1. 11
JAGANNATH GUPTA INST.OF ENGG.JAGANNATH GUPTA INST.OF ENGG.
&TECH.&TECH.
By:- 1)AMIT KUMAR
2)ABHISHEK KUMAR
3)ALOK NARUKA
4)ANIL KUMAR
BRANCH:- E&C
SECTION:- A
Electronics
devices
and
circuit lab
II
m
osfet
R
evolutionizing
the
electronic
devices
2. 22
An Introduction to Mosfet
Mosfet works on the principal of field effect transistor. It is a three terminal device in
which conduction is due to only one type of carriers. And this current is controlled by
the effect of electric field. FET is simpler to fabricate and it takes less space on a chip
as compared to bipolar junction transistor. Mosfet is a type of FET. Mosfet stands for
metal oxide semiconductor fet, it is also called insulated gate FET.
Mosfet consists of source, drain and gate. The flow of charges or holes takes place
through n-channel or p-channel.
The gate is separated from n or p
substrate by metal oxide layer
like silicon dioxide and the drain
and source are directly is in
contact with the substrate. On
the basic mode of operation
MOSFET is classified into two
types. They are as follows:-
(i) Depletion type mosfet
(ii) Enhancement type mosfet
The working and
construction of both types of
mosfet are entirely different.
3. 33
Depletion type MosfetDepletion type Mosfet
CONSTRUCTION:CONSTRUCTION:
The basic construction of n-channel depletion type mosfet-The basic construction of n-channel depletion type mosfet-
It consists of a lightly doped p-type bar in to which two highly doped n-regions areIt consists of a lightly doped p-type bar in to which two highly doped n-regions are
diffused. These highly doped regions are called as source and drain. A thin layer ofdiffused. These highly doped regions are called as source and drain. A thin layer of
insulating silicon dioxide is grown over the surface of the substrate and holes areinsulating silicon dioxide is grown over the surface of the substrate and holes are
etched in to the insulating layer, allowing metallic contacts with source and drain foretched in to the insulating layer, allowing metallic contacts with source and drain for
electrical connection. The terminal is gate.electrical connection. The terminal is gate.
Here an N-channel is deliberately diffused
between the source and drain. With such a
construction of depletion type MOSFET
since a channel is provided there will be an
appreciable drain current.
Gate remains completely
insulated from diffused N- channel. In
some cases substrate is connected
internally to source terminal. Due to the
insulating layer of silicon dioxide there is
very desirable input impedance. This is the
basic construction of depletion type mosfet.
4. 44
OperationOperation andand
characteristicscharacteristicsIf we apply drain to source voltage and gate to source voltage equal to zero byIf we apply drain to source voltage and gate to source voltage equal to zero by
making direct combination from gate to source terminal . The result is that ismaking direct combination from gate to source terminal . The result is that is
that free electrons from N-channel get attracted towards positive potential ofthat free electrons from N-channel get attracted towards positive potential of
drain terminal, which causes current to flow through the channel . Now, if wedrain terminal, which causes current to flow through the channel . Now, if we
apply negative gate voltage instead of 0v the negative charges on the gateapply negative gate voltage instead of 0v the negative charges on the gate
repel electrons from the channel and attract holes from p substrate. Thisrepel electrons from the channel and attract holes from p substrate. This
causes recombination of repelled electrons and attracted holes, this reducescauses recombination of repelled electrons and attracted holes, this reduces
drain current.drain current.
If we apply positive gate voltage, positive charges on the gate terminal attractsIf we apply positive gate voltage, positive charges on the gate terminal attracts
more electrons from the p type substrate. Thus new carriers are establishedmore electrons from the p type substrate. Thus new carriers are established
through collision between accelerated particles. As the positive gate to sourcethrough collision between accelerated particles. As the positive gate to source
voltage increases the drain current also increases.voltage increases the drain current also increases.
The positive gate voltage enhances the
level of conduction electrons in the
channel and negative gate voltage
reduces the number of free electrons in
the channel. Due to this fact MOSFET
with negative gate voltage is called to be
working in depletion mode and MOSFET
with positive gate voltage is called to be
working in enhancement mode.
5. 55
Enhancement typeEnhancement type MosfetMosfet
The mechanical structure of this device is shown to the right. In an IC, we would place
two n-type regions side by side within a p-type area and then place the gate between the
n-type regions. However, the important region still consists of the two n-type regions and
the p-type area between them. This is the portion we have depicted to the right.
With no applied bias, we have what amounts to an npn transistor with no base
connection. The two n-type regions are isolated from each other, and are electrically
separate. Even with a voltage applied between the two n-type regions, there is no
channel present and no current flow.
While we still apply the usual positive voltage to the drain with respect to the source, this
time we will also apply a positive voltage to the gate regionit will attract.
You might not think this would have any effect on the p-type region, where the majority
current carriers are holes. However, there are some free electrons here as well. In
addition, the source junction is forward biased, so the positive gate voltage can attract
electrons across this junction towards the gate.
The net result is that the electrons attracted towards the gate actually enhance a channel
within the p-type region, as shown to the left. This is a channel formed of free electrons,
and actually bridges the gap between source and drain. Now we have a channel, which
can conduct current from source to drain through the device.
. This has the effect of attracting free electrons towards the gate. The larger the positive
gate voltage, the wider its electric field and the more free electrons
6. 66
Because these devices operate by having a channel enhanced
in the semiconductor material where no channel was
constructed, they are known as enhancement-mode
MOSFETs. It is just as easy to construct p-channel versions of
these devices as n-channel versions. Indeed CMOS logic ICs
consist of nothing but these devices, constructed and used in
pairs such that one will be turned off while the other is turned
on. This is the source of the designation CMOS:
Complementary MOS.
Enhancement-mode MOSFETs have the same advantages
and disadvantages as their depletion-mode cousins. However,
when they are constructed as part of an IC rather than as
individual devices, they are not readily subject to random static
charges. Such ICs are constructed with input protection
circuitry for any MOSFET input that must be made accessible
to external circuitry.
Enhancement-type β-SiC MOSFETs have been fabricated on
a single crystalline β-
SiC layer grown on a 3-inch Si(100) substrate by chemical
vapor deposition. The MOSFETs fabricated by applying the
reactive ion etching technique show reasonable I-V
characteristics at room temperature. The saturation tendency
of the drain currents has been observed at a drain voltage as
high as 18 V. The MOSFETs operate even at 350°C.
7. 77
Characteristics of Enhancement Type MOSFETCharacteristics of Enhancement Type MOSFET
The drain characteristics of enhancement type MOSFET is given below.This depicts theThe drain characteristics of enhancement type MOSFET is given below.This depicts the
variation of drain current(ID)with drain to source voltage(VDS)for different values of gatevariation of drain current(ID)with drain to source voltage(VDS)for different values of gate
to source voltage(VGS).to source voltage(VGS).
The lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almostThe lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almost
zero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device canzero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device can
operate in the ohmic,active or cutoff(break down)region.The rising part of curveoperate in the ohmic,active or cutoff(break down)region.The rising part of curve
(fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a(fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a
resistor,when operated in this region.The drain current is almost constant when theresistor,when operated in this region.The drain current is almost constant when the
device operates in the active region.when VDS exceeds the rated value,avalanchedevice operates in the active region.when VDS exceeds the rated value,avalanche
breakdown occurs and the device is in the breakdown region.breakdown occurs and the device is in the breakdown region.
Transconductance curve of enhancement MOSFET is shown belowTransconductance curve of enhancement MOSFET is shown below
This curve start from VGS(Th) because the device is off and drain current is zero whenThis curve start from VGS(Th) because the device is off and drain current is zero when
VGS<VGS(Th).In addition two other quantities specified are VGS(ON) andVGS<VGS(Th).In addition two other quantities specified are VGS(ON) and
ID(ON).Drain current is given byID(ON).Drain current is given by
ID =[(VGS -VGS(TH)) / (VGS(ON)- VGS(TH))] × ID(ON)ID =[(VGS -VGS(TH)) / (VGS(ON)- VGS(TH))] × ID(ON)
MOSFEts have a very thin silicon dioxide layer.This layer is kept very thin to ensure thatMOSFEts have a very thin silicon dioxide layer.This layer is kept very thin to ensure that
the gate has good control over the gate current.This layer could be destroyed if a voltagethe gate has good control over the gate current.This layer could be destroyed if a voltage
higher than rated value is applied to the gate.If the MOSFET has a rated VGS ofhigher than rated value is applied to the gate.If the MOSFET has a rated VGS of
-30V,we should never apply a voltage higher than +30V or lower than -30V.Moreover-30V,we should never apply a voltage higher than +30V or lower than -30V.Moreover
they should not be connected or disconnected in the circuit when the circuit is ON.they should not be connected or disconnected in the circuit when the circuit is ON.
9. 99
Operation of enhancement type MosfetOperation of enhancement type Mosfet
Since the channel is absent here, if we apply drain to source voltage and keep equal to
zero volt, there is no conduction and hence drain current is zero in enhancement type
mosfet, just opposite to depletion type mosfet where channel was diffused between
source and drain terminal.
If we increase the gate to source voltage from its zero volt value the concentration of
electrons near the silicon dioxide layer increases. The flow of electrons between source
and drain induces in N-channel. As vgs is increased the number of electrons reaching
the drain increases and hence conductivity also increases and at some particular value
of vgs current flows from source to drain through the induced channel. This value of
vgs is called as threshold voltage.
Thus, the current is enhanced by the positive gate voltage and such a device is known
asenhancement type MOSFET.
