MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

Prepared by Mr. Shaswata Mohanta(HOD E & TC Engineering Department)

Subject – VLSI & EMBEDDED SYSTEM

Chapter – 1–Introduction to VLSI & MOS Transistor

Topic – MOS Transistor

Semester – 5^th

Branch – Electronics & Telecommunication

AY-2021-2022, WINTER-2021

Outline

• Introduction
• Silicon, pn-junctions and transistors
• A Brief History
• Operation of MOS Transistors
• CMOS circuits
• Fabrication steps for CMOS circuits

​

Introduction

• Integrated circuits: many transistors on one chip.
• Very Large Scale Integration (VLSI)
• Complementary Metal Oxide Semiconductor (CMOS)
• Fast, cheap, “low-power” transistors circuits

​

WHY VLSI DESIGN?

Money, technology, civilization

Digression: Silicon Semiconductors

• Modern electronic chips are built mostly on silicon substrates
• Silicon is a Group IV semiconducting material
• crystal lattice: covalent bonds hold each atom to four neighbors

http://onlineheavytheory.net/silicon.html

Dopants

• Silicon is a semiconductor at room temperature
• Pure silicon has few free carriers and conducts poorly
• Adding dopants increases the conductivity drastically
• Dopant from Group V (e.g. As, P): extra electron (n-type)
• Dopant from Group III (e.g. B, Al): missing electron, called hole (p-type)

p-n Junctions

• First semiconductor (two terminal) devices
• A junction between p-type and n-type semiconductor forms a diode.
• Current flows only in one direction

​

A Brief History �Invention of the Transistor

• Vacuum tubes ruled in first half of 20^th century Large, expensive, power-hungry, unreliable
• 1947: first point contact transistor (3 terminal devices)
• Shockley, Bardeen and Brattain at Bell Labs

�

A Brief History, contd..

• 1958: First integrated circuit
• Flip-flop using two transistors
• Built by Jack Kilby (Nobel Laureate) at Texas Instruments
• Robert Noyce (Fairchild) is also considered as a co-inventor

​

​

smithsonianchips.si.edu/ augarten/

Kilby’s IC

�

A Brief History, contd.

• First Planer IC built in 1961

​

​

​

​

​

• 2003
• Intel Pentium 4 μprocessor (55 million transistors)
• 512 Mbit DRAM (> 0.5 billion transistors)
• 53% compound annual growth rate over 45 years
• No other technology has grown so fast so long
• Driven by miniaturization of transistors
• Smaller is cheaper, faster, lower in power!
• Revolutionary effects on society

• 1970’s processes usually had only nMOS transistors

Inexpensive, but consume power while idle

• 1980s-present: CMOS processes for low idle power

MOS Integrated Circuits

Intel 1101 256-bit SRAM

Intel 4004 4-bit μProc

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
• First patent in the ’20s in USA and Germany
• Not widely used until the ’60s or ’70s

MOS Transistors

• Four terminal device: gate, source, drain, body
• Gate – oxide – body stack looks like a capacitor
• Gate and body are conductors (body is also called the substrate)
• SiO₂ (oxide) is a “good” insulator (separates the gate from the body
• Called metal–oxide–semiconductor (MOS) capacitor, even though gate is mostly made of poly-crystalline silicon (polysilicon)

NMOS

PMOS

NMOS Operation

• Body is commonly tied to ground (0 V)
• Drain is at a higher voltage than Source
• 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

NMOS Operation Cont.

• When the gate is at a high voltage: Positive charge on gate of MOS capacitor
• Negative charge is attracted to body under the gate
• Inverts a channel under gate to “n-type” (N-channel, hence

called the NMOS) if the gate voltage is above a threshold voltage (VT)

• Now current can flow through “n-type” silicon from source through channel to drain, transistor is ON

PMOS Transistor

• Similar, but doping and voltages reversed
• Body tied to high voltage (V_DD)
• Drain is at a lower voltage than the Source
• Gate low: transistor ON
• Gate high: transistor OFF
• Bubble indicates inverted behavior

Power Supply Voltage

• GND = 0 V
• In 1980’s, V_DD = 5V
• V_DD has decreased in modern processes
• High V_DD would damage modern tiny transistors
• Lower V_DD saves power
• V_DD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0,
• Effective power supply voltage can be lower due

to IR drop across the power grid.

Transistors as Switches

• In Digital circuits, MOS transistors are electrically controlled switches
• Voltage at gate controls path from source to drain

THANK YOU