Topic 2.6.3 – enhancement mode n-channel MOSFET’s
Learning Objectives:
At the end of this topic you will be able to;
 describe the switching action of n-channel MOSFET’s;
 recognise that MOSFET’s have a very high input resistance;
 perform calculations using I D  g M  VGS ;
 understand that rDS decreases from a very high value to a very low
value as VGS is increased and is at a minimum value of rDS
ON
at
saturation;
 perform calculations on MOSFET switching circuits;
 compare the performance of MOSFET and transistor switches.
1
Module ET2
Electronic Circuits and Components.
MOSFET’s.
In our previous topic we looked the use of the NPN transistor as an
electronic switch. This device was essentially current controlled, requiring a
small base current to enable a much larger collector current to flow. Our
work revealed a potentially disastrous problem with the loading effect of the
transistor circuit. Unless great care was taken in the design of the sensing
circuit it was possible that the transistor switch would not work as it was
intended.
In this topic we are going to investigate the operation of a different type of
transistor which will overcome this loading problem. The transistor is called a
MOSFET, which stands for Metal Oxide Semiconductor Field Effect
Transistor, which is a bit of a mouthful, so we will simply refer to it as a
MOSFET. There are many different types of MOSFET, available but we will
be concentrating only on one type in this course, which is the n-channel
enhancement MOSFET. You will not be asked about any other version in the
examination.
The Field Effect Transistor
Detailed knowledge of how a MOSFET is constructed are not required, the
important thing is that the Field Effect Transistor, or simply FET, uses the
voltage that is applied to the input terminal to control the output current,
since their operation relies on the electric field (hence the name field
effect) generated by the input voltage. This then makes the Field Effect
Transistor a VOLTAGE operated device.
The Field Effect Transistor has very similar properties to those of the NPN
Transistor i.e., high efficiency, instant operation, robust and cheap. However
they can be made much smaller than an equivalent NPN transistor and along
with their low power consumption and dissipation make them ideal for use in
integrated circuits such as the CMOS range of chips.
2
Topic 2.6.3 – enhancement mode n-channel MOSFET’s
The Field Effect Transistor has one major advantage over its standard NPN
transistor cousins, in that their input impedance is very high, (Thousands of
Ohms) making them very sensitive to input signals, but this high sensitivity
also means that they can be easily damaged by static electricity.
Enhancement-mode MOSFET
The symbol, and picture for an n-channel enhancement mode MOSFET is
shown below.
D, Drain
G, Gate
S, Source
The leads for this type of transistor are
labelled as Gate (G), Drain (D) and Source (S).
The Enhancement-mode MOSFET has the property of being normally "OFF"
when the gate bias voltage is equal to zero.
A drain current will only flow when a gate voltage (VGS) is applied to the gate
terminal. This positive voltage reduces the overall resistance of the device
allowing current to flow between the Drain (D) and Source (S). Increasing
this positive gate voltage will cause an increase in the drain current, ID
through the channel. The MOSFET, can also saturate when VGS is increased
sufficiently, when this occurs the resistance of the MOSFET reaches its
lowest value and will be written in data sheets as rDS .
ON
3
Module ET2
Electronic Circuits and Components.
The transfer characteristic of the MOSFET is similar to that of the NPN
transistor, with one major difference, the linear region is very small, making
it very unlikely that the MOSFET will operate in this region, as shown below.
VOUT (V)
Cut-off
Linear Region
Saturation
6
4
2
0
0
1
2
3
4
5
6
VGS (V)
The exact voltages at which cut-off ends and saturation ends are functions
of the device itself and therefore the above characteristic is given only for
illustrative and comparative purposes to the transistor characteristic.
There only two formulae we need in order to design MOSFET circuits. First is
the transconductance formula, which relates the Drain current to the input
voltage VGS. The transconductance of a MOSFET is given the symbol gM, and
defined as
gM 
ID
VGS
Secondly, the formula for the power dissipated in a MOSFET when it is
saturated is defined as
P  I D2  rDS
ON
4
Topic 2.6.3 – enhancement mode n-channel MOSFET’s
Enhancement-mode MOSFET's make excellent electronics switches due to
their low "ON" resistance and extremely high "OFF" resistance and
extremely high gate resistance. Enhancement-mode MOSFET's are used in
integrated circuits to produce CMOS type Logic Gates and power switching
circuits as they can handle large currents and can be driven directly by digital
logic levels.
So let us look at how the MOSFET is used in a circuit.
Example : The following circuit shows a MOSFET being used to switch on a
high powered lamp from a light sensing circuit.
12V
12V
48W
0V
An extract from the datasheet for the MOSFET is shown below:
VDS/V
(max)
VGS /V
(max)
ID /A
(max)
PTOT /W
(max)
gM /S
(typical)
rDS /Ω
50
15
8
50
1.3
0.24
ON
(a)
Calculate the minimum value of voltage from the light sensing subsystem to allow the load to operate at it’s rated power.
(b)
Calculate the power dissipated in the MOSFET when the lamp is
operating at full power.
5
Module ET2
Electronic Circuits and Components.
Solution : (a)
First calculate the current needed by the load to operate at
full power.
ID 
P 48

 4A
V 12
The minimum value of VGS can now be calculated using the
transconductance formula as shown below.
gM 
ID
VGS
VGS  g M  I D
 1.3  4  5.2V
(b)
The power dissipated in the MOSFET is given by ID squared
multiplied by rDS(ON)
P  I D2  rDS
ON
 4 2  0.24
 3.84W
Hopefully you can see that the circuit calculations relating to the MOSFET
are much more straightforward than those for an NPN transistor.
The last question here illustrates the need to have rDS as a low value,
ON
because the power dissipated in the MOSFET is reliant on this value. If this
value was high then the power dissipated in the MOSFET would be excessive.
Even with it having a low value the power dissipated in a MOSFET is
significant, and they usually run hot, and need to be mounted on heatsinks.
They are capable however of handling much larger currents than the NPN
transistor, and are therefore most suited to switching high powered loads
like motors and solenoids.
6
Topic 2.6.3 – enhancement mode n-channel MOSFET’s
Comparision between NPN Transistors and MOSFET’s








Transistors are "Current Operated Devices" where a much smaller
Base current causes a larger Collector to Emitter current, which
themselves are nearly equal, to flow.
A transistor can also be used as an electronic switch to control
devices such as lamps, motors and solenoids etc.
The NPN transistor requires the Base to be more positive than the
Emitter.
Field Effect Transistors, or FET's are "Voltage Operated Devices"
FET's have very high input resistances so very little or no current
(MOSFET types) flows into the input terminal making them ideal for
use as electronic switches.
The high input impedance makes the design of the sensing sub-system
easier, since we do not have to worry about loading effects.
The input impedance of the MOSFET means that static electricity
can easily damage MOSFET devices so care needs to be taken when
handling them.
They can be used as ideal switches due to their very high channel
"OFF" resistance, low "ON" resistance.
There now follows a number of examination style questions which will allow
you to practice some numerical problems.
7
Module ET2
Electronic Circuits and Components.
Examination Style Questions.
1.
The circuit below shows a MOSFET being used to interface a CMOS logic system to a lamp rated
at 24V, 12A.
24V
24V
12A
Output from
CMOS logic
system
0V
An extract from the datasheet for the MOSFET is shown below:
(a)
VGS /V
(max)
ID /A
(max)
PTOT /W
(max)
gM /S
(typical)
15
15
90
1.2
rDS
ON
/Ω
0.15
Calculate the minimum value of VGS required to enable the lamp to operate at its rated
current.
………………………………………………………………………………………………
(b)
………………………………………………………………………………………………
[2]
Calculate the power dissipated in the MOSFET when the lamp is operating at its rated
current.
………………………………………………………………………………………………
(c)
………………………………………………………………………………………………
[2]
The maximum output current available from the logic system is 30mA. Explain why an
NPN transistor would be unsuitable for this application.
………………………………………………………………………………………………
………………………………………………………………………………………………
[2]
8
Topic 2.6.3 – enhancement mode n-channel MOSFET’s
2.
The circuit below shows a MOSFET being used as a transducer driver to interface a logic system
to a lamp rated at 18V, 9A.
18V
18V
9A
Output from
logic system
0V
An extract from the datasheet for the MOSFET is shown below:
(a)
VGS /V
(max)
ID /A
(max)
gM /S
(typical)
10
16
1.5
rDS
ON
/Ω
0.2
Calculate the minimum value of VGS required to enable the lamp to operate at its rated
current.
………………………………………………………………………………………………
(b)
………………………………………………………………………………………………
[2]
Why is it important for the MOSFET to have a small value of rDS ?
ON
………………………………………………………………………………………………
………………………………………………………………………………………………
[1]
9
Module ET2
Electronic Circuits and Components.
3.
The following circuit is setup to check some parameters of a MOSFET.
15V
I1
VOUT
I2
VIN
A
0V
The following results were obtained with the lamp bright and the MOSFET saturated.
VIN /V
VOUT /V
I2 /A
3
0.6
1.5
(a)
Estimate the value of I1.
(b)
………………………………………………………………………………………………
[1]
Use the results to calculate the value of rDSON .
………………………………………………………………………………………………
(c)
………………………………………………………………………………………………
[2]
What is the main disadvantage of a MOSFET compared with a NPN transistor switch.
………………………………………………………………………………………………
………………………………………………………………………………………………
[1]
10
Topic 2.6.3 – enhancement mode n-channel MOSFET’s
4.
The circuit below shows a MOSFET being used to interface a 24V, 5A.load to a logic system.
24V
LOAD
24V, 5A
Output from
logic system
0V
An extract from the datasheet for the MOSFET is shown below:
VDS /V
(max)
VGS /V
(max)
ID /A
(max)
PTOT /W
(max)
gM /S
(typical)
50
12
10
80
1.6
(a)
rDS
ON
/Ω
0.3
Calculate the minimum value of VGS required to enable the lamp to operate at its rated
current.
………………………………………………………………………………………………
(b)
………………………………………………………………………………………………
[2]
What is the advantage of the MOSFET having a small value of rDS .
ON
………………………………………………………………………………………………
(c)
………………………………………………………………………………………………
[1]
Explain why a MOSFET interface is the most suitable interface between a CMOS logic
system and a high-power load.
………………………………………………………………………………………………
………………………………………………………………………………………………
[2]
11
Module ET2
Electronic Circuits and Components.
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



describe the switching action of nchannel MOSFET’s;
recognise that MOSFET’s have a
very high input resistance;
perform calculations using
I D  g M VGS ;
understand that rDS decreases from
a very high value to a very low value
as VGS is increased and is at a
minimum value of rDS
ON
at saturation;
perform calculations on MOSFET
switching circuits;
compare the performance of
MOSFET and transistor switches.
Targets:
1.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
2.
………………………………………………………………………………………………………………
………………………………………………………………………………………………………………
12