Microelectronics

1
Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 3
The Field Effect Transistor
Neamen
Microelectronics, 4e
Chapter 3-1
McGraw-Hill
2
In this chapter, we will
? Study and understand the operation and
characteristics of the various types of
MOSFETs.
? Understand and become familiar with the dc
analysis and design techniques of MOSFET
circuits.
? Examine three applications of MOSFET circuits.
? Investigate current source biasing of MOSFET
circuits, such as those used in integrated
circuits.
? Analyze the dc biasing of multistage or
multitransistor circuits.
? Understand the operation and characteristics of
the
junction field-effect transistor, and analyze the
dc response of JFET circuits.
Neamen
Microelectronics, 4e
Chapter 3-2
McGraw-Hill
3
Basic Structure of MOS Capacitor
Neamen
Microelectronics, 4e
Chapter 3-3
McGraw-Hill
4
MOS Capacitor Under Bias Electric Field and
Charge
Parallel plate capacitor
Negative gate bias
Positive gate bias
Holes attracted to gate
Electrons attracted to gate
Neamen
Microelectronics, 4e
Chapter 3-4
McGraw-Hill
5
Schematic of n-Channel
Enhancement Mode MOSFET
Neamen
Microelectronics, 4e
Chapter 3-5
McGraw-Hill
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Basic Transistor Operation
After electron inversion layer is
Before electron inversion layer is
formed
formed
Neamen
Microelectronics, 4e Chapter 3-6 McGraw-Hill
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Basic Transistor Operation
Neamen
Microelectronics, 4e
Chapter 3-7
McGraw-Hill
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Current Versus Voltage Characteristics
Enhancement-Mode nMOSFET
Neamen
Microelectronics, 4e
Chapter 3-8
McGraw-Hill
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Family of iD Versus vDS Curves Enhancement-Mode
nMOSFET
Neamen
Microelectronics, 4e
Chapter 3-9
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p-Channel Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
Chapter 3-10
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Symbols for n-Channel
Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
Chapter 3-11
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Symbols for p-Channel
Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
Chapter 3-12
McGraw-Hill
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n-Channel Depletion-Mode MOSFET
Neamen
Microelectronics, 4e
Chapter 3-13
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Family of iD Versus vDS Curves Depletion-Mode
nMOSFET
Symbols
Neamen
Microelectronics, 4e
Chapter 3-14
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p-Channel Depletion- Mode MOSFET
Symbols
Neamen
Microelectronics, 4e
Chapter 3-15
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Cross-Section of nMOSFET and pMOSFET
Both transistors are used in the fabrication of
CMOS circuitry.
Neamen
Microelectronics, 4e
Chapter 3-16
McGraw-Hill
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Summary of I-V Relationships
Neamen
Microelectronics, 4e
Chapter 3-17
McGraw-Hill
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Enhancement and Depletion Mode
? In field-effect transistors (FETs), depletion
mode and enhancement mode are two major
transistor types, corresponding to whether the
transistor is in an ON state or an OFF state at
zero gate-source voltage.
? Enhancement-mode MOSFETs are the common
switching elements in most MOS logic families.
These devices are OFF at zero gate-source
voltage, and can be turned on by pulling the
gate voltage in the direction of the drain
voltage that is, toward the VDD supply rail,
which is positive for NMOS logic and negative
for PMOS logic.
? In a depletion-mode MOSFET, the device is
normally ON at zero gate-source voltage. Such
devices are used as load "resistors" in logic
circuits (in depletion-load NMOS logic, for
example).
Neamen
Microelectronics, 4e
Chapter 3-18
McGraw-Hill
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Enhancement and Depletion Mode
? For an n-channel enhancement -mode MOSFET, a
positive gate to source voltage greater than
the threshold voltage VTN must be applied to
induce an electron inversion layer. For VGS gt
VTN the device is turned on.
? For an n-channel depletion-mode MOSFET, a
channel between source and drain exists even
for VGS0. The threshole voltage is negative,
so that a negative voltage is required to turn
the device off.
? Current voltage relations are the ideal
relations for long channel devices. Long
channel devices are the ones whose channel
length is 2um. However we are in the order of
0.2um nowadays.
Neamen
Microelectronics, 4e
Chapter 3-19
McGraw-Hill
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Non-ideal Current Voltage Characteristics
? Finite output resistance ? Body effect
? Subthreshold conduction ? Breakdown effects ?
Temperature effects
Neamen
Microelectronics, 4e
Chapter 3-20
McGraw-Hill
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Channel Length Modulation Early Voltage
Neamen
Microelectronics, 4e
Chapter 3-21
McGraw-Hill
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Body Effect
The occupancy of the energy bands in a
semiconductor is set by the position of the Fermi
level relative to the semiconductor energy-band
edges. Application of a source-to-substrate
reverse bias of the source-body pn-junction
introduces a split between the Fermi levels for
electrons and holes, moving the Fermi level for
the channel further from the band edge, lowering
the occupancy of the channel. The effect is to
increase the gate voltage necessary to establish
the channel, as seen in the figure. This change
in channel strength by application of reverse
bias is called the body effect.
Neamen
Microelectronics, 4e
Chapter 3-22
McGraw-Hill
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Subthreshold Condition
As MOSFET geometries shrink, the voltage that can
be applied to the gate must be reduced to
maintain reliability. To maintain performance,
the threshold voltage of the MOSFET has to be
reduced as well. As threshold voltage is
reduced, the transistor cannot be switched from
complete turn-off to complete
turn-on with the limited voltage swing available
the circuit design is a compromise between
strong current in the "on" case and low current
in the "off" case, and
the application determines whether
to favor one over the other. Subthreshold leakage
(including subthreshold conduction, gate-oxide
leakage and reverse-biased junction leakage),
which was ignored in the past, now can consume
upwards of half of the total power consumption
of modern high-performance VLSI chips
Neamen
Microelectronics, 4e
Chapter 3-23
McGraw-Hill
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NMOS Common-Source Circuit
Neamen
Microelectronics, 4e
Chapter 3-24
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PMOS Common-Source Circuit
Neamen
Microelectronics, 4e
Chapter 3-25
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Load Line and Modes of Operation NMOS
Common-Source Circuit
Neamen
Microelectronics, 4e
Chapter 3-26
McGraw-Hill
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Problem-Solving Technique NMOSFET DC Analysis
1. Assume the transistor is in saturation.
a. VGS gt VTN, ID gt 0, VDS VDS(sat)
2. Analyze circuit using saturation I-V
relations.
3. Evaluate resulting bias condition of
transistor.
a. If VGS lt VTN, transistor is likely in cutoff
b. If VDS lt VDS(sat), transistor is likely in
nonsaturation region
4. If initial assumption is proven incorrect,
make new assumption and repeat Steps 2 and 3.
Neamen
Microelectronics, 4e
Chapter 3-27
McGraw-Hill