VLSI DESIGN
UNIT-I:
INTRODUCTION AND BASIC ELECTRICAL
PROPERTIES OF MOS CIRCUITS: VLSI Design
Flow, Introduction to IC technology, Fabrication process:
nMOS, pMOS and CMOS. Ids versus Vds Relationships,
Aspects of MOS transistor Threshold Voltage, MOS
transistor Trans, Output Conductance and Figure of Merit.
nMOS Inverter, Pull-up to Pull-down Ratio for nMOS
inverter driven by another nMOS inverter, and through one
or more pass transistors. Alternative forms of pull-up, The
CMOS Inverter, Latch-up in CMOS circuits, Bi-CMOS
Inverter, Comparison between CMOS and BiCMOS
technology, MOS Layers, Stick Diagrams, Design Rules
and Layout, Layout Diagrams for MOS circuits
UNIT-1
•
INTRODUCTION AND BASIC ELECTRICAL PROPERTIES OF MOS
CIRCUITS:
• VLSI Design Flow, Introduction to IC technology, Fabrication
process: nMOS, pMOS and CMOS. Ids versus Vds Relationships,
Aspects of MOS transistor Threshold Voltage, MOS transistor Trans,
Output Conductance and Figure of Merit. nMOS Inverter, Pull-up to
Pull-down Ratio for nMOS inverter driven by another nMOS
inverter, and through one or more pass transistors. Alternative forms
of pull-up, The CMOS Inverter, Latch-up in CMOS circuits, BiCMOS Inverter, Comparison between CMOS and BiCMOS
technology, MOS Layers, Stick Diagrams, Design Rules and
Layout, Layout Diagrams for MOS circuits
VLSI DESIGN FLOW
Fig.1 :
Simplified VLSI Design Flow
IC Technology
• Electronics as we know it today is
characterized by reliability, low power
dissipation, extremely low weight and volume,
and low cost, coupled with an ability to cope
easily with a high degree of sophistication and
complexity
• Up until the 1950s electronic active device
technology was dominated by the vacuum
tube technology.
IC Fabrication Technology: Brief History
• 1940s - setting the stage - the initial inventions that made
integrated circuits possible.
• In 1945, Bell Labs established a group to develop a
semiconductor replacement for the vacuum tube. The group
led by William Shockley, included, John Bardeen, Walter
Brattain and others.
•
In 1947 Bardeen and Brattain and Shockley succeeded in
creating an amplifying circuit utilizing a point-contact
"transfer resistance" device that later became known as a
transistor.
• In 1951 Shockley developed the junction transistor, a more
practical form of the transistor.
• By 1954 the transistor was an essential component of the
telephone system and the transistor first appeared in
hearing aids followed by radios.
13
The transistor invented at Bell lab. in 1947
In 1956 the importance of the invention of the transistor by Bardeen, Brattain and
Shockley was recognized by the Nobel Prize in physics.
14
• History:
• …Vacuum tubes, discrete components
• 1947 The first integrated transistor (Bell Telephone
Laboratories)
• 1959 The first bipolar planar transistor
• 1958 The first integrated circuit available as a
monolithic chip (flip-flop)
• 1965 The first op-amp
• 1971 The first 4bit microprocessor (Intel 4004)
• 1972 The first 8bit microprocessor (Intel 8008)
• 1981 The first IBM PC
Moore's Law
• The number of transistors per square inch on integrated circuits had
doubled every year since their invention (1958).
(Prediction - This trend will continue into the foreseeable future)
• New Definition –
The number of transistors per square inch has doubled
approximately every 18 months that is computing power doubles
every 18-months.
Level of Integration
CLASSIFICATION OF ICS
Based on
Fabrication Techniques
Based on
Application
Based on
Technology
Based on
Device Count
SSI
Linear
IC
MSI
Digital
IC
LSI
VLSI
Monolithic
IC
Hybrid
IC
BJT
ULSI
MOSFET
Cross section of CMOS inverter
(p-well)
BASIC MOS TRANSISTOR
Steps:
• The substrate is used of p-type with moderate doping
level
• Through suitable masks source and drain are formed by
diffusing n- type impurities.
• For connections metal is deposited on source and drain
• An oxide layer is formed between source and drain
• Polysi is deposited on this layer and later metal is
deposited for gate contact.
• For D-MOSFET a channel is established during the
manufacturing process it self between source and
drain.
Circuit Symbol (NMOS)
D
ID = IS
G
B (IB=0, should be reverse biased)
IG= 0
IS
S
Output Characteristics
Transfer Characteristics
Circuit Symbol (NMOS)
D
ID = IS
G
B (IB=0, should be reverse biased)
IG= 0
IS
S
Output Characteristics
Transfer Characteristics
MOSFET SWITCHES
NMOS Inverter
An inverter circuit is a very important circuit for producing a complete range of logic
circuits. This is needed for restoring logic levels, for Nand and Nor gates, and for
sequential and memory circuits of various forms .
A simple inverter circuit can be constructed using a transistor with source connected
to ground and a load resistor of connected from the drain to the positive supply rail
VDD· The output is taken from the drain and the input applied between gate and ground
But, during the fabrication resistors are not conveniently produced on the silicon
substrate and even small values of resistors occupy excessively large areas .Hence
some other form of load resistance is used. A more convenient way to solve this
problem is to use a depletion mode transistor as the load, as shown in Fig. Below.
The salient features of the n-MOS inverter are
 For the depletion mode transistor, the gate is connected to the source so it is always
on .

In this configuration the depletion mode device is called the pull-up (P.U) and the
enhancement mode device the pull-down (P.D) transistor.

With no current drawn from the output, the currents Ids for both transistors must be
equal.
Transfer characteristics
The transfer characteristic is drawn by taking Vds on x-axis and Ids on Y-axis for both
enhancement and depletion mode transistors. So,to obtain the inverter transfer
characteristic for Vgs = 0 depletion mode characteristic curve is superimposed on the
family of curves for the
enhancement mode device and from the graph it can be seen that , maximum voltage
across the enhancement mode device corresponds to minimum voltage across the
depletion mode transistor
From the graph it is clear that as Vin(=Vgs p.d. Transistor) exceeds the Pulldown
threshold voltage current begins to flow. The output voltage Vout thus decreases and
the subsequent increases in Vin will cause the
saturation and become resistive.
Pull down transistor to come out of