Introduction to CMOS VLSI Design

Introduction to CMOS VLSI Design Slides adapted from: N. Weste, D. Harris, CMOS VLSI Design, © Addison-Wesley, 3/e, 2004

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Introduction Integrated Circuits: many transistors on one chip Very Large Scale Integration (VLSI): very many transistors on one chip Complementary Metal Oxide Semiconductor (CMOS): fast, cheap, low power

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Outline A Brief History MOS transistors CMOS Logic CMOS Fabrication and Layout Chip Design Challenges System Design Logic Design Physical Design Design Verification Fabrication, Packaging and Testing 3

A Brief History T-R-A-N-S-I-S-T-O-R = TRANsfer resiSTOR 1947: John Bardeen, Walter Brattain and William Schokley at Bell laboratories built the first working point contact transistor (Nobel Prize in Physics in 1956) 1958: Jack Kylby built the first integrated circuit flip flop at Texas Instruments (Nobel Prize in Physics in 2000) 1925: Julius Lilienfield patents the original idea of field effect transistors 1935: Oskar Heil patents the first MOSFET 1963: Frank Wanlass at Fairchild describes the first CMOS logic gate (nMOS and pMOS) 1970: Processes using nMOS became dominant 1980: Power consumption become a major issue. CMOS process are widely adopted. 4

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A Brief History Integrated Circuits enabled today’s way of life 1018 transistors manufactured in 2003 100 million for every human on the planet

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Moore’s Law In 1963 Gordon Moore predicted that as a result of continuous miniaturization transistor count would double every 18 months 53% compound annual growth rate over 45 years No other technology has grown so fast so long Transistors become smaller, faster, consume less power, and are cheaper to manufacture

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Clock Frequencies of Intel Processors Transistor count is not the only factor that has grown exponentially, e.g. clock frequencies have doubled roughly every 34 months

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Chip Integration Level SSI = small-scale integration ( up to 10 gates) MSI = medium-scale integration ( up to 1000 gates) LSI = large-scale integration (up to 10000 gates) VLSI = very large-scale integration (over 10000 gates) 8

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Technology Scaling 1971: Intel 4004 transistors with minimum dimension of 10um 2003: Pentium 4 transistors with minimum dimension of 130 nm Scaling cannot go on forever because transistors cannot be smaller than atoms ☺ 9

The Productivity Gap Designers rely increasingly on design automation software tools: • to seek productivity gains • to cope with increased complexity

Source: SEMATECH

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Silicon Lattice Silicon is a semiconductor Transistors are built on a silicon substrate Silicon is a Group IV material Forms crystal lattice with bonds to four neighbors

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Dopants Pure silicon has no free carriers and conducts poorly Adding dopants increases the conductivity Group V: extra electron (n-type) Group III: missing electron, called hole (p-type)

Si

Si

Si

Si

Si

Si

As

Si

Si

B

Si

Si

Si

Si

Si

-

+

+ -

Si Si Si

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Transistor Types Bipolar transistors npn or pnp silicon structure Small current into very thin base layer controls large currents between emitter and collector Base currents limit integration density Metal Oxide Semiconductor Field Effect Transistors nMOS and pMOS MOSFETS Voltage applied to insulated gate controls current between source and drain Low power allows very high integration 13

MOS Transistors Four terminals: gate, source, drain, body (= bulk = substrate)

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nMOS Operation Body is commonly tied to ground (0 V) When the gate is at a low voltage: P-type body is at low voltage Source-body and drain-body diodes are OFF No current flows, transistor is OFF Source

Gate

Drain Polysilicon SiO2

n+

0

n+ p

bulk Si

S

D

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nMOS Operation Cont. When the gate is at a high voltage: Positive charge on gate of MOS capacitor Negative charge attracted to body channel under gate gets “inverted” to n-type Now current can flow through n-type silicon from source through channel to drain, transistor is ON Source

Gate

Drain Polysilicon SiO2

n+

1

n+ p

bulk Si

S

D

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pMOS Transistor Similar, but doping and voltages reversed Body tied to high voltage (VDD) Gate low: transistor ON Gate high: transistor OFF Bubble indicates inverted behavior Source

Gate

Drain

Polysilicon SiO2

p+

p+ n

bulk Si

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Power Supply Voltage GND = 0 V In 1980’s, VDD = 5V VDD has decreased in modern processes High VDD would damage modern tiny transistors Lower VDD saves power

VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, … 18

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MOS Transistors as switches We can model MOS transistors as controlled switches Voltage at gate controls path from source to drain

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CMOS Technology CMOS technology uses both nMOS and pMOS transistors. The transistors are arranged in a structure formed by two complementary networks Pull-up network is complement of pull-down Parallel -> series, series -> parallel

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CMOS Logic Inverter

1= 0=

A

Y

0

1

1 ON OFF =1 =0 OFF ON

0

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CMOS Logic NAND

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CMOS Logic NOR

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CMOS Logic Gates (a.k.a. Static CMOS)

Pull-up network is complement of pull-down Parallel series, series parallel

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Compound Gates Example: Y = (A+B+C) D ABCD

Y

---0 0001 1--1 -1-1 --11

1 1 0 0 0 25

Compound Gates

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How good is the output signal ? Signal Strength Strength of signal How close it approximates ideal voltage source VDD and GND rails are strongest 1 and 0 sources nMOS and pMOS are not ideal switches nMOS pass strong 0, but degraded or weak 1 pMOS pass strong 1, but degraded or weak 0 Thus: nMOS are best for pull-down network pMOS are best for pull-up network 27

Pass Transistors Transistors can be used as switches

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Transmission Gates Pass transistors produce degraded outputs Transmission gates pass both 0 and 1 well

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Static CMOS gates are fully restored In static CMOS, the nMOS transistors only need to pass 0’s and the pMOS only pass 1’s, so the output is always strongly driven and the levels are never degraded This is called a fully restored logic gate

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Static CMOS is inherently inverting CMOS single stage gates must be inverting To build non inverting functions we need multiple stages

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Tristates Tristate buffer produces Z when not enabled EN 0 0 1 1

A 0 1 0 1

Y Z Z 0 1

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Nonrestoring Tristates

Transmission gate acts as tristate buffer Only two transistors But nonrestoring A is passed on to Y as it is (thus, Y is not always a strong 0’s or 1’s) 33

Tristate Inverter Tristate inverter produces restored output For a non inverting tristate add an inverter in front

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Multiplexers 2:1 multiplexer chooses between two inputs S 0 0 1 1

D1 X X 0 1

D0 0 1 X X

Y 0 1 0 1

S D0

0

D1

1

Y

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Gate-Level Mux Design Y = S D0 + S D1 How many transistors are needed? = 20 = = Too Many !!

D1 S D0

D1 S D0

Y

4

2

4

2

4 2

2

Y 36

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Transmission Gate Mux Nonrestoring mux uses two transmission gates Only 4 transistors

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Inverting Mux Inverting multiplexer Use compound gate or pair of tristate inverters Essentially the same thing For noninverting multiplexer add an inverter

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D Latch When CLK = 1, latch is transparent D flows through to Q like a buffer When CLK = 0, the latch is opaque Q holds its old value independent of D a.k.a. transparent latch or level-sensitive latch

Latch

CLK D

CLK D

Q

Q 39

D Latch Design Multiplexer chooses D or hold Q CLK D

1

CLK

Q Q

Q

D

Q

0 CLK

CLK

CLK 40

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D Latch Operation Q D

Q

CLK = 1

Q D

Q

CLK = 0

CLK D Q 41

D Flip-flop When CLK rises, D is copied to Q At all other times, Q holds its value a.k.a. positive edge-triggered flip-flop, masterslave flip-flop CLK

CLK D

Flop

D

Q Q 42

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D Flip-flop Design Built from master and slave D latches CLK

CLK CLK

D CLK

QM

Latch

Latch

CLK D

QM CLK

Q CLK

CLK

Q CLK

CLK

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D Flip-flop Operation D

QM

Q

CLK = 0

D

QM

Q

CLK = 1

CLK D Q

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Summary “If the automobile had followed the same development cycle as the computer, a Rolls Royce would today cost $100, get one million miles per gallon, and explode once a year …” Robert X. Cringely, InfoWorld Magazine

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