MT 01 Ch01 1 Introduction

8/12/2019 MT 01 Ch01 1 Introduction

1/13

1

Semiconductors: A general introduction

Insulators 1010 1018 cm

Semiconductors 104 108 cm

Conductors 106 104 cm

Classification of Materials in terms of electrical resistivity:

The uniqueness of semiconductors is that their conductivitycan be varied by usover a wide range, e.g. by

adding minute quantities of impurities

by applying electric field

illumination

8/12/2019 MT 01 Ch01 1 Introduction

2/13

2

Introduction

Conducting(on)

Non conducting(off)

We can use this as a switch. (Example: Digital computers)

Current,I

Voltage, V

8/12/2019 MT 01 Ch01 1 Introduction

3/13

3

Abbreviated periodic table of the elements

4

Be5

B6

C7

N8

O

12

Mg13

Al14

Si

15

P16

S

30

Zn

31

Ga

32

Ge

33

As

34

Se48

Cd49

In50

Sn51

Sb52

Te

80

Hg

81

Tl

82

Pb

83

Bi

84

Po

The most common semiconductor is Silicon

ElementsESi = 1.1 eVEGe= 0.67eV

CompoundsEGaAs= 1.43eVEGaSb= 0.7eVEGaN = 3.4eV

8/12/2019 MT 01 Ch01 1 Introduction

4/13

4

Periodic table of the elements

8/12/2019 MT 01 Ch01 1 Introduction

5/13

5

Semiconductor materials

8/12/2019 MT 01 Ch01 1 Introduction

6/13

6

Crystalline solids

The fact that one can alter the properties of semiconductors

over a wide range may have something to do with theatomic arrangement of atoms in these materials. So, let uslook at the crystal structure.

8/12/2019 MT 01 Ch01 1 Introduction

7/137

Amorphous Si thin film transistors used as switching devicesin LCDs

Polycrystalline Si used as gate in MOSFETs

Actual active region of MOSFET is fabricated in crystalline Si

Lattice: Periodic arrangement of atoms. The atomic arrangementdetermines the macro-properties of the crystal.

Crystalline solids

Examples:

8/12/2019 MT 01 Ch01 1 Introduction

8/138

The unit cel lis a small portion of any given crystal that could beused to reproduce a crystal.

Two differentways ofrepresenting aunit cell

Unit cell concept

8/12/2019 MT 01 Ch01 1 Introduction

9/139

Simple 3D unit cells

8/12/2019 MT 01 Ch01 1 Introduction

10/1310

2 FCC lattices displaced by ((1/4) a, (1/4) a, (1/4) a)alongbody diagonal*

8 atoms per unit cell

Diamond lattice (also called zincblende if interpenetrating

FCC lattices are made of different elements like in GaAs)

Each atom is bonded to 4 other atoms (tetrahedral bondingstructure)

Crystal structure of Si and Ge and other commonsemiconductors

*The lattice constant or cubic edge is a. Generally aisexpressed in Angstroms. 1 = 108 cm = 1010 m

8/12/2019 MT 01 Ch01 1 Introduction

11/1311

Diamond and zincblende lattice unit cells

8/12/2019 MT 01 Ch01 1 Introduction

12/1312

Diamond lattice (detail)

8/12/2019 MT 01 Ch01 1 Introduction

13/13

13

What is the number of Si atoms in 1 cm3of Si?

Given is the lattice constant: a= 5.43

What is the density of Si?

Atomic weight of Si = 28.1 i.e. 1 mole (NA= 6.023 x 1023atoms)

of Si has a mass of 28.1 g

323

322

cm

g2.33

moleatoms

106.02

molegm

28.1cm

atoms105

Density

Example

3

22

3 cm

atoms105

atoms8

a