COMSATS Institute of Information Technology Virtual campus Islamabad
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Transcript of COMSATS Institute of Information Technology Virtual campus Islamabad
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Dr. Nasim Zafar
Electronics 1 - EEE 231
Fall Semester – 2012
COMSATS Institute of Information TechnologyVirtual campus
Islamabad
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Current -Voltage CharacteristicsI-V Characteristics
Lecture No. 29 Contents:
Qualitative theory of operation
Quantitative ID-versus-VDS characteristics
Large-signal equivalent circuits.
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Lecture No. 29
Current-Voltage CharacteristicsReference:
Chapter-4.2
Microelectronic Circuits
Adel S. Sedra and Kenneth C. Smith.
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Circuit Symbol (NMOS)Enhancement-Type:
G
D
S
B
ID= IS
IS
IG= 0 G-GateD-DrainS-SourceB-Substrate or Body
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Circuit Symbol (NMOS)Enhancement-Type
The spacing between the two vertical lines that represent the gate and the channel, indicates the fact the gate electrode is insulated from the body of the device.
The drain is always positive relative to the source in an n-channel FET.
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Qualitative Theory of Operation
Modes of MOSFET Operation
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Modes of MOSFET Operation
MOSFET can be categorized into three modes of operation, depending on VGS:
VGS < Vt: The cut-off Mode
VGS > Vt and VDS < (VGS − Vt): The Linear Region
VGS > Vt and VDS > VGS − Vt: The Saturation Mode
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MOSFET-Structure Enhancement Type-NMOSFET
pn+n+
metal
LW
SourceS
Gate: metal or heavily doped poly-Si G
DrainD
BodyB
oxide
IG=0
ID=ISIS
x
y
(bulk or substrate)
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VGS<0
n+p n+ Structure ID ~ 0
pn+n+
n++
LW
SourceS
GateG Drain
Dbody
B
oxide
+-
VD=Vs
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VGS < Vt The Cut-off Mode:
n+-depletion-n+ structure ID ~ 0
pn+n+
n++
LW
sourceS
gateG
drainD
bodyB
oxide
+-
+++
VD=Vs
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VGS > VT The Linear Mode of Operation: n+-n-n+ structure inversion
pn+n+
n++
LW
sourceS
gateG
drainD
bodyB
oxide
+-
+++++++++
- - - - -
VD=Vs
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VGS > VT
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Quantitative ID-versus-VDS Relationships
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Quantitative ID-VDS Relationships
QN = inversion layer charge
VDV
G (VG)S D (VDS)
For VG < VT, Inversion layer charge is zero (Slide11).For VG > VT, Qn(y) = QG = Cox (VG V VT) (Slide12)
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Nasim Zafar.
Quantitative ID-VDS Relationships
In the MOSFET, the gate and the channel region form a parallel-plate capacitor for which the oxide layer serves as a dielectric.
If the capacitance per unit gate area is denoted Cox and
the thickness of the oxide layer is tox, then
Cox=εox/ tox (4.2)
Where εox is the permittivity of the silicon oxide
ε= 3.9 ε0= 3.9×8.854×10-12= 3.45×10-11F/m
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Quantitative ID-VDS Relationships
Current and Current Density:
In general, Jn= q n n E , for the drift current
Here, current ID is the same everywhere, but Jn (current density) can vary from position to position.
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ynqnqJJ y d
dnnnn
E sincey
yd
d)(
E
Let “ ” be the potential along the channel
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Quantitative ID-VDS Relationships
To find current, we have to multiply the above with area, but Jny,n, etc. are functions of x and z. Hence,
areaunitcharge)()(d
d
dd
dddd
nnn
nnnD
/yQyQy
Z
xqny
ZxJZzxJI yy
Integrating the above equation, and noting that ID is constant, we get
d)(n0nDDS yQ
L
ZI
VSince we know expression for Qn(y) in terms of , we can integrate this to get ID
Current and Current Density:
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Quantitative ID-VDS Relationships
2
2DS
DSTGoxn
DV
VVVCL
ZI satDS,DS0 VV TG VV ;
ID will increase as VDS is increased, but when VG – VDS = VT, pinch-off of channel occurs, and current saturates when VDS is increased further. This value of VDS is called VDS,sat. i.e., VDS,sat = VG – VT and the current when VDS= VDS,sat is called IDS,sat.
2TGox
satD, 2VV
L
CZI
satDS,D VV TG VV ;
Here, Cox is the oxide capacitance per unit area, Cox = ox / xox
Current and Current Density:
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Current-Voltage Characteristics
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Current-Voltage Characteristics
A
B C DIDS
VDS
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The iD-VDS Characteristics
Figure 4.11(a) shows an n-channel enhancement-type MOSFET with voltages VGS and VDS applied and with the normal directions of current flow indicated.
Fig. 4.11 (a): An n-channel enhancement type MOSFET
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The iD-VDS Characteristics
Figure 4.11 (b) shows a typical set of iD-VDS Characteristics.
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The iD–vDS Characteristics for a MOSFET Device with k’n(W/L) = 1.0 mA/V2.
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The iD-VDS Characteristics
Current-Voltage characteristics of Fig. 4.11 (b) show that there are three distinct regions of operation:
The Cutoff Region,
The Triode Region, and
The Saturation Region.
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The iD–vDS Characteristics for a MOSFET Device.
The iD-VDS Characteristics
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The iD-VDS Characteristics
Saturation Region: The saturation region is used if the MOSFET is to operate as
an amplifier.
Cutoff and Triode Regions: For operation as a switch, the cut-off and triode regions are
utilized.
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Operation in the Triode Region
To operate the MOSFET in the triode region we must first induce a channel:
VGS Vt (Induced channel) ≧
VDS< VGS – Vt (Continuous Channel)
The n-channel enhancement-type MOSFET operates in the triode region when VGS is greater than Vt and the drain voltage is lower than the gate voltage by at least Vt volts.
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The iD-VDS Characteristics
The Triode Mode:
In the triode region, the iD-VDS characteristics can be described by the following equation:
ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS
2] (4.11)
Where kn’= μnCox is the process transcondctance parameter, its value is determined by the fabrication technology
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The iD-VDS Characteristics
The Triode Mode:
• If VDS is sufficiently small
• ID = kn’(W/L)[(VGS-VT)VDS] (4.12)
This linear relationship represents the operation of the MOSFET as a linear resistance rDS whose value is controlled by VGS.
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Operation in the Saturation Region
To operate the MOSFET in the Saturation Region we must first induce a channel.
vGS ≧ Vt (Induced channel) (4.16)
vGD Vt≦ (Pinched-off channel) (4.17)
vDS v≧ GS-Vt (Pinched-off channel) (4.18)
The n-channel enhancement-type MOSFET operates in the saturation region when vGS is greater than Vt and the drain voltage does not fall below the gate voltage by more than Vt.
The boundary between the triode region and the saturation region is characterized by
vDS= vGS-Vt (Boundary) (4.19)
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The iD-VDS Relationship
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Saturation Mode
In the Saturation region, the iD-VDS characteristics can be described by eq. (4. 20):
Nasim Zafar.
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The iD–vGS characteristic
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The iD–vGS Characteristic for an NMOS Transistor in Saturation
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Summary: MOSFET I-V Equations
The Cut-off Region: VGS< VT
ID = IS = 0
The Triode Region: VGS>VT and VDS < VGS-VT
ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS
2]
The Saturation Region: VGS>VT and VDS > VGS-VT
ID = 1/2kn’(W/L)(VGS-VT)2
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Output Characteristics of MOSFET
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Large-Signal Equivalent-Circuit Model
In saturate mode, MOSFET provides a drain current whose value is independent of the drain-voltage VDS and is determined by the gate-voltage VGS
Thus, the Saturated MOSFET behaves as an ideal current source whose value is controlled by VGS according to the nonlinear relationship in Eq. (4.20).
Figure 4.13 shows a circuit representation of this view of MOSFET operation in the saturation region. Note that this is a large-signal equivalent-circuit model.
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Large-signal equivalent-circuit model of an n-channel MOSFET operating in the saturation region.
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MOSFET Summary
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I-V Characteristics of MOSFET
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A majority-carrier device: fast switching speed
Typical switching frequencies: tens and hundreds of kHz
On-resistance increases rapidly with rated blocking voltage
The device of choice for blocking voltages less than 500V
1000V devices are available, but are useful only at low power
levels (100W)
MOSFET: Summary
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MOSFET Summary
Importance for LSI/VLSI– Low fabrication cost– Small size– Low power consumption
Applications– Microprocessors– Memories– Power Devices
Basic Properties– Unipolar device– Very high input impedance– Capable of power gain– 3/4 terminal device, G, S, D, B– Two possible channel types: n-channel; p-channel
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MOSFET: Merits/ Demerits
Advantages• Voltage controlled device• Low gate losses• Parameters are less sensitive to junction temperature• No need for negative voltage during turnoff Limitations• One disadvantage of MOSFET devices is their extreme sensitivity to
electrostatic discharge (ESD) due to their insulated gate-source regions.
• The SiO2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge.
• High-on-state drop as high as 10V• Lower off-state voltage capability• Unipolar voltage device.
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