Download Chap007-2011

Chapter 7
Complementary MOS (CMOS) Logic
Design
Microelectronic Circuit Design
Richard C. Jaeger
Travis N. Blalock
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 1
Chapter Goals
•
•
•
•
•
•
•
•
•
•
Introduce CMOS logic concepts
Explore the voltage transfer characteristics of CMOS inverters
Learn to design basic and complex CMOS logic gates
Discuss the static and dynamic power in CMOS logic
Present expressions for dynamic performance of CMOS logic
devices
Present noise margins for CMOS logic
Introduce dynamic logic and domino CMOS logic techniques
Introduce design techniques for “cascade buffers”
Explore layout of CMOS logic gates
Discuss the concept of “latchup”
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 2
CMOS Inverter Technology
• Complementary MOS, or CMOS, needs both
PMOS and NMOS devices for the logic gates to
be realized
• The concept of CMOS was introduced in 1963 by
Wanlass and Sah, but it did not become common
until the 1980’s as NMOS microprocessors were
dissipating as much as 50 W and alternative
design technique was needed
• CMOS dominates digital IC design today
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 3
CMOS Inverter Technology
• The CMOS inverter consists of a PMOS device stacked on
top on an NMOS device, but they need to be fabricated on
the same wafer
• To accomplish this, the technique of “n-well” implantation is
needed as shown in this cross-section of a CMOS inverter
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 4
CMOS Inverter
(a) Circuit schematic for a CMOS inverter
(b) Simplified operation model with a high input applied
(c) Simplified operation model with a low input applied
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 5
CMOS Inverter Operation
• When vI is pulled high (to VDD), the PMOS
transistor is turned off, while the NMOS device is
turned on pulling the output down to VSS
• When vI is pulled low (to VSS), the NMOS
transistor is turned off, while the PMOS device is
turned on pulling the output up to VDD
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 6
CMOS Inverter Layout
• Two methods of laying
out a CMOS inverter
are shown
• The PMOS transistors
lie within the n-well,
whereas the NMOS
transistors lie in the psubstrate
• Polysilicon is used to
form common gate
connections, and metal
is used to tie the two
drains together
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 7
Static Characteristics of the CMOS
Inverter
• The figure shows the two
static states of operation
with the circuit and
simplified models
• Notice that VH = 5V and
VL = 0V, and that ID =
0A which means that
there is no static power
dissipation
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 8
CMOS Voltage Transfer Characteristics
The VTC shown is for a
CMOS inverter that is
symmetrical (Kp = Kn).
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 9
CMOS Voltage Transfer Characteristics
• The simulation
results show the
varying VTC of the
inverter as VDD is
changed
• The minimum
voltage supply for
CMOS technology is
VDD = 2VT ln(2) V
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 10
CMOS Voltage Transfer Characteristics
• Simulation results
show the varying
VTC of the inverter
as KN/KP = KR is
changed
• For KR > 1 the
NMOS current
drive is greater, and
it forces transition
region vI < VDD/2
• For KR < 1 the
PMOS current drive
is greater, and it
forces transition
region vI > VDD/2
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 11
Noise Margins for the CMOS Inverter
Noise margins
are defined by
the points shown
in the given
figure
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 12
Noise Margins for the CMOS Inverter
KR 

KN
KP
NML  VIL VOL
NM H  VOH VIH
2K R VDD  VTN  VTP  VDD  K RVTN  VTP 
VIH 

K R 1
K R 1 1 3K R
VOL
K R  1VIH  VDD  K RVTN  VTP


2K R
2 K R VDD  VTN  VTP  VDD  K RVTN  VTP 
VIL 

K R 1
K R 1 K R  3
VOH
K R  1VIL  VDD  K RVTN  VTP


2
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 13
Propagation Delay Estimate
• The two modes of capacitive charging/discharging that
contribute to propagation delay
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 14

Propagation Delay Estimate
 PHL
RonN
p 

  V H  VTN  

2VTN 
 RonNCln 4

1

  V H  VL   V H  VTN 

1

K n V H  VTN 
 PHL   PLH
2
  PHL  1.2RonNC
• If it is assumed the inverter in “symmetrical” with (W/L)P
= 2.5 (W/L)N, then PLH =  PHL
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 15
Rise and Fall Times
• The rise and fall times are given by the following
approximate expressions:
t f  3 PHL
t r  3 PLH

Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 16
Reference Inverter Design Example
• Design a reference inverter to achieve a delay of
250ps with a 0.1pF load given the following
information:
VDD  3.3V
C  0.1 pF
 p  250 ps
VTN  VTP  0.75V
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 17
Gate Device Geometry Scaling based
Upon Reference Circuit Simulation
• State-of-the-art short gate length technologies are
hard to analyze
• Scaling can be used to properly set W/L for a
given load capacitance relative to reference gate
simulation with a reference load.

W / L  CL
P 

W / L'  CLref


  Pref or  W '   W    Pr ef
 

L
L




 P

  CL
  

  CLref




Scaling allows us to calculate a new geometry (W/L)' in terms
of a target load and delay.
Jaeger/Blalock
3/15/10
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 67 - 18
Performance Scaling
• Consider a reference inverter with a delay of 3.16
ns.
• What is the delay if an inverter has a W/L 4x
larger than the transistors of the reference inverter
and twice the load capacitance.
2 /1 2 pF' 
P 
 
 3.16 ns 1.58 ns
8 /1' 1pF 

Scaling allows us to calculate a new geometry (W/L)' or delay
relative to a reference design.
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 19
Reference Inverter Design Example
• Assuming the inverter is symmetrical and using
the values given in Table 7.1:
K  100
'
n
A
2
V
A
'
K p  40 2
V
 p   PHL   PLH  250 ps
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 20
Reference Inverter Design Example
• Solving for RonN:
RonN 
 PHL

  VDD  VTN   1 

Cln 4
1 

  VDD  VL   2 

 1040 
• Then solve for the transistor ratios:

W 
1
3.77

   '
 L n K n RonN VDD  VTN 
1
W  K n' W 
W  9.43
   '    2.5  
 L p K p  L n
 L n
1
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 21
Delay of Cascaded Inverters
• An ideal step was used to derive the previous
delay equations, but this is not possible to
implement
• By using putting the following circuit in SPICE, it
is possible to produce more accurate equations
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 22
Delay of Cascaded Inverters
• The simulated output of the previous circuit appears below,
and it can be seen that the delay for the nonideal step input
is approximately twice than the ideal case:
 PHL  2.4RonNC
 PLH  2.4RonPC
t f  2 PHL
t r  2 PLH
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 23
Static Power Dissipation
• CMOS logic is considered to have no static power
dissipation
• This is not completely accurate since MOS transistors have
leakage currents associated with the reverse-biased drainto-substrate connections as well as sub-threshold leakage
current between the drain and source
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 24
Dynamic Power Dissipation
•
There are two
components that add
to dynamic power
dissipation:
1) Capacitive load
charging at a
frequency f given by:
PD = CV2DDf
2) The current that
occurs during
switching which can
be seen in the figure
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 25
Power-Delay Product
• The power-delay product is given as:
PDP  Pav P
2
Pav  CVDD
f
1
f 
T
The figure shows a symmetrical
inverter switching waveform
2tr 22 P 
T  t r  t a  t f  tb 

 5 P
0.8
0.8
2
2
CVDD
CVDD
PDP 
P 
5 P
5
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 26
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 27
IrDA, Infrared Data Association
Audio DAC (Digital to Analog Converter)
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 28
CMOS NOR Gate
Basic CMOS logic
gate structure
CMOS NOR gate
implementation
Microelectronic Circuit Design, 4E
McGraw-Hill
Reference
Inverter
Chap 7 - 29
CMOS NOR Gate Transistor Sizing
• When sizing the transistors, we attempt to keep
the delay times the same as the reference inverter
• To accomplish this, the on-resistance in the PMOS
and NMOS branches of the NOR gate must be the
same as the reference inverter
• For a two-input NOR gate, the (W/L)p must be
made twice as large
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 30
CMOS NOR Gate Body Effect
• Since the bottom PMOS body contact is not
connected to its source, its threshold voltage
changes as VSB changes during switching
• Once vO = VH is reached, the bottom PMOS is not affected
by body effect, thus the total on-resistance of the PMOS
branch is the same
• However, the rise time is slowed down slightly due to |VTP|
being a function of time
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 31
Two-Input NOR Gate Layout
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 32
Three-Input NOR Gate Layout
• It is possible to extend this same design technique to create
multiple input NOR gates
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 33
Shorthand Notation for NMOS and
PMOS Transistors
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 34
CMOS NAND Gates
CMOS NAND gate
implementation
Reference Inverter
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 35
CMOS NAND Gate Transistor Sizing
• The same rules apply for sizing the NAND gate
devices as for the NOR gate, except now the
NMOS transistors are in series
• (W/L)N will be twice the size of that of the
reference inverter
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 36
Multi-Input CMOS NAND Gates
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 37
Complex CMOS Logic Gate
Design Example – Euler path
• Design a CMOS logic gate for (W/L)p,ref = 5/1 and for (W/L)n,ref = 2/1
that yields the function: Y = A + BC +BD
• By inspection (knowing Y), the NMOS branch of the gate can drawn
as the following with the corresponding graph, while considering the
longest path for sizing purposes:
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 38
Complex CMOS Logic Gate
Design Example
• By placing nodes in the interior of each arc, plus two more outside the
graph for VDD (3) and the complementary output (2’), the PMOS
branch can be realized as shown on the left figure
• Connect all of the nodes in the manner shown in the right figure, and
the NMOS arc’s that the PMOS arc’s intersect have the same inputs
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 39
Complex CMOS Logic Gate
Design Example
• From the PMOS
graph, the PMOS
network can now
be drawn for the
final CMOS logic
gate while once
again considering
the longest PMOS
path for sizing
Two equivalent forms of the final circuit
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 40
Complex CMOS Gate with a Bridging
Transistor - Design Example
• Design a CMOS gate that implements the following logic function
using the same reference inverter sizes as the previous example:
Y = AB +CE + ADE + CDB
• The NMOS branch can be realized in the following manner using
bridging NMOS D to implement Y. The corresponding NMOS graph
is shown to the right.
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 41
Complex CMOS Gate with a Bridging
Transistor - Design Example
• By using the same technique as before, the PMOS
graph can now be drawn
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 42
Complex CMOS Gate with a Bridging
Transistor - Design Example
• By using the PMOS
graph, the PMOS
network can now be
realized as shown
(considering the
longest path for sizing)
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 43
Minimum Size Gate Design and
Performance
• With CMOS technology, there is an area/delay
tradeoff that needs to be considered
• If minimum feature sized are used for both devices,
then the PLH will be increased compared to the
symmetrical reference inverter
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 44
Minimum Size Complex Gate and
Layout
• The following shows the layout of a complex minimum size
logic gate
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 45
Minimum Size Complex Gate and
Layout
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 46
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 47
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 48
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 49
Dynamic Domino CMOS Logic
• One technique to help decrease power in MOS
logic circuits is dynamic logic
• Dynamic logic uses different precharge and
evaluation phases that are controlled by a system
clock to eliminate the dc current path in single
channel logic circuits
• Early MOS logic required multiphase clocks to
accomplish this, but CMOS logic can be operated
dynamically with a single clock
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 50
Dynamic Domino CMOS Logic
• The figure demonstrates the basic concept of domino CMOS
logic operation
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 51
Simple Dynamic Domino Logic Circuit
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 52
Dynamic Domino CMOS Logic
• It should be noted that domino CMOS circuits only produce
true logic outputs, but this problem can be overcome by using
registers that have both true and complemented outputs to
complete the function shown by the following
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 53
Cascade Buffers
• In some circuits, the logic must be able to drive
large capacitances (10 to 50 pF)
• By cascading a number of increasingly larger
inverters, it is possible to drive the loads
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 54
Cascade Buffers
• The taper factor  determines the increase of the cascaded
inverter’s size in manner shown of the previous image.
CL
 
Co
N
where Co is the unit inverter’s load capacitance
• The delay of the cascaded buffer is given by the following:
1/ N
 CL 
 B  N  
 Co 
o
where o is the unit inverter’s
propagation delay
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 55
Optimum Design of Cascaded Stages
• The following expressions can aid in the design of
an optimum cascaded buffer
N opt
 CL 
 ln  
 Co 
1
C
ln L
 Co
 opt
 CL 
  
 Co 
 Bopt
 CL 
 ln   o
 Co 




Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 56
The CMOS Transmission Gate
• The CMOS
transmission gate
(T-gate) is a useful
circuits for both analog
and digital applications
• It acts as a switch that
can operate up to VDD
and down to VSS
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 57
The CMOS Transmission Gate
• The main consideration that needs to be
considered is the equivalent on-resistance which is
given by the following expression:
REQ 
Ronp Ronn
Ronp  Ronn
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 58
CMOS Latchup
• There is one major downfall to the CMOS logic
gate – Latchup
• There are many safeguards that are done during
fabrication to suppress this, but it can still occur
under certain transient or fault conditions
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 59
CMOS Latchup
• Latchup occurs due to parasitic bipolar transistors
that exist in the basic inverter as shown below
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 60
CMOS Latchup
• The configuration of
these bipolar
transistors creates a
positive feedback
loop, and will cause
the logic gate to
latchup as shown at
the left
• By using heavily
doped material
where Rn and Rp
exist, their resistance
will be lowered,
thereby reducing the
chance of latchup
occurring
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 61
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 62
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 63
End of Chapter 7
Microelectronic Circuit Design, 4E
McGraw-Hill
Chap 7 - 64