Basics I Unit

8/11/2019 Basics I Unit

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Atomic Structure

Atoms go around the nucleolus in their orbitsdiscrete distances

Each orbit has some energy level

The closer the orbit to the nucleus the less energy ithas

Group of orbits called shell

Electrons on the same shell have similar energy level

Valence shellis the outmost shell Valence shell has valence electronsready to be

freed

Number of electrons (Ne) on each shell (n)

First shell has 2 electrons Second shell has 8 electrons (not shown here)

Ne = 2n2


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Atomic Structure

Elements are made of atoms 110 Elements; each has an atomic

structure

Today, quarksand leptons, and theirantiparticles, are candidates for being the

fundamentalbuilding blocks from whichall else is made!

Bohr Model

Atoms have planetary structure

Atoms are made of nucleus(Protons (+)

& Neutrons) and electrons(-)

110 th element is called Darmstadtium (Ds)


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Valence Shell

Atoms are made of valence shell

and core

Core includes nucleus and other

inner shells

For a Carbon atom the atomic

number is 6

Core charge = 6 P + 2 e = (+6)+(-

2)=(+4)

Remember the first shell has 2

electrons


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Elements

Basic categories Conductors

Examples: Copper, silver One valence electron , the ecan

easily be freed

Insulators Valence electrons are tightly

bounded to the atom

Semiconductors Silicon, germanium (single

element) Gallium arsenide, indium

phosphide (compounds) They can act as conductors or

insulators Conduction bandis where

the electron leaves the

valence shell and becomes

free

Valence bandis where the

outmost shell is

Always free

electrons

Free electrons


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Semiconductors Remember the further away from the

nucleus the less energy is required tofree the electrons

Germanium is less stable Less energy is required to make the

electron to jump to the conduction band

When atoms combine to form a solid,they arrange themselves in asymmetrical patterns

Semiconductor atoms (silicon) formcrystals

Intrinsiccrystals have no impurities


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Conduction Electrons and Holes

Electrons exist only withinprescribed energy bands

These bands are separated byenergy gaps

When an electron jumps to the

conduction band it causes a hole When electron falls back to its

initial valence recombinationoccurs

Consequently there are twodifferent types of currents

Hole current (electrons are theminority carriers)

Electron current (holes are theminority carriers)

Remember: We are interested in electrical current!


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Doping

By adding impurities to the intrinsicsemiconductor we can change theconductivity of the materialthis is calleddoping

N-type doping

P-type doping

N-type:pentavalent (atom with 5 valence

electrons) impurity atoms are added [Sb(Antimony) + Si]

Negative charges (electrons) aregenerated

N-type has lots of free electrons

P-type: trivalent (atom with 3 valence

electrons) impurity atoms are added [B(Boron) + Si]

Positive charges (holes) are generated

P-type has lots of holes


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What is a Semiconductor?

Low resistivity => conductor

High resistivity => insulator

Intermediate resistivity => semiconductor

conductivity lies between that of conductors and insulators

generally crystalline in structure for IC devices

In recent years, however, non-crystalline semiconductors have

become commercially very important

polycrystalline amorphous crystalline


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Semiconductor Materials

Gallium

(Ga)

Phosphorus

(P)


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Silicon

Atomic density: 5 x 1022atoms/cm3

Si has four valence electrons. Therefore, it can form

covalent bonds with four of its nearest neighbors.

When temperature goes up, electrons can becomefree to move about the Si lattice.


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Electronic Properties of Si

Silicon is a semiconductor material. Pure Si has a relatively high electrical resistivity at room temperature.

There are 2 types of mobile charge-carriers in Si: Conduction electronsare negatively charged; Holesare positively charged.

The concentration (#/cm3) of conduction electrons & holes in asemiconductor can be modulated in several ways:

1. by adding special impurity atoms ( dopants )

2. by applying an electric field3. by changing the temperature

4. by irradiation


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Electron-Hole Pair Generation

When a conduction electron is thermally generated,

a hole is also generated.

A hole is associated with a positive charge, and is

free to move about the Si lattice as well.


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Carrier Concentrations in Intrinsic Si

The band-gap energy Egis the amount of energy

needed to remove an electron from a covalent bond.

The concentration of conduction electrons in intrinsic

silicon, ni, depends exponentially on Egand theabsolute temperature (T):

600Kat/101

300Kat/101

/2

exp102.5

315

310

32/315

cmelectronsn

cmelectronsn

cmelectronskT

ETn

i

i

g

i


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Doping (N type)

Si can be doped with other elements to change its

electrical properties.

For example, if Si is doped with phosphorus (P), each

P atom can contribute a conduction electron, so thatthe Si lattice has more electrons than holes, i.e.itbecomes N type: Notation:

n= conduction electron

concentration


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Doping (P type)

If Si is doped with Boron (B), each B atom cancontribute a hole, so that the Si lattice has moreholes than electrons, i.e.it becomes P type:

Notation:

p= hole concentration


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Summary of Charge Carriers


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Electron and Hole Concentrations

Under thermal equilibrium conditions, the productof the conduction-electron density and the hole

density is ALWAYS equal to the square of ni:2

innp

P-type material

A

i

A

Nnn

Np

2

D

i

D

N

np

Nn

2

N-type material


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Terminology

donor: impurity atom that increases nacceptor: impurity atom that increasesp

N-type material: contains more electrons than holes

P-type material: contains more holes than electrons

majority carrier: the most abundant carrier

minority carrier: the least abundant carrier

intrinsic semiconductor: n=p= ni

extrinsic semiconductor: doped semiconductor


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Summary

The band gap energy is the energy required to free anelectron from a covalent bond.

Egfor Si at 300K = 1.12eV

In a pure Si crystal, conduction electrons and holes are

formed in pairs. Holes can be considered as positively charged mobile particles

which exist inside a semiconductor.

Both holes and electrons can conduct current.

Substitutional dopants in Si: Group-V elements (donors) contribute conduction electrons

Group-III elements (acceptors) contribute holes

Very low ionization energies (

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Basics I Unit