C13 MOSFET operation - utcluj.ro. MOSFET operation.pdf · 2019-05-28 · C13 –MOSFET operation...
Transcript of C13 MOSFET operation - utcluj.ro. MOSFET operation.pdf · 2019-05-28 · C13 –MOSFET operation...
ELECTRONIC DEVICES
Assist. prof. Laura-Nicoleta IVANCIU, Ph.D.
C13 – MOSFET operation
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Contents
➢ Symbols
➢ Structure and physical operation
➢ Operating principle
➢ Transfer and output characteristics
➢ Quiescent point
➢ Operating regions
➢ Examples
C13 – MOSFET operation
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Transistors
= active semiconductor devices, with three terminals
- used to amplify or switch signals
- essential components of electronic circuits
- discrete or integrated
Operating principle:
The voltage applied between two terminals (command) controls the current through the third terminal
Previously on ED (C11):
C13 – MOSFET operation
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SymbolsC13 – MOSFET operation
n-channel enhancement-type p-channel enhancement-type
general symbols
(w/ B substrate terminal)
simplified symbols
(B internally connected to S)
other symbols
Metal-oxide-semiconductor field effect transistors (MOSFETS)
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SymbolsC13 – MOSFET operation
D – drain (similar to C)
G – gate (similar to B)
S – source (similar to E)
n-channel enhancement-type p-channel enhancement-type
Metal-oxide-semiconductor field effect transistors (MOSFETS)
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Structure and physical operation
C13 – MOSFET operation
▪ creation of an n- type channel between the drain and source terminals
▪ existence of a positive potential difference between the drain and source to move the carriers
In order to have a drain-to-source current, two conditions must be fulfilled:
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Structure and physical operation
C13 – MOSFET operation
No possibility for the current to flow from drain to source
No channel
VTh – threshold voltage
0
0
=
=
D
DS
ThGS
I
V
VV
OPTIONAL
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Structure and physical operation
C13 – MOSFET operation
0
0
=
=
D
DS
ThGS
I
V
VV
OPTIONAL
Due to the electric field created by VGB > 0 (VGS > 0), the electrons in the substrate will be attracted and accumulated just under the oxide (gate region).
When VGS is increased further, the electron concentration becomes larger than the hole concentration; this process is called population inversion.
Inversion creates a conducting n-channel between the drain and the source.
The gate voltage VGS at which inversion produces an n concentration equal to the unbiased p concentration is called the threshold voltage VTh.
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Structure and physical operation
C13 – MOSFET operation
OPTIONAL
0
0
D
DSsatDS
ThGS
I
VV
VV
The current flows through the channel under the action of VDS >0. The transistor operates in the linear region.
The channel will became shallower at the drain end, because the electrons from the close vicinity of the drain region are attracted by the positive drain region.
The current depends linearly on VGS, but also depends on VDS.
( )
( ) 2
2
22
2
DSDSThGSD
DSDSThGSD
VVVVL
WKI
VVVVI
−−=
−−=
β – constructive parameter [μA/V2]K – transconductance parameter [μA/V2]W, L – physical dimensions of channel [μm]
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Structure and physical operation
C13 – MOSFET operation
OPTIONAL
The current depends non-linearly (quadratic) on VGS
0
D
DSsatDS
ThGS
I
VV
VV( ) ( )
( ) ( )22
22
21
2
1
ThGS
A
DSThGSD
ThGS
A
DSThGSD
VVL
WK
V
VVV
L
WKI
VVV
VVVI
−
+−=
−
+−=
The VDS voltage is greater than the VDSsat voltage.
All the electrons from the close vicinity of the drain region are attracted by the more positive drain region.
The channel depth becomes zero at the drain end. The transistor is now in the pinch-off region, or saturationregion or active region.
ID is almost independent of the VDS. Conduction from the source to drain still occurs with current passing through the depletion region next to the drain.
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Structure and physical operation
C13 – MOSFET operation
▪ VGS - conducting channel building (enhancement)
▪ VDS – controlled movement of the carriers (free electrons between D and S terminals)
The current depends on
),( DSGSD VVfI =
➢ The current through MOSFET
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Operating principleC13 – MOSFET operation
➢ n-channel enhancement type MOSFET
SD
G
SGD
ii
i
iii
=
=
=+
0PSDS
CoGS
Vv
vv
=
=
▪ Family of transfer characteristics
DSGSD vvi ,)(
▪ Family of output characteristics
GSDSD vvi ,)(
as parameter
as parameter
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Transfer and output characteristics
C13 – MOSFET operation
➢ Transfer characteristics
DSGSD vvi ,)( as parameter
DSsatDS Vv ])(2[2
DSDSThGSD vvVvi −−=
• Linear region (exc)
iD – linear dependence on vGS
- also influenced by the output voltage vDS
VTh= 0.58 V, β = 104 μA/V2
ThGSDSsat VvV −=
2)( ThGSD Vvi −=
• Saturation region (aF)
DSsatDS Vv
iD – quadratic dependence on vGS
drain-source saturation voltage
vDS< vDSsat
active (saturation)
region
linear region
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Transfer and output characteristics
C13 – MOSFET operation
➢ Output characteristics
GSDSD vvi ,)( as parameter
DSsatDS Vv ])(2[2
DSDSThGSD vvVvi −−=
• Linear region (exc)
iD – linear dependence on vGS
- also influenced by the output voltage vDS
2)( ThGSD Vvi −=
• Saturation region (aF)
DSsatDS Vv
iD – quadratic dependence on vGS
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Quiescent pointC13 – MOSFET operation
IDex
Quiescent point Q = a point on the output characteristic iD(vDS) of MOSFET
▪ Q is defined by VDS and ID
▪ Q(VDS, ID) is at the intersection between the load line and the output characteristic corresponding to vGS
Load line:
vDS = VPS - RD iD
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Operating regionsC13 – MOSFET operation
Modes of useswitching mode: (off) / (exc)
as an amplifier: (aF)
voltage-controlled linear resistance (small vDS)
T – (aF): VTh < VGS < VGS3
T – (off): VGS < VTh
T – (exc): VGS > VGS3
])(2[2
DSDSThGSD vvVvi −−=
2)( ThGSD Vvi −=
0=Di
IDex
Boundary (aF) - (exc): vDS,sat = vGS - VTh
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Operating regionsC13 – MOSFET operation
Linear region (exc):
])(2[2
DSDSThGSD vvVvi −−=
Saturation (active) region (aF):
2)( ThGSD Vvi −=
L
WK
2= ( )2
2ThGSD Vv
L
WKi −=
For integrated transistors:
In the active region
K – transconductance parameter [µA/V2]W - the width of the channel [µm]L - the length of the channel [µm]
β – parameter of the MOSFET - beta factor
- measured in µA/V2, mA/V2, A/V2
- constructive parameter
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➢ Example - 1
Compute the values of VGS, ID, VDS, VGD, VDSsat for:i) vCo = 0.5 V; ii) vCo = 2.5 V; iii) vCo= 2.75 V.
Place the quiescent points Q(VDS, ID) in the plan of output characteristics.
Find the operating regions of T.
ExamplesC13 – MOSFET operation
V5
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➢ Example - 2
a) What is the operating region of T for:i) vGS = 0.8 V; ii) vGS = 2.5 V; iii) vGS= 4 V.
b) What is the minimum value of vGS so that T stays in (exc)?
ExamplesC13 – MOSFET operation
VPS
Β = 2 mA/V2
VTh = 1 V
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➢ Example - 2
a)
i) Because vGS < VTh => T - (off)
ii) vGS > VTh T is on, in (aF) or in (exc)
▪ Assume T in (aF), so that iD = β(vGS – VTh)2
▪ Compute vDS
▪ Compare vDS with vDSsat :
If vDS > vDSsat, the assumption was true, and T is in (aF)
If vDS < vDSsat, the assumption was false, and T is in (exc)
ExamplesC13 – MOSFET operation
VPS
Β = 2 mA/V2
VTh = 1 V
Solution:
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➢ Example - 2
a) ii)
ExamplesC13 – MOSFET operation
VPS
Β = 2 mA/V2
VTh = 1 V
Solution:
iD = 2(2.5 -1)2= 4.5mA
vDS = VPS - RD iD
vDS = 20 - 3·4.5 = 6.5 V
vDS sat= vGS – VTh = 2.5 - 1= 1.5 V
vDS > vDssat, T is in (aF)
vGD = vGS – vDS = 2.5 - 6.5 = -4V < VTh
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➢ Example - 2
a) iii) vGS > VTh T is on, in (aF) or in (exc)
▪ Assume T in (aF), so that iD = β(vGS – VTh)2
▪ Compute vDS
▪ Compare vDS with vDSsat :
ExamplesC13 – MOSFET operation
VPS
Β = 2 mA/V2
VTh = 1 V
Solution:
vDS = 20 - 3·18 = -34 V
vDS sat= vGS – VTh = 4 - 1= 3 V vDS < vDssat, T is in (exc)
iD = 2(4 -1)2= 18 mA
Alternative method: compare the value of iD in (aF) with iDex
mA67.63
20,==
−=
R
V
R
vVi PSexDSPS
Dex iD = 18 mA > iDex = 6.67 mA T - (exc)
What is the actual value of iD?
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➢ Example - 2
b) Minimum value of vGS so that T stays in (exc)
ExamplesC13 – MOSFET operation
VPS
Β = 2 mA/V2
VTh = 1 V
Solution:
The operating point of T is placed on the vDSsat curve:
vDSsat = VPS – RiD
vDSsat = vGSmin - VTh
R β(vGSmin – VTh)2+ (vGSmin - VTh) - VPS= 0
Two solutions for vGSmin, choose the convenient one: vGSmin >VTh
VGSmin = 2.744 V
VPS – RiD = vGSmin – VTh
iD = β(vGSmin - VTh)2
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➢ Example - 3
ExamplesC13 – MOSFET operation
a) Determine the operating region of the transistor for:i) R1 = 4.5 MΩ; R2 = 0.5 MΩ; ii) R1 = 12 MΩ; R2 = 6 MΩ;
b) What can be the range for R1 with R2 = 0.5 MΩ so that the transistor is (on)?
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SummaryThe MOSFET is no longer a random acronym, after digging into:
➢ Symbols➢ Structure and physical operation➢ Operating principle➢ Transfer and output characteristics➢ Quiescent point ➢ Operating regions➢ Examples
Next week: Recap. Preparation for exam.
To do: Homework 10
C13 – MOSFET operation