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VLSI DESIGN
Dr
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Abirami
Assistant Professor / EEE
BSACIST
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EED 3104 – VLSI DESIGN (LIT)
COURSE OBJECTIVES:
COB 1: To apply the concepts of modeling in digital system using Verilog HDL
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COB 3: To understand the design procedures of digital logic circuits
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INTRODUCTION TO CMOS
❑ Complementary MOSFET (CMOS) technology is
widely used today to form circuits in numerous and
varied applications
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❑ CMOS offers low power dissipation, relatively high
speed, high noise margins in both states, and will
operate over a wide range of source and input
voltages (provided the source voltage is fixed)
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MOS LAYERS
Metal-Oxide-Semiconductor
Field
Effect
Transistor (MOSFET)
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Structure of a MOSFET

•The Metal Gate Electrode
•The Insulating Oxide Layer (SiO2)
•P – type Semiconductor (Substrate)

MOS transistors are built on top of silicon wafers
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Geometric shapes are obtained via photolithography and chemical
etching
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Substrate: Both NMOS and PMOS transistors are typically fabricated on a silicon substrate
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The gate is insulated from the semiconductor substrate by a thin layer of insulating material (usually
silicon dioxide) known as the gate oxide
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In NMOS transistors, electrons flow from the source to the drain (hence the name "N-channel"),
while in PMOS transistors, holes flow from the drain to the source (hence the name "P-channel")
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Channel: The channel is the region between the source and drain terminals that is controlled by the
voltage applied to the gate
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In PMOS transistors, the
channel consists of a p-type semiconductor material, allowing holes to flow when a voltage is applied to
the gate
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❑ The thickness of dielectric material (SiO2) is usually between
10 nm and 50 nm
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❑ MOS transistors are classified into two types PMOS & NMOS
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This transistor has a controlling gate that controls the electron flow
between the source & drain terminals
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❑ A positive (+ve) voltage on the gate terminal turns on the device
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❑ As compared to the PMOS transistor, this transistor is very faster, so
more transistors can be placed on a single chip
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❑ This alternation results in a depletion region that blocks any current
between drain and source, which explains the cut-off region
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❑ When voltage is applied to the gate, the electric field attracts minority
carriers to the region below the SiO2 layer
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❑ When enough charge is accumulated in that region, the minority
carriers become the majority carriers, forming a channel with the
same type as the drain and source
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❑ The gate-to-source voltage that results in channel inversion is the
threshold voltage VTH
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❑ The channel presents ohmic resistance for a fixed VGS (linear
operation mode) as long as VDS remains smaller than the VGS – VTH
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❑ This is called channel pinch-off, and marks the division between the
saturation and linear regions
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Drain and Transfer Characteristics of NMOS
Transfer Characteristics

Drain Characteristics

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Drain and Transfer Characteristics of PMOS
Transfer Characteristics

Drain Characteristics

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CMOS Inverter
The term “CMOS” stands for “complementary metal–oxide–semiconductor”
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What is CMOS Inverter?
CMOS inverter definition is a device that is used to generate logic
functions and is the essential component in all integrated circuits
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These inverters are used in most electronic devices which are
accountable for generating data n small circuits
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CMOS Inverter Schematic Diagram
Once the input voltage of
CMOS changes between
0 to 5 volts, then both the
transistors state will be
changed accordingly
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The NMOS transistor is connected at the drain (D) & gate (G) terminals, a voltage
supply (VDD) is connected at the source terminal of PMOS & a GND terminal is
connected at the source terminal of NMOS
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CMOS Inverter Operation & Working
❑ When the low input voltage is given to the CMOS inverter, then the
PMOS transistor is switched ON whereas the NMOS transistor will
switch OFF by allowing the flow of electrons throughout the gate
terminal & generating high logic output voltage
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DC Transfer Characteristics of CMOS Inverter

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The DC input – output transfer characteristic is also called as Voltage Transfer
Characteristics (VTC)
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Saturation

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Saturation

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