Download MOSFETs

Microelectronics
Circuit Analysis and Design
Donald A. Neamen
Chapter 3
The Field Effect Transistor
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-1
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
McGraw-Hill
Chapter 3-2
Basic Structure of MOS Capacitor
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-3
MOS Capacitor Under Bias:
Electric Field and Charge
Parallel plate capacitor
Negative gate bias:
Holes attracted to gate
Neamen
Microelectronics, 4e
McGraw-Hill
Positive gate bias:
Electrons attracted to gate
Chapter 3-4
Schematic of n-Channel
Enhancement Mode MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-5
Basic Transistor Operation
After electron
inversion layer is
formed
Before electron
inversion layer is
formed
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Microelectronics, 4e
McGraw-Hill
Chapter 3-6
Basic Transistor Operation
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-7
Current Versus Voltage Characteristics:
Enhancement-Mode nMOSFET
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Microelectronics, 4e
McGraw-Hill
Chapter 3-8
Family of iD Versus vDS Curves:
Enhancement-Mode nMOSFET
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Microelectronics, 4e
McGraw-Hill
Chapter 3-9
p-Channel Enhancement-Mode
MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-10
Symbols for n-Channel
Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-11
Symbols for p-Channel
Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-12
n-Channel Depletion-Mode MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-13
Family of iD Versus vDS Curves:
Depletion-Mode nMOSFET
Symbols
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Microelectronics, 4e
McGraw-Hill
Chapter 3-14
p-Channel DepletionMode MOSFET
Symbols
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-15
Cross-Section of nMOSFET and pMOSFET
Both transistors are used in the fabrication of CMOS circuitry.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-16
Summary of I-V Relationships
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-17
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
McGraw-Hill
Chapter 3-18
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 > VTN the device is
turned on.
 For an n-channel depletion-mode MOSFET, a channel
between source and drain exists even for VGS=0. 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
McGraw-Hill
Chapter 3-19
Non-ideal Current Voltage
Characteristics
Finite output resistance
 Body effect
 Subthreshold conduction
 Breakdown effects
 Temperature effects

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Microelectronics, 4e
McGraw-Hill
Chapter 3-20
Channel Length Modulation:
Early Voltage
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Microelectronics, 4e
McGraw-Hill
Chapter 3-21
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
McGraw-Hill
Chapter 3-22
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
McGraw-Hill
Chapter 3-23
NMOS Common-Source Circuit
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Microelectronics, 4e
McGraw-Hill
Chapter 3-24
PMOS Common-Source Circuit
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Microelectronics, 4e
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Chapter 3-25
Load Line and Modes of Operation:
NMOS Common-Source Circuit
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Microelectronics, 4e
McGraw-Hill
Chapter 3-26
Problem-Solving Technique:
NMOSFET DC Analysis
1. Assume the transistor is in saturation.
a. VGS > VTN, ID > 0, & VDS ≥ VDS(sat)
2. Analyze circuit using saturation I-V relations.
3. Evaluate resulting bias condition of transistor.
a. If VGS < VTN, transistor is likely in cutoff
b. If VDS < 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
McGraw-Hill
Chapter 3-27