lecture2.pdf all aaj wo ek ek will wo wo so will

2

1. Lecture 2
https://www.electronicsforu.com/videos-slideshows/videos/mosfet-
working
https://www.youtube.com/watch?v=rkbjHNEKcRw

2. •WhatisMOSFET?
•
•MOSFETstandsforMetalOxideSemiconductorFieldEffectTransistor.ItisatypeofFieldEffectTransistoranditisvoltagecontrolleddevice.ItisalsocalledasInsulatedGateFieldEffectTransistor(IGFET).Itisusedforswitchingoramplifyingelectronicsignalsintheelectronicdevices.Itisthemostcommonlyusedtransistoranditcanbeusedinbothanaloganddigitalcircuits.

3. Types of MOSFET:

4. Symbol of MOSFET:

5. Construction of MOSFET:
•The metallic gate terminal in the MOSFET is insulated
from the semiconductor layer by a SiO2 layer or
dielectric layer.
•The MOSFET consists of three terminals, they are
source(S), Gate (G), Drain (D) and the body which is
called as substrate. The substrate is connected to the source internally.

6. Construction of N channel and P channel Enhancement MOSFET

7. •In N channel Enhancement MOSFET the source and
drain are of N type semiconductor which is heavily
doped and the Substrate is of P type semiconductor.
•Majority charge carriers are electrons. The source and
drain terminals are physically separated in
Enhancement mode.
•In P channel Enhancement MOSFET the source and
drain are of P type semiconductor which is heavily
doped and the Substrate is of N type semiconductor. Majority charge carriers are holes.

8. Construction of N channel and P channel
DepletionMOSFET

9. •IntheNChanneldepletionMOSFETasmallstripofsemiconductorofNtypeconnectsthesourceanddrain.ThesourceanddrainareofNtypesemiconductorandtheSubstrateisofPtypesemiconductor.Majoritychargecarriersareelectrons.Thesourceanddrainareheavilydoped.
•
•InthePChanneldepletionMOSFETasmallstripofsemiconductorofPtypeconnectsthesourceanddrain.ThesourceanddrainareofPtypesemiconductorandtheSubstrateisofNtypesemiconductor.Majoritychargecarriersareholes.
•

10. Working of Enhancement MOSFET:

11. •IntheEnhancementMOSFETthesourceandthedrain
arenotconnectedphysicallysointhesymbollinesare
brokenandintheDepletionmodelineiscontinuous.In
theNtypethearrowpointsinsideandinthePtypearrowpointsoutside.

12. N-Channel Enhancement MOSFET Working
•To get the drain current first we have to create a channel for the free
movement of electrons.
•To create a channel we have to apply a voltage between the gate and
the source terminal keeping the Gate at a higher potential. This
voltage is called VGS.
•Now the gate is at higher potential. The free electrons will move
toward the gate terminal. As discussed earlier we have a Silicon
Dioxide layer at the top. Hence these free electrons will accumulate
near the Gate region and will not escape. The silicon dioxide layer also
acts as a dielectric. It will allow more free electrons to accumulate
near the gate terminal in less applied voltage at the gate terminal.

13. •Now on increasing VGSfurther, a high electric field is developed..
•The free electrons generated will fill the holes near the gate region.
This way holes are pushed away from the gate terminal increasing N-
type behaviornear the gate terminal.
•A time will come when an N-channel is created between the two N
wells. The VGS voltage at which the channel is created is called the
threshold voltage or VT. We can conclude from this discussion when
VGS > VT an N channel is induced near the gate terminal as shown in
the figure below.

15. •A channel is created still we are not getting any current.
•Let us see how to get the drain current. Apply a voltage source
between the drain and the source keeping the drain at a higher
potential.
•This voltage is called VDS. On applying this voltage current will start
flowing from drain to source. This current is called drain current or
ID.
•We can conclude from this discussion, when VGS > VT and VDS> 0,
the current ID flows from drain to source as shown in the figure
below.

17. •On increasing the positive voltage at the drain terminal a reverse bias is formed at the PN junction near the drain terminal.
•This will result in a thick depletion region near the PN junction. Hence on increasing VDS further, you will see the channel near the drain terminal is becoming narrow.
•The drain current will face more resistance near the drain terminal. A situation will reach when the drain current becomes constant and will not increase further.
•This situation is called the pinch-off situation and the drain current is called the saturation current. The voltage at which we will get saturation current is called saturation voltage. We can conclude from this discussion, that pinch-off is reached when VGS> 0(constant)and VDS = VDS(SAT), ID = ID(SAT)as shown in the figure below.

18. N Channel Enhancement MOSFET Working

19. •Now is there any way to increase the drain current beyond
saturation? The answer is yes. Increasing ID further increases the
value of VGS. This will increase the width of the complete N-channel.
Hence VGS is controlling voltage

20. Characteristics of N-Channel E-MOSFET
•Characteristicsofn-channelE-MOSFETreferstothecurveswhichrelatethecurrentandvoltageofdevicewitheachother.Therearemainlytwotypesofcharacteristicsinn-channelE-MOSFET:
•Drain Characteristics:These curves provide the relationship between drain current (ID) and drain-to-source voltage (VDS). When different values of drain current and drain-to-source voltage are plotted on graph, it gives respective values of gate-to-source voltage (VGS). These characteristics are also called as V-I characteristics of a curve.
•Fromthegraphshownbelow,itisobservedthatwhenthepositivevalueofVGSisincreased,thecurrentIDwillalsoincrease.Thisgraphconsistsoftworegions:non-saturatedregionandsaturatedregion.Thenon-saturatedregionofthecurveisalsocalledasohmicregion,inthisregionwhendraincurrentisincreasedthensubsequentlythevalueofdrain-to-sourcevoltagealsoincreases.
•Ohmic region lasts till when the value of drain-to-source voltage reaches a threshold value called as threshold voltage (VTN). After this voltage saturation of n-channel E-MOSFET takes place. Hence, the region of curve after threshold voltage is achieved is called as saturated region

22. •Cutoffregion:
•NocurrentflowsthroughitandtheMOSFETisoff.
•Ohmicregion:
•Draincurrentincreaseswhenthedrainsourcevoltageincreases.Usedasamplifierinthisregion.
•Saturationregion:
•Draincurrentisconstantfordrainsourcevoltage.Used
asswitchinthisregion.Thisoccurswhenthedrainsourcevoltagereachespinchoffvoltage.

23. N-channel Enhancement MOSFET V-I Characteristic Now we can plot VI
characteristics very easily. In the VI characteristics, you will see the plots of VDS vs ID
for various values of VGS. From the graph, it is clear that
the current ID will become
constant at a specific value of
VDS. current ID increases only
when the value of VGS is
increased.

24. •Transfer characteristics:These curves provide the relationship
between drain current (ID) and gate-to-source voltage (VGS). When
different values of drain current and gate-to-source voltage are
plotted on X-axis and Y-axis respectively, it provides different values
of drain-to-source voltage (VDS). These curves are also called as
transconductance curves.
•Fromthetransfercharacteristicsofn-channelE-MOSFETshown
belowitisobservedthatwhenthevalueofgate-to-sourcevoltageis
belowthethresholdvoltage(VTN)thennodraincurrentflows.When
gate-to-sourcevoltageisincreased,anditreachestothreshold
voltagethendraincurrent(ID)startsflowing.

26. N-Channel Depletion MOSFET Structure

27. •Since the channel is already present. connect the Gate terminal to the
ground and apply a positive voltage at the drain and source. On
applying positive VDS, the electrons in the N channel will move
towards the positive drain terminal, and the drain current will start
flowing from drain to source. On increasing VDS further, keeping VGS
0, a time will come where ID will become constant, and that value of
drain current is called saturation current.
•We can conclude from this discussion that when VGS = 0and VDS > 0,
current ID flows from drain to source, and on increasing VDS further,
ID = IS = IDSS, as shown in the figure below.

29. •The effect of gate voltage on N channel depletion MOSFET.
•ApplyVGS < 0. Holes from the P-type substrate will attract towards
the negative gate terminal and recombine with electrons in the N
channel, forming electron-hole pairs.
•On increasing negative potential at the gate, more electron-hole
combinations will occur, decreasing the number of free electrons in
the N channel. As a result, ID decreases. A time will come when the
drain current will become zero. The negative gate voltage at which
the drain current is zero is called pinch-off voltage or VP.
•We can conclude from this discussion at pinch-off, VGS = VP, VDS > 0,
and ID = 0, as shown in the figure below.

30. Effect of Gate Voltage on MOSFET

31. Now we will see the effect of positive gate voltage on the drain
current. •On applyingVGS > 0, the minority carriers in the p-type substrate, i.e.
electrons, will get attracted towards the gate terminal, thereby
increasing the concentration of electrons in the N-channel. As a
result, the drain current will increase and exceed the saturation
current. We can conclude from this discussion, when VGS > 0and
VDS > 0, thenID > IDSS, as shown in the figure below.

33. •N channel Depletion MOSFET V-I Characteristics
•As discussed earlier, depletion-type MOSFET worked for both
positive and negative gate voltages. Now we can plot VI
characteristics very easily. In the VI characteristics, you will
see the plots of VDS vs ID for various values of VGS.

35. •The graph shows that the current ID will flow for both positive and
negative values ofVGS. You can see from the graph that the drain
current is less than the saturation current for the negative value of
gate voltage, whereas for the positive value of gate voltage, the drain
current exceeds the saturation current. VGS = VPis also represented
in this graph for which drain current is zero irrespective of drain to
source voltage.

36. Difference Between Depletion MOSFET and Enhancement MOSFET
Difference between D-MOSFET (Depletion MOSFET) and E-MOSFET
(Enhancement MOSFET) is given below:

38. •A depletion-mode MOSFET (D-MOSFET) operates by naturally having a conducting channel when no gate
voltage is applied, and applying a negative gate voltage "depletes" the channel, reducing its conductivity,
while in enhancement mode, a positive gate voltage is applied to create a conducting channel, essentially
"enhancing" the current flow through the device; essentially, a D-MOSFET can operate in both depletion and
enhancement modes depending on the polarity of the gate voltage applied.
•Key points about D-MOSFET in Depletion and Enhancement modes:
•Depletion Mode:
•When no gate voltage is applied, the channel is already formed and current can flow freely.
•Applying a negative gate voltage repels electrons from the channel, creating a depletion region and reducing
the channel's conductivity, thus lowering the current flow.
•This mode is useful for applications where a high initial current is needed, like in power supplies.
•Enhancement Mode:
•Applying a positive gate voltage attracts electrons to the channel, creating a stronger conducting path and
increasing the current flow.
•This mode is similar to a typical enhancement-mode MOSFET, where the channel is induced by the gate
voltage