is applied: It is a transistor “turn-on” state.
The I D increases with increase in V GS .
(a) V DS > V DS (sat) : I D becomes constant and independent of V DS (Drain to source
voltage) and the region is known as saturation region.
(b) V DS < V DS (sat) : The region is known as unsaturated state.
6 Designing of FET Based Sensors
In relevance to the above discussion, we can easily understand that, current flowing
through FETs is a function of biased gate voltage applied to it. Therefore, sensing of
organic and/or inorganic species is performed by adjusting gate material or by placing a
selective membrane or by using (bio/chemical)-recognition component onto it. The
resulting sensor is known as CHEMFET. When ion selective material is used then it is
known as ISFET. When organic sensing platform is used as sensing material then it is
known as OFET [37]. ISFET based sensors have numerous possible applications in
diversified fields of chemistry, sensor designing and development, microbiology,
flexible and wearable electronic devices and many more [38]. As of now, we are
assuming that, readers got the clear picture of how a MOSFET based devices works. In
the next section we will include the various nanomaterials reported in the literature
towards water pollutants.
Materials for Electrical Detection of Water Pollutants
115