Mosfet

Mosfet
MOSFETs have characteristics similar to JFETs and additional characteristics that make them very useful.

There are 2 types:
• Depletion-Type MOSFET
• Enhancement-Type MOSFET

1. MOSFET

2. MOSFETs
MOSFETs have characteristics similar to JFETs and additional characteristics that make
them very useful.
There are 2 types:
• Depletion-Type MOSFET
• Enhancement-Type MOSFET

3. Depletion-Type MOSFET Construction
The Drain (D) and Source (S) connect to the to n-doped regions. These N-doped regions
are connected via an n-channel. This n-channel is connected to the Gate (G) via a thin
insulating layer of SiO
2. The n-doped material lies on a p-doped substrate that may have an
additional terminal connection called SS.

4. Basic Operation

5. Basic Operation
A Depletion MOSFET can operate in two modes: Depletion or Enhancement mode.

6. •In this figure V
GShas been set at a negative voltage such as 1 V. The negative potential at the gate will tend to
pressure electrons toward the p-type substrate (like charges repel) and attract holes from the p-type substrate
(opposite charges attract).
•Depending on the magnitude of the negative bias established by V
GS, a level of recombination between electrons
and holes will occur that will reduce the number of free electrons in the n-channel available for conduction. The more
negative the bias, the higher the rate of recombination.
•The resulting level of drain current is therefore reduced with increasing negative bias for V
GSfor V
GS =1 V, 2 V, and
so on, to the pinch-off level of 6 V. The resulting levels of drain current and the plotting of the transfer curve proceeds
exactly as described for the JFET.

7. Depletion-type MOSFET in Depletion Mode
Depletion mode
The characteristics are similar to the JFET.
When V
GS= 0V, I
D= I
DSS
When V
GS < 0V, I
D< I
DSS
The formula used to plot the Transfer Curve still applies: 2
P
GS
DSSD )
V
V
(1II 

8. Enhancement Mode
ForpositivevaluesofV
GS,thepositivegatewilldrawadditionalelectrons(freecarriers)
fromthep-typesubstrateduetothereverseleakagecurrentandestablishnewcarriers
throughthecollisionsresultingbetweenacceleratingparticles.
Asthegate-to-sourcevoltagecontinuestoincreaseinthepositivedirection,thedrain
currentwillincreaseatarapidrate
Theapplicationofapositivegate-to-sourcevoltagehas“enhanced”theleveloffree
carriersinthechannelcomparedtothatencounteredwithV
GS=0V.Forthisreasonthe
regionofpositivegatevoltagesonthedrainortransfercharacteristicsisoftenreferred
toastheenhancementregion,
TheregionbetweencutoffandthesaturationlevelofI
DSSreferredtoasthedepletion
region.

9. Depletion-type MOSFET in Enhancement Mode
Enhancement mode
V
GS> 0V, I
Dincreases above I
DSS
The formula used to plot the
Transfer Curve still applies:
(note that VGS is now a positive polarity)2
P
GS
DSSD )
V
V
(1II 

10. Slide 26
p-Channel Depletion-Type MOSFET
The p-channel Depletion-type MOSFET is similar to the n-channel except that the voltage
polarities and current directions are reversed.

11. Symbols

12. Enhancement-Type MOSFET Construction
The Drain (D) and Source (S) connect to the to n-doped regions. These n-doped regions
are connected via an n-channel. The Gate (G) connects to the p-doped substrate via a thin
insulating layer of SiO
2. There is no channel. The n-doped material lies on a p-doped
substrate that may have an additional terminal connection called SS.

13. Basic Operation and Characteristics
IfV
GSissetat0Vandavoltageappliedbetweenthedrainandsourceofthedevice,the
absenceofann-channel(withitsgenerousnumberoffreecarriers)willresultinacurrent
ofeffectivelyzeroamperes—quitedifferentfromthedepletion-typeMOSFETandJFET
whereI
D=I
DSS.
Itisnotsufficienttohavealargeaccumulationofcarriers(electrons)atthedrainand
source(duetothen-dopedregions)ifapathfailstoexistbetweenthetwo.
IfbothV
DSandV
GSissetatsomepositivevoltagegreaterthan0V,thenthepositive
potentialatthegatewillpressuretheholes(sincelikechargesrepel)inthep-substrate
alongtheedgeoftheSiO
2layertoleavetheareaandenterdeeperregionsofthep-
substrate.
However, the electrons in the p-substrate (the minority carriers of the material) will be
attracted to the positive gate and accumulate in the region near the surface of the SiO
2
layer. The SiO
2layer and its insulating qualities will prevent the negative carriers from
being absorbed at the gate terminal.

14. Continued…
AsV
GSincreasesinmagnitude,theconcentrationofelectronsneartheSiO
2surfaceincreasesuntileventuallythe
inducedn-typeregioncansupportameasurableflowbetweendrainandsource.ThelevelofV
GSthatresultsinthe
significantincreaseindraincurrentiscalledthethresholdvoltageandisgiventhesymbolV
T.
SincethechannelisnonexistentwithV
GS=0Vand“enhanced”bytheapplicationofapositivegate-to-source
voltage,thistypeofMOSFETiscalledanenhancement-typeMOSFET.

15. Continued…
•AsV
GSisincreasedbeyondthethresholdlevel,thedensityoffreecarriersinthe
inducedchannelwillincrease,resultinginanincreasedlevelofdraincurrent.
•However,ifweholdV
GSconstantandincreasethelevelofV
DS,thedraincurrentwill
eventuallyreachasaturationlevelThelevellingoffofI
Disduetoapinching-off
processdepictedbythenarrowerchannelatthedrainendoftheinducedchannel
•By applying KVL we get -
IfV
GSisheldfixedatsomevaluesuchas8VandV
DSisincreasedfrom2to5V,the
voltagewilldropfrom-6to-3V.Thisreductioningate-to-drainvoltagewillinturn
reducetheattractiveforcesforfreecarriers(electrons)inthisregionoftheinduced
channel,causingareductionintheeffectivechannelwidth.
Eventually,thechannelwillbereducedtothepointofpinch-offandasaturation
conditionwillbeestablished.

16. Continued…

17. Slide 30
Basic Operation
The Enhancement-type MOSFET only operates in the enhancement mode.
V
GSis always positive
As V
GSincreases, I
Dincreases
But if V
GSis kept constant and V
DSis increased, then I
Dsaturates (I
DSS)
The saturation level, V
DSsatis reached.TGSDsat VVV 

18. Slide 31
Transfer Curve
To determine I
Dgiven V
GS:
where V
T= threshold voltage or voltage at which the MOSFET turns on.
k = constant found in the specification sheet
k can also be determined by using values at a specific point and the formula:
V
DSsatcan also be calculated:2
)(
TGSD VVkI  2
TGS(ON)
D(on)
)V(V
I
k

 TGSDsat VVV 

19. Slide 32
p-Channel Enhancement-Type MOSFETs
The p-channel Enhancement-type MOSFET is similar to the n-channel except that the
voltage polarities and current directions are reversed.

20. Symbols

21. MOSFET Handling
MOSFETs are very static sensitive. Because of the very thin SiO
2layer between the
external terminals and the layers of the device, any small electrical discharge can stablish
an unwanted conduction.
Protection:
• Always transport in a static sensitive bag
• Always wear a static strap when handling MOSFETS
• Apply voltage limiting devices between the Gate and Source, such as back-to-
back Zeners to limit any transient voltage.

22. CMOS –Complementary MOSFET p-channel and n-channel MOSFET on the same
substrate.
Advantage:
• Useful in logic circuit designs
• Higher input impedance
• Faster switching speeds
• Lower operating power levels
Application:
CMOS Inverter
CMOS

23. CMOS Inverter
•Aninverterisalogicelementthat“inverts”theappliedsignal.Thatis,ifthelogiclevelsofoperation
are0V(0-state)and5V(1-state),aninputlevelof0Vwillresultinanoutputlevelof5V,andvice
versa.
•Here,bothgatesareconnectedtotheappliedsignalandbothdraintotheoutputV
o.Thesourceofthe
p-channelMOSFET(Q
2)isconnecteddirectlytotheappliedvoltageV
SS,whilethesourceofthen-
channelMOSFET(Q
1)isconnectedtoground.
•Here,theapplicationof5Vattheinputshouldresultinapproximately0Vattheoutput.With5Vat
V
i(withrespecttoground),V
GS1=V
iandQ
1is“on,”resultinginarelativelylowresistancebetween
drainandsource
•SinceV
iandV
SSareat5V,V
GS2=0V,whichislessthantherequiredV
Tforthedevice,resultingin
an“off”state.TheresultingresistancelevelbetweendrainandsourceisquitehighforQ
2

24. CMOS Inverter
•Asimpleapplicationofthevoltage-dividerrulewillrevealthatV
oisverycloseto0V
orthe0-state,establishingthedesiredinversionprocess.
•ForanappliedvoltageV
iof0V(0-state),V
GS1=0VandQ
1willbeoffwithV
SS2=-5V,
turningonthep-channelMOSFET.TheresultisthatQ
2willpresentasmallresistance
level,Q
1ahighresistance,andV
o=V
SS=5V(the1-state).

25. Summary Table