figure 8.1 (a) schematic cross section and (b) the circuit symbol of an n-channel mosfet
DESCRIPTION
1. 8.1 MOSFET PRINCIPLES. 8.1.1 MOSFET Structure. Figure 8.1 (a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET. Figure 8.2 Types of MOSFETs. 8.1.2 MOSFET as a Voltage – Controlled Switch. Figure 8.3 Cross-sectional illustrations of a MOSFET in (a) off - PowerPoint PPT PresentationTRANSCRIPT
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Figure 8.1 (a) Schematic cross section and (b) the circuit symbol of an N-channel MOSFET.
8.1 MOSFET PRINCIPLES
8.1.1 MOSFET Structure
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Figure 8.2 Types of MOSFETs.
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Figure 8.3 Cross-sectional illustrations of a MOSFET in
(a) off (b) on modes, along with(c) the corresponding currentvoltage characteristics.
chnch Wx
L
qnWx
LR
1
(8.1)
8.1.2 MOSFET as a Voltage – Controlled Switch
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Figure 8.4 Two-dimensional energy-band diagram for the semiconductor part of an N-channel MOSFET in the flat-band condition. The two different colors in the conduction band indicate the two different types of doping: N type in the source and drain regions,and P type in the body. Darker colors indicate higher carrier concentration, and the nearly white areas indicate the depletion region.
W
L
VVCR
TGSoxn )(
1
DSTGSoxnDS
D VVVL
WC
R
VI )(
(8.2)
(8.3)
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Figure 8.5 Two-dimensional energy-band diagrams for an N-channel MOSFET acting as a switch in (a) off mode and (b) on mode. Thetwo colors in the conduction band indicate the concentrations of electrons and holes: darker colors correspond to higher carrierconcentrations, whereas the nearly white areas indicate depleted regions. Note that the depleted regions correspond to the areas withsloped energy bands and therefore with existence of built-in and externally applied electric field.
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Figure 8.6 (a) The circuit of CMOS inverter. (b) Typical input/output signals.
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Figure 8.7 Illustration of the body effect.
(a) VSB voltage increases the barrier between the electrons in the source and the drain.
(b) The surface potential of 2F does not reduce the barrier sufficiently for the electrons to be able to move into the channel.
(c) The surface potential needed to form the channel is 2F VSB .
8.1.3 The Threshold Voltage and the Body Effect
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SBFs V 2
)for( TGSFBox
dsFBSBGS VVV
C
QVVV
effective gate-to-body voltage
voltage across the gate oxide
ox
dSBFFBSBT C
QVVVV )2(
ox
dFFBT C
QVV 2
(8.4)
(8.5)
(8.6)
(8.7)
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)2(2 SBFAsd VqNQ
SBFFFBT VVV 22
)22()0()( FSBFSBTSBTT VVVVVV
BSFFFBT VVV 22
(8.8)
(8.9)
(8.10)
(8.11)
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Figure 8.8 (a) Cross section of a MOSFET in the saturation region. (b) The corresponding IDVDS characteristics.
8.1.4 MOSFET as a Voltage – Controlled Current Source : Mechanisms of Current Saturation
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Figure 8.9 Two-dimensional energy-band diagrams for an N-channel MOSFET in saturation due to channel pinch off. A comparisonof the smaller VDS bias in (a) to the larger VDS value in (b) shows that the channel is shortened by the increased drain-to-source bias,but the concentration of electrons in the channel is not changed. As in the waterfall analogy, the drain current is limited by theconcentration of the electrons in the channel and not by the height of the fall.
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Figure 8.10 (a) Output and (b) transfer characteristics of a MOSFET.
)(saturation)(
)0(region triode)(
)(cutoff of ldsubthresho0
,,
,.
.
DSsatDSDSsatSBGS
DSsatDSDSSBGS
TGSD
VVVVVf
VVVVVf
VVI
(8.12)
8.2 PRINCIPAL CURRENT – VOLTAGE CHARACTERISTICS AND EQUATIONS
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Ej
nEqj 0
effDS
IDLV
mCQ
ch
AI
ch EWqnxWxj ][
0
][ 2
DSIeff
D VQL
WI 0
(8.13)
(8.14)
(8.15)
(8.16)
8.2.1 SPICE LEVEL 1 Model
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)Vfor()( T GSoxTGSII VCVVQQ
oxTGSI CvVVvQ )()(
oxDS
TGS
V
TGSDS
ox
V
IDS
I
CV
VV
dvvVVV
C
dvvQV
Q
DS
DS
2
)(
)(1
0
0
2)(
2DS
DSTGSD
VVVVI
effox L
WC0
(8.17)
(8.18)
(8.19)
(8.20)
(8.21)
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Figure 8.11 Output characteristics corresponding to SPICE LEVEL 1 model. Solid lines, Eq. (8.20);dashed lines, saturation current.
)()(0 effeffox LWKPLWC
TGSDSsat VVV
TGSDSsatDS
D VVVV
I
0
2)(2
)( TGSDSsatDDsat VVVII
(8.22)
(8.23)
(8.24)
(8.25)
DSsatDSTGS
DSsatDSDSDSTGSD
VVifVV
VVifVVVVI
2
2
)(2
0]2)[(
(8.26)
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Figure 8.12 N-channel MOSFET diagram, indicating the surface potential at the source and the drain ends of the channel.
8.2.2 SPICE LEVEL 2 Model
ss
SBFSBFSBFBT VVVVV
22
ssSBFBsT VVv )(
(8.27)
(8.28)
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dyCvVL
dyyQL
Qeffeff L
oxsTGSeff
L
Ieff
I 00
)]([1
)(1
s
VV
V oxsTGSDS
s
VV
V sISBFDSSBF
I
dCvVV
dQVVV
Q
DSSBF
SBF
DSSBF
SBF
2
2
2
2
)]([1
)()2()2(
1
(8.29)
(8.30)
2323 22
3
2
22 SBFDSSBFDS
DSFFBGS
DS
oxI VVVV
VVV
V
CQ
(8.31)
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2323 22
3
2
22 SBFDSSBFDS
DSFFBGSD VVVV
VVVI
(8.32)
1)(4
12
20
2
2
SBFBGS
FFBGSDSsatDS
D
VVV
VVVV
I
(8.33)
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2)0()0()0(
2DS
DDSDDD
VIVIII
21
212
2
21
21
)2(2
1)0(
)2(2
1
)2()2()0(
)2()2(
0)0(
SBFD
DSSBFDS
D
DSDS
DD
SBFFFBGSD
DSSBFDSFFBGSDS
DD
D
VI
VVV
I
VV
II
VVVI
VVVVVV
II
I
(8.34)
(8.35)
8.2.3 SPICE LEVEL 3 Model : Principal Effects
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DSDS
F
SBFV
SBFFFBGSD VVV
VVVI
B
T
221
2
1)22(
SBF
BV
F
22
(8.36)
(8.37)
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2)()1(2
)( TGSB
DSsatDDsat VVF
VII
DSsatDSTGSB
DSsatDSDS
BDSTGSD
VVifVVF
VVifV
FVVVI
2
2
)()1(2
02
1
B
TGSDSsat F
VVV
1
(8.39)
(8.40)
(8.41)
B
TGSDSsat
DS
D
F
VVV
V
I
10 (8.38)
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Figure 8.13 Comparison of LEVEL 1, LEVEL 2, and LEVEL 3 models.
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Figure 8.14 Influence of mobility reduction with gate voltage on (a) transfer and (b) output characteristics.
)(10
TGSs VV
mobilitysurface
(8.42)
8.3 SECOND – ORDER EFFECTS8.3.1 Mobility Reduction with Gate Voltage
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eff
DSs
seff
LV
v
mobilityeffective
max
1
(8.43)
effeff
oxeff
L
WKP
L
WC
(8.44)
8.3.2 Velocity Saturation ( Mobility Reduction with Drain Voltage)
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Figure 8.15 Energy-band diagrams along the channel of a MOSFET illustrating the drain-induced barrier lowering ( DIBL).
pincheffpincheff
oxeff
LL
WKP
LL
WC
DSDSBFsFFBT VVFVV 22
(8.45)
(8.46)
8.3.3 Finite Output Resistance
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Figure 8.16 Output characteristics with (solid lines) and without (dashed lines) the influence of VDS on VT .
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Figure 8.17 Illustration of the threshold voltage related short-channel effect.
SBFsFFBT VFVV 22 (8.47)
Figure 8.18 Threshold voltage dependence on the MOSFET channel length.
8.3.4 Threshold – Voltage – Related Short – Channel Effects
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Figure 8.19 Illustration of the narrow channel effect. Fringing electric field wastes the gate voltage, causing a threshold voltage increase in narrow-channel MOSFETs.
)2(22 SBFnDSDSBFsFFBT VFVVFVV (8.48)
8.3.5 Threshold – Voltage – Related Narrow – Channel Effects
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Figure 8.20
The use of a logarithmic drain axis emphasizes the subthreshold region of a transfer characteristic (the subthreshold current cannot be seen with a linear axis). The subthreshold swing is 60 mV/decade, corresponding to ns 1 in Eq. (8.49).
kTnVDsubthD
sGSeII /0 (8.49)
8.3.6 Subthreshold Current
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8.4 NANOSCALE MOSFETs8.4.1 Down – Scaling Benefits and Rules
Figure 8.21 Deep-submicron MOSFET structure.
Table 8.1 General Down-Scaling Rules and Their Effects
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Figure 8.22 Transfer characteristics of 100-nm MOSFETs (the channel width is 1 m).
(a) Devices with similar characteristics wereexperimentally demonstrated as early as 1987, but they exhibit pronounced short-channel effects and very high off current.
(b) As known from the down-scaling rules, an increase in the substrate doping eliminates the short-channel effects, but also increases the threshold voltage,in this case above the 1.0-V supply voltage.
(c) Down-scaling rules require a 1.1-nm gate oxide that leaks a very high input current.
8.4.2 Leakage Currents
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Figure 8.23 (a) Energy-band diagram and (b) equipotential contours for the MOSFET with the transfer characteristics shownin Fig. 8.22a (VGS 0 V, VDS 1.0 V).
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Figure 8.24 Energy-band diagram (a) and equipotential contours (b) for the MOSFET with the transfer characteristics shown in Fig. 8.22c (VGS 0 V, VDS 1.0 V).
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d
d
tC
gate-dielectric capacitance per unit area : (8.50)
Figure 8.25 MOSFET with engineered doping profile in the substrate.Figure 8.26 SOI MOSFET with ultra-thin body.
8.4.3 Advanced MOSFETs
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Figure 8.27 FinFET: the most promising structure for nanoscale MOSFETs.
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Figure 8.28 Failure causes and mechanisms leading to changes of oxide charge, and border and interface trap densities.
Figure 8.29 The threshold voltage “turn-around” effect is a manifestation of the link between hole trapping and the creation of interface traps.
8.4.4 Reliability Issues
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Figure 8.30 Mechanisms of positive gate-oxide charge creation by a high gate-oxide field.
(a) Negative gate bias. (b) Positive gate bias.
oxEEAer /0 (8.51)
High Oxide Field
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Figure 8.31 Mechanism of channel hot-electron injection into the gate oxide.
(a) Illustration at the device cross section. (b) Energy bands at the point of hot electron injection.
Hot Carriers
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Figure 8.32 Mechanism of avalanche hot-carrier injection into the gate oxide.
msubIAt )(
max/EBsub CeI
(8.52)
(8.53)
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Table 8.2 Results of Hot-Carrier Accelerated Testing
Example 8.5
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Figure 8.33 The architecture of DRAMs with 1C1T memory cells.
8.5.1 1C1T DRAM Cell
Figure 8.34 The cross section of a 1C1T memory cell.
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Figure 8.35 Flash-memory MOSFET.
(a) The cross section. (b) The transfer characteristics. (c) The energy bands in erased state. (d) The energy bands in programmed state.
8.5.2 Flash – Memory Cell