Semiconductors

Question 1

Atoms

Answer 1

Smallest Particle of an element that retains characteristics of that element

Every element has unique atomic structure

Atom has a planetary type of structure.

• Central nucleus
• Orbiting electrons

Question 2

Nucleus

Answer 2

Positively charged particles (Protons)
Uncharged particles (Neutrons)

Question 3

Electrons

Answer 3

Basic particles of negative charge

Question 4

Free Electrons

Answer 4

Valence Electron gains sufficient energy from surrounding medium

Valence electrons are in outermost shell

Can break away from parent atom

Becomes free electron

Free electrons capable of moving from one atom to another in material - drift randomly from atom to atom within copper
- Free electrons make electric current possible

Question 5

Materials used in Electronics

Answer 5

Conductors
Semiconductors
Insulators

Question 6

Conductors

Answer 6

Materials in which current can be established with relative ease

Large number of free electrons

One to three valence electrons in atomic structure

Most metals are good conductors

Silver is best but expensive

Copper most widely used

Question 7

Periodic Table Groups based on Valence Electrons

Answer 7

Group 3, B (Boron), AI (Aluminium)

Group 4, Si (Silicon), Ge (Germanium)

Group 5, P (Phosphorus), Sb (Antimony)

Question 8

Semiconductors

Answer 8

Classed below conductors in ability to carry current

Fewer free electrons

Four valence electrons

Unique Characteristics

Basis for semiconductor devices
- Diode, Transistor, Integrated Circuit (IC)
Examples, Silicon & Germanium

Question 9

Insulators

Answer 9

Poor conductors of electric current

Used to prevent current where it is not wanted

Very few free electrons

More than four valence electrons in atomic structure

Question 10

Covalent Bonds

Answer 10

When atoms combine to form a solid crystalline material they arrange themselves in a symmetrical pattern

Atoms within crystal structure held together by covalent bonds

Covalent bonds created by interaction of valence electrons in atoms

Silicon is a crystalline material

Important terms - Valence Shell, Nucleus, Core

Question 11

Covalent Bonds - Silicon

Answer 11

Silicon Atom with 4 valence electrons

Shares electrons with each of it’s four neighbours

Effectively creates 8 valence electrons for each atom

Produces state of chemical stability

Sharing produces covalent bonds that hold atoms together

Each shared atom attracted equally by two adjacent atoms which share it

Intrinsic crystal; crystal with no impurities

Question 12

Conduction Electrons and Holes

Answer 12

Intrinsic silicon crystal at room temperature

Sufficent heat (thermal energy) for some valence electrons to jump into conduction band

Become free electrons (conduction electrons)

Vacancy in valence band called a hole

For every conduction electron there is a hole left in valence band

Called electron-hole pair

Question 13

Recombination

Answer 13

Occurs when conduction-band electron loses energy

Electron falls back into hole in valence band

At room temp, at any instant, intrinsic Si has a number of ‘free’ electrons (conduction band) drifting randomly through the material.

Question 14

Electron Current

Answer 14

Apply voltage across piece of intrinsic silicon

Free electrons in conduction band easily attracted towards +

Movement of free electrons called electron current

Question 15

Hole Current

Answer 15

At valence level holes created by free electrons

Valence electron can move into nearby hole with little change in energy level leaving another hole where it came from

Called hole current

Question 16

N-type and P-type Semiconductors

Answer 16

Intrinsic semiconductors don’t conduct well

Modified to increase conductivity

Increase number of free electrons

Addition of impurities

Two types of extrinsic semiconductive materials
P-type
N-type

Key building blocks of most electronic devices

Question 17

Doping

Answer 17

Controlled addition of impurities to intrinsic semiconductive materials produces an extrinsic semiconductor.

Identify intrinsic materials

Doping increases number of current carriers (increases conductivity)
Electrons
Holes

Two types of impurities
P-type
N-type

Question 18

N-Type Semiconductor

Answer 18

Pentavalent impurity atoms added to intrinsic silicon

Increases number of conduction band electrons

Atoms have 5 valence electrons
Arsenic (As)
Phosphorus (P)
Bismuth (Bi)
Antimony (Sb)

Find pentavalent elements on periodic table

Question 19

N-Type: Doping Silicon

Answer 19

Pentavalent atom forms covalent bonds with 4 adjacent Si atoms

4 of Sb atom’s valence electrons used for covalent bonds

Extra electron becomes conductive electron

Pentavelent atom gives up electron- called a donor atom

Number of conductive electrons controlled by number of added impurity atoms

Conductive electrons created by doping does not leave a hole because its is excess of the no. reuired to fill the valence band.

Question 20

P-Type Semiconductor

Answer 20

Increase number of holes in intrinsic Silicon by adding trivalent impurity atoms

Aluminium (Al), Boron (B), Gallium (Ga)

Question 21

P-Type: Doping Silicon

Answer 21

Each trivalent atom forms covalent bonds with 4 adjacent Si atoms

All three of the Boron atom’s valence electrons used for covalent bonds
Four electrons required

Hole formed with each trivalent atom

Number of holes controlled by amount of added trivalent impurity

Question 22

Majority and Minority Carriers

Answer 22

Silicon or germanium doped with pentavalent atoms

Most current carriers are electrons

N-type semiconductor
N stands for Negative charge on electron

Electrons called majority carriers

Some holes created when electron-hole pairs are thermally generated

Holes in n-type material called minority carriers