BASIC ELECTRONICS

Fundamental of Solid States Principles

Shell

Nucleus

Electron

Basic Structure of Atom

Silicon

Carbon2, 4

VALENCE SHELL

• Determine the conductivity of atom• Contain maximum of 8 electrons• If one electron - perfect conductor• If eight electrons (complete valence shell) – insulator

Conductivity decreases with an increase the number of valence electron.

SEMICONDUCTOR (Neither conductor nor insulator)

If valence shell has 4 electrons It is neither conductor nor a good insulator Commonly used semiconductor material

Silicon ( Si ) Germanium (Ge ) Carbon ( C )

Silicon and Germanium - used in production of solid state components Carbon – used in production of resistor and potentiometer.

CHARGE AND CONDUCTION

When no outside force to cause conduction

electrons = protons net-charge = zero

If atom losses one valence electron - therefore atom has has fewer electron, hence net charge is POSITIVE

If atom in valence shell gains one electron -> atom contains more electron -> net charge is NEGATIVE

Fundamental Law on Relationship Between Electrons and Orbital Shell

1. Electron travel in orbital shell. They cannot orbit the nucleus in the space that exist between any two orbital shells. 2. Each orbital shell relates the specific energy. Thus, all the electrons travelling in a given orbital shell contain the same relative amount of energy . [ Greater the distance from nucleus , the greater the energy level that is associates with a given orbital shell. Valence electron – having higher energy levels].3. For electron to jump from one shell to another, it must absorb enough energy to make up the difference its initial energy level and that of the shell to which it is jumping.4. If an electron absorb enough energy to jump from one shell to another, it will eventually given up energy it absorb and return to a lower-energy shell.

Atom Energy Level

To jump from energy level E3 (0.7 eV) to conduction band E4 (1.8 eV), the energy required is

1.8 eV – 0.7 eV = 1.1 eV.

Types Energy

Conductors 0.4 eV

Semiconductor 1.1 eV

Insulator 1.8 eV

When electron absorbs enough energy to jump from valence shell to the conduction band - electrons is said to be in an excited state. An excited electron will eventually give up the energy it absorb and return to its original energy level (in the form of light or heat)

Covalent Bonding

Covalent bonding is the method by atoms complete their valence shell by ‘sharing’ valence electrons with another atoms

Basis of Covalent

Si

Si

Si

Si

Si

Silicon Covalent Bonding

Result of Bonding

Atom held together forming solid substance

Atom – electrically stable

The complete shell cause the silicon to act as insulator

Silicon or Germanium - form as crystal

Conduction

Valence electrons absorb enough energy jump from valence band to conduction band

As result a GAP is left in the covalent band (known as ‘hole’)

For every conduction band electrons there must exist a valence band hole.

Valence band

Conduction band

Recombination - when free electron return to holes in covalent bondLife time – time between generation of electron-hole pair and recombination.

Conduction Versus Temperature

At room temperature

Thermal energy (heat) cause the constant creation of electron – hole pairs with their subsequent recombination.

Therefore a semiconductor always has some number of free electrons even when no voltage is applied

Increase temperature -- more electron absorb energy to break free of their covalent bond (increase free electrons)

Decrease temperature -- less free electrons

No free electron at absolute temperature – 273.16 C (0 Kelvin).

Conductivity in a semiconductor varies directly with temperature.

Doping

Doping is the process of adding impurity atoms to intrinsic (pure) silicon or germanium to improve conductivity of the semiconductor.

A doped semiconductor is called extrinsic semiconductor

Two types of doping

trivalent - element that has 3 free electrons pentavalent --- element that has 5 free electrons

When trivalent atom are added to intrinsic semiconductor; the resulting material is called P-type material.

If pentavalent atom are added to intrinsic semiconductor ; the resulting material is N-type material.

Trivalent Pentavalent

Al P

Ga As

Boron (B) Antimony (Sb)

Indium (In) Bismuth (Bi)

N-type Material

As

Si Si

SiSi

Extra covalent bond electron

Pentavalent impurities

add to

Silicon or Germanium

Result in access ofone electron in covalent bond

Extra electron can be made to flow through the material with little difficulty

Even with extra electron, the material still electrically neutral because atom still have equal photons as electrons

Energy

Conduction band

Valence band

hole

electron

In N-type material : More electrons in conduction band than holes in valence band

The electrons are called majority carriers while valence band holes are called minority carriers

Energy Diagram – N-type

P-type Material

Al

Si Si

SiSi

hole

Intrinsic silicon

add to

trivalent element

P – type material

Energy

Conduction band

Valence band

hole

electron

In P-type material : Less electrons in conduction band than holes in valence band

The holes are called majority carriers while valence band electrons are called minority carriers

Energy Diagram – P-type

END