Download Chapter 5 Lecture Notes - the GMU ECE Department

ECE 333 Linear Electronics
Chapter 5 MOS Field-Effect Transistors
(MOSFETs)
Why MOSFETs  Device Structure  Physical
Operation  I-V Characteristics  MOSFET Circuits at
DC  The Body Effect and Other Topics
1
Introduction
•
•
•
•
•
•
The invention of MOSFETs (page 248)
MOSFET vs. BJT
MOSFET in Integrated Circuits
VLSI
Digital Circuit and Analog Circuit
Memory
2
5.1 Device Structure and Physical Operation
• 5.1.1 Device Structure
3
5.1.1 Device Structure
(b) cross section. Typically L = 0.03 μm to 1 μm, W = 0.05 μm to 100
μm, and the thickness of the oxide layer (tox) is in the range of 1 to 10
nm.
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5.1.2 Operation with Zero Gate Voltage
• Two pn junctions back-to-back
• Prevent current conduction from drain to
source when vD is applied
• Very high resistance (of the order of 1012 Ω)
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5.1.3 Creating a Channel for Current Flow
Figure 5.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate.
An n channel is induced at the top of the substrate beneath the gate.
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5.1.3 Creating a Channel for Current Flow
• Overdrive voltage vOV
𝑣𝑂𝑉 = 𝑣𝐺𝑆 − 𝑉𝑇
• Electron Charge in the channel
|𝑄| = 𝐶𝑜𝑥 (𝑊𝐿)𝑣𝑂𝑉
• Oxide Capacitance
𝐶𝑜𝑥
𝜖𝑜𝑥
=
𝑡𝑥𝑜
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5.1.4 Applying a Small vDS
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5.1.4 Applying a Small vDS
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5.1.4 Applying a Small vDS
|𝑄|
= 𝐶𝑜𝑥 𝑊𝑣𝑂𝑉
𝑢𝑛𝑖𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
Electron drift velocity = 𝜇𝑛 𝐸 =
𝑣𝐷𝑆
𝜇𝑛
𝐿
𝑊
𝑖𝐷 = [(𝜇𝑛 𝐶𝑜𝑥 )( )𝑣𝑂𝑉 ]𝑣𝐷𝑆
𝐿
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5.1.4 Applying a Small vDS
|𝑄|
= 𝐶𝑜𝑥 𝑊𝑣𝑂𝑉
𝑢𝑛𝑖𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑙𝑒𝑛𝑔𝑡ℎ
Electron drift velocity = 𝜇𝑛 𝐸 =
𝑣𝐷𝑆
𝜇𝑛
𝐿
𝑊
𝑖𝐷 = [(𝜇𝑛 𝐶𝑜𝑥 )( )𝑣𝑂𝑉 ]𝑣𝐷𝑆
𝐿
𝑟𝐷𝑆 =
1
𝜇𝑛 𝐶𝑜𝑥 𝑊 𝐿 𝑣𝑂𝑉
𝑟𝐷𝑆 =
1
𝜇𝑛 𝐶𝑜𝑥 𝑊 𝐿 (𝑣𝐺𝑆 −𝑉𝑡 )
𝑘𝑛 = 𝜇𝑛 𝐶𝑜𝑥 𝑊 𝐿
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5.1.5 Operation as vDS is Increased
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13
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5.1.5 Operation as vDS is Increased
1
[𝑉𝑂𝑉 + 𝑉𝑂𝑉 − 𝑣𝐷𝑆 ] ∙ 𝑣𝐷𝑆
2
𝑊
1
𝑖𝐷 = 𝑘′𝑛 ( )(𝑉𝑂𝑉 − 𝑣𝐷𝑆 )𝑣𝐷𝑆
𝐿
2
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5.1.5 Operation as vDS is Increased
𝑊
1
𝑖𝐷 = 𝑘′𝑛 ( )(𝑉𝑂𝑉 − 𝑣𝐷𝑆 )𝑣𝐷𝑆
𝐿
2
𝑖𝐷 =
𝑘′
𝑛
𝑊
1
[(𝑣𝐺𝑆 −𝑉𝑡 )𝑣𝐷𝑆 − 𝑣𝐷𝑆 2 ]
𝐿
2
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5.1.6 Operation for vDS>VOV: Channel
Pinch-off and Current Saturation
𝑣𝐷𝑆 = 𝑉𝑂𝑉
Triode region
1
𝑎𝑟𝑒𝑎 = 𝑉𝑂𝑉 ∙ 𝑣𝐷𝑆 | at this point
2
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
𝑣𝑂𝑉 2
2
𝐿
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5.1.6 Operation for vDS>VOV: Channel
Pinch-off and Current Saturation
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
𝑣𝑂𝑉 2
2
𝐿
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
(𝑣𝐺𝑆 − 𝑉𝑡 )2
2
𝐿
Example 5.1 Consider a process technology for which Lmin=0.4um,
tox=8nm, un=450 cm2/Vs, and Vt=0.7V.
(a)Find Cox and k’n.
(b)For a MOSFET with W/L=8 um / 0.8 um, calculate VOV, VGS and VDSmin
needed to operate the transistor in the saturation region with a dc
current ID=100uA.
(c)For the device in (b), find the values of VOV and VGS required to cause
the device to operate as a 1000-Ω resistor for very small vDS.
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5.1.7 The p-Channel MOSFET
To form a channel, it must have
𝑣𝐺𝑆 ≤ 𝑉𝑡𝑝
|𝑣𝐺𝑆 | ≥ |𝑉𝑡𝑝 |
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5.1.8 Complementary MOS or CMOS
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5.2 Current-Voltage Characteristics
• 5.2.1 Circuit Symbol
Figure 5.11 (a) Circuit symbol for the n-channel enhancement-type
MOSFET. (b) Modified circuit symbol with an arrowhead on the source
terminal to distinguish it from the drain and to indicate device polarity
(i.e., n channel). (c) Simplified circuit symbol to be used when the source
is connected to the body or when the effect of the body on device
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operation is unimportant.
5.2.2 i-v
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5.2.2 i-v
Figure 5.12 The relative levels of the terminal voltages of the
enhancement NMOS transistor for operation in the triode region
and in the saturation region.
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5.2.2 i-v
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5.2.3 The iD – vGS Characteristics
• When the MOSFET is used to design an
amplifier, it is operated in the saturation
region.
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
(𝑣𝐺𝑆 − 𝑉𝑡𝑛 )2
2
𝐿
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
𝑣𝑂𝑉 2
2
𝐿
Nonlinear  linear amplifier? (chapter 7)
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5.2.3 The iD – vGS Characteristics
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5.2.3 The iD – vGS Characteristics
Figure 5.15: Large-signal, equivalent-circuit model of an
n-channel MOSFET operating in the saturation region.
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Example 5.2 Consider an NMOS transistor fabricated in a 0.18um process with L=0.18 um and W=2um. The process technology
is specified to have Cox=8.6 fF/um, un=450cm2/Vs and Vtn=0.5V.
(a)Find VGS and VDS that result in the MOSFET operating at the
edge of saturation with iD=100uA.
(b) If VGS is kept constant, find VDS that results in iD=50uA.
(c) To investigate the use of the MOSFET as a linear amplifier, let
it be operating in saturation with VDS=0.3V. Find the change in iD
resulting from vGS changing from 0.7 V by 0.01V and by -0.01V.
Solution:
Kn=unCox(W/L)=4.3 mA/V2
1 ′ 𝑊
2
(a) With nMOSFET in saturation: 𝑖𝐷 = 2 𝑘 𝑛 𝐿 𝑣𝑂𝑉
So, vOV=0.22V
VGS=vOV + Vtn = 0.72 V
At the edge: VDS = VGS – Vtn = 0.22 V
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(b) With VGS kept constant at 0.72V, iD reduced  the nMOSFET
will now be operating in triode region
𝑊
1
𝑖𝐷 = 𝑘′𝑛 ( )(𝑉𝑂𝑉 − 𝑣𝐷𝑆 )𝑣𝐷𝑆
𝐿
2
Because VOV is 0.22 V with VGS = 0.72V,
We have: VDS = 0.06 V or 0.39 V (two solutions from the above eq.
Because 0.39 V is above VOV, not in triode region, so only
VDS=0.06V is correct solution.
(c) For vGS=0.7V, VOV=0.2V and VDS=0.3V, the transistor is
1
operating in saturation region:
𝑖𝐷 = 𝑘𝑛 𝑣𝑂𝑉 2 = 86 𝜇𝐴
2
Now with VGS=0.71V, VOV=0.21V, iD=94.8 uA
With VGS=0.69V, VOV=0.19V, iD=77.6 uA.
The change of + or – 0.01V, the change of iD is similar 8.8 ~ 8.4 uA
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5.2.4 Finite output resistance in Saturation
• Channel-length modulation (real MOSFET)
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
2
𝐿
𝑣𝐺𝑆 − 𝑉𝑡𝑛
2
(1 + 𝜆𝑣𝐷𝑆 )
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5.2.4 Finite output resistance in Saturation
1 ′ 𝑊
𝑖𝐷 = 𝑘 𝑛
2
𝐿
𝑣𝐺𝑆 − 𝑉𝑡𝑛
2
(1 + 𝜆𝑣𝐷𝑆 )
ro: output resistance
𝑑𝑖𝐷
𝑟𝑜 =
𝑑𝑣𝐷𝑆
−1
𝑤𝑖𝑡ℎ 𝑣𝐺𝑆 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
VA: A device parameter in V
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5.2.4 Finite output resistance in Saturation
𝑑𝑖𝐷
𝑟𝑜 =
𝑑𝑣𝐷𝑆
−1
𝑤𝑖𝑡ℎ 𝑣𝐺𝑆 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝑟𝑜 =
𝑉𝐴
𝐼′𝐷
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5.2.5 Characteristics of the p-Channel MOSFET
Figure 5.19 (a) Circuit symbol for the p-channel enhancement-type
MOSFET. (b) Modified symbol with an arrowhead on the source lead.
(c) Simplified circuit symbol for the case where the source is
connected to the body.
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5.3 MOSFET Circuits at DC
• Simple model: 𝜆 = 0
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5.4 The Body Effect and Other Topics
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• Temperature Effects:
(i) Vt decreases by about 2 mV for every 1oC rise
in temperature
(ii) K’ decreases with temperature increase –
dominant effect
So current decreases with increasing
temperature
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• Breakdown and input protection
• Velocity Saturation (107 cm/s limit)
• Depletion-type MOSFET
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