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Lecture 10. Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). Dr Tarek Abdolkader Dr Tarek AbdolkaderDr Tarek Abdolkader Dr Tarek Abdolkader. Dr Tarek Abdolkader Dr Tarek AbdolkaderDr Tarek Abdolkader Dr Tarek Abdolkader. By: Dr Tarek Abdolkader. OUTLINE. - PowerPoint PPT PresentationTRANSCRIPT
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المملكة العربية السعوديةوزارة التعليم العالي - جامعة
أم القرىكلية الهندسة و العمارة
اإلسالميةقسم الهندسة الكهربائية
802311-4 ELECTRONIC DEVICES
KINGDOM OF SAUDI ARABIAMinistry of Higher Education
Umm Al-Qura UniversityCollege of Engineering and Islamic Architecture
Electrical Engineering Department
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Lecture 10
By: Dr Tarek Abdolkader
7/6/1433 Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader Dr T
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OUTLINE
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At the end of this lecture, the student should be able to:
1. Describe the construction of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)
2. Differentiate between Enhancement and Depletion MOSFETs and their symbols
3. Explain the operation of D-MOSFET and E-MOSFET
4. Demonstrate D-MOSFET and E-MOSFET characteristics
5. Define the different parameters of MOSFET
6. Describe the different methods of MOSFET biasing
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Dr T
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Depletion-type MOSFET (D-MOSFET)
Enhancement-type MOSFET (E-MOSFET)
p
D
S
G
Substrate
Silicon dioxide (SiO2)insulating layer
Metal
n
D
S
G
p
D
S
G
Substrate
Silicon dioxide (SiO2)insulating layer
Metal n
n
D
S
G
• There are basically two types of MOSFETs: Depletion-type MOSFET (D-MOSFET) and Enhancement-type MOSFET(E-MOSFET)
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MOSFET CONSTRUCTION AND SYMBOLS
D-MOSFET
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MOSFET CONSTRUCTION AND SYMBOLS
E-MOSFET
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D- MOSFET CHARACTERISTICS
p
D
S
Gn
5mAID
VDS
VGS
ID
IDSS=5mA
Depletion(ID < IDSS)
Enhancement(ID > IDSS)
VGS(off)
p
D
S
Gn
ID
VDS
0.8mA
VGS
p
D
S
Gn
ID
VDS
7.4mA
VGS
2
off
1 GSD DSS
GS
VI I
V
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D- MOSFET CHARACTERISTICS
A D-MOSFET has parameters of VGS(off)= -6V and IDSS = 1 mA. Plot the transconductance curve for the device.
-2-4-6 42VGS(V)
0.5
1.0
1.5
2.0
2.5
ID(mA)
IDSS
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D- MOSFET CHARACTERISTICS
• D-MOSFET is very similar to JFET:
1. In D-MOSFET no gate current at all because of the insulator between gate and channel
2. If VGS < 0, both devices have the same characteristics.3. For VGS > 0, JFET cannot be used. However, D-MOSFET can be
used with drain currents larger than IDSS.
p
D
S
Gn
5mAID
VDS
VGS
ID
IDSS=5mA
Depletion(ID < IDSS)
Enhancement(ID > IDSS)
VGS(off)
p
D
S
Gn
ID
VDS
0.8mA
VGS
p
D
S
Gn
ID
VDS
7.4mA
VGS
• n-channel D-MOSFET with VGS < 0, is said to be in Depletion mode, while n-channel D-MOSFET with VGS > 0, is said to be in Enhancement mode
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D- MOSFET CHARACTERISTICS
• More negative VGS depletes more electrons from the n-channel and decrease its conductivity. So, lower VGS leads to less drain current.
• VGS(off) for n-channel D-MOSFET is negative
• More positive VGS attracts more electrons to the n-channel and increase its conductivity. So, higher VGS leads to more drain current.
n-channel p-channel
• More positive VGS depletes more holes from the p-channel and decrease its conductivity. So, higher VGS leads to less drain current.
• VGS(off) for p-channel D-MOSFET is positive
• More negative VGS attracts more holes to the p-channel and increase its conductivity. So, lower VGS leads to more drain current.
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D- MOSFET CHARACTERISTICS
For a certain D-MOSFET, IDSS = 10 mA and VGS(off) = ‒ 8 V.
(a) Is this an n-channel or a p-channel?
(b) Calculate ID at VGS = ‒3 V.
(c) Calculate ID at VGS = +3 V.
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E- MOSFET CHARACTERISTICS
p
D
S
G
n
n
0 mA
VDS p
D
S
GVDS
VGS
>0 mA
n
n
• There is no built-in channel in E-MOSFET.• A positive gate bias has to be applied on the n-channel.• Minimum value of VGS needed to form the n-channel and passes
drain current is called “threshold voltage Vth “
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E- MOSFET CHARACTERISTICS
VGS(V)
ID(mA)
VGS(th)0V
2
(th)D GS GSI k V V
on
2
(on) th
D
GS GS
Ik
V V
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E- MOSFET CHARACTERISTICS
• Vth for n-channel E-MOSFET is positive
n-channel p-channel
• Vth for p-channel E-MOSFET is negative
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E- MOSFET CHARACTERISTICSFor a 2N7002 E-MOSFET, ID(on) = 500 mA (for a point well above the threshold voltage) at VGS = 10 V and VGS(th) = 1 V. Determine the drain current for VGS = 5 V.
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MOSFET BIASING
• D-MOSFET can be operated with either positive or negative gate bias (VGS).
• A simple method is to set VGS = 0.
• RG is used for protecting ac signal input from being shorted
• Remember that at VGS = 0, the drain current ID = IDSS
DS DD DSS DV V I R
• Remember that for JFET, there was a resistance RS at the source terminal to make VGS negative.
D-MOSFET
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MOSFET BIASINGDetermine the drain-to-source voltage in the circuit shown. IDSS = 12 mA and VGS(off) = ‒8 V.
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MOSFET BIASING
• For E-MOSFET, VGS must be greater than threshold voltage VGS(th). So, zero bias cannot be used.
• Two methods of biasing are shown: (a) voltage-divider bias, (b) Drain-feedback bias.
• Note that here also RS is not used because it will raise the potential of source terminal and decrease VGS. In BJT, we were using RE to increase the input impedance Zin but here we do not need it because Zin is already high
DS DD D DV V I R
E-MOSFET
2
1 2GS DD
RV V
R R
2
(th)D GS GSI k V V
GS DSV V
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MOSFET BIASINGDetermine VGS and VDS for the E-MOSFET circuit shown. Assume that ID(on) = 200 mA at VGS = 4 V and VGS(th) = 2 V.
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MOSFET BIASINGDetermine the values of ID and VDS for the circuit shown. The data sheet for this particular MOSFET gives ID(on) = 10 mA when VGS = VDS
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FET AC analysisGraphical Picture
• Change in VGS around the VGSQ leads to change in ID around IDQ
• We use the symbol Vgs , Id , … for the change in VGS , ID , … around the Q point (ac components)
• The transconductance:dD
mGS gs
IIg
V V
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FET AC analysisEquivalent Circuit
• In ac analysis we replace the transistor by its equivalent circuit:
• rgs represents the resistance between gate and source which is infinitely large
• rds represents the dependence of drain current on drain to source voltage which is very small.
• Although the dc resistance of channel RDS = IDQ/VDSQ is small, the ac resistance rds is very high
(a) Complete equivalent circuit (b) Simplified equivalent circuit
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General Knowledge
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Electrostatic Discharge• Excess static charge may be accumulated on
the gate of a MOSFET.• All MOS devices are subject to damage due
to Electro-Static Discharge (ESD)
Humans don't feel ESD transfers less than 3,500 volts, but MOS devices are sensitive to charges at less than half of this level. So, when you work on MOS devices, you may damage the components in the computer via ESD, and not even feel it.
• MOSFETs are usually shipped in conductive foam.• MOSFETs are usually shipped with a wire ring
around the leads, which is removed just prior to installing the MOSFET in a circuit.
• All instruments and metal benches used in assembly or testing of MOSFETs should be connected to earth ground.
• Handler’s rest should be connected to a grounding strap
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MOSFET scaling