EC8095 VLSI Design-UNIT-1-Part 1

 Introduction

Level of integrations in IC’s :

Source: wikipedia 

ULSI, WSI, SoC and 3D-IC

• Conductor - Low Resistance 
• SemiConductor - Resistance changes 
• Insulator - High Resistance  

 

Karl Ferdinand Braun developed the crystal detector, the first semiconductor device, in 1874.

SemiConductor

A semiconductor material has an electrical conductivity value falling between that if a conductor, such as metallic copper, and an insulator, such as glass.

Its conducting properties may be altered in useful ways by introducing impurities ("doping") . 

Semiconductor - Types

1. Intrinsic (pure)

2. Extrinsic (Doped)

1. N type 

2. Ptype

 

	Intrinsic semiconductor	Donor atoms (n-Type Semiconductor)	Acceptor atoms (p-Type Semiconductor)
Group IV semiconductors	Silicon, Germanium	Phosphorus, Arsenic, Antimony	Boron, Aluminium, Gallium
Group III–V semiconductors	Aluminum phosphide, Aluminum arsenide, Gallium arsenide, Gallium nitride	Selenium, Tellurium, Silicon, Germanium	Beryllium, Zinc, Cadmium, Silicon, Germanium

         “Transistor count doubling every 18 months “

Gorden Moore (1965)

MOS Transistors - Types

If the MOSFET is normally OFF, and is turned ON by applying voltage at the gate terminal, then the MOSFET is known as enhancement-type MOSFET

If the MOSFET is normally ON, and is turned OFF by applying voltage at the gate terminal, then the MOSFET is known as depletion type MOSFET.

MOS Transistors -  Enhancement MOS Types

sources: © Oxford University Press 2010, 2015

MOS Transistors -  Enhancement MOS Types

MOS Transistors - Structure

• Each transistor consists of a stack of the conducting gate, an insulating layer of silicon dioxide (SiO2, better known as glass), and the silicon wafer, also called the substrate, body, or bulk. 

• Gate  - M (metal / polysilicon)

• Silicon dioxide - O - (SiO2)  

• Substrate -S-  P type/N type  

• Transistor operation is controlled by electric fields so the devices are also called Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) or simply FETs.

 

N MOS	P MOS
P- Substrate connected to GND	N-Substrate connected to VDD
N+  Source / Drain	P+  Source / Drain
Source to Body & Drain to Body are Reverse Biased	Source to Body & Drain to Body are Reverse Biased
Gate G Input =1 ON Gate G Input =0 OFF	Gate G Input =0 ON Gate G Input =1 OFF

POWER SUPPLY RAILS : VDD - Logic 1  

GND - Logic 0 

MOS Transistors - Operations  

• If a voltage is applied at the gate terminal, a conducting channel is formed underneath the oxide layer between the source and drain. 

• A MOSFET is called n channel or simply nMOS when the channel is formed with electrons. 

• Depending on the voltage applied at the gate terminal, the operation of MOSFET is divided into three conditions: 

• Accumulation

• Depletion 

• Inversion

Accumulation

If a negative voltage is applied at the gate terminal, the majority carriers (holes) from the p-type substrate are attracted towards the gate terminal and are accumulated underneath the gate oxide layer. This condition is known as accumulation.

Depletion

✉️ if a small positive voltage is applied, the holes will be repelled into the substrate. The repelled holes will move into the substrate and create negatively charged fixed acceptor ions underneath the gate oxide layer. 

✉️  These fixed negatively charged ions form the depletion layer.

Inversion

✉️ If the positive gate voltage is large enough, the majority carrier holes are repelled into the substrate and the small numbers of minority carrier electrons are attracted towards the gate oxide surface. 

✉️ The attracted electrons accumulate underneath the gate oxide layer and form a conducting path between the source and drain. This conducting path is called channel and the condition is known as “strong inversion”

Summary