lesson 1 electron theory

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Lesson 1 Electrons USC ELCT 220 1

ELCT 220Introduction to Electrical

Engineering

2008 Charles W. Brice


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Lesson 1 Electron Theory

Model of copper atom:

Copper is a good conductor

Bohr model shown below depicts 29protons (and some neutrons) in thenucleus, with 29 electrons in orbits

Quantum theory shows that theorbit picture is oversimplified


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Nucleus has 29 protons

each positively charged Fourth shell has oneelectron, which is avalence electron

First shell has 2 electrons

Second shell has 2+6=8 electrons

Third shell has 2+6+10=18 electrons


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s orbitalpx orbital

one s orbital per shell with 2 electrons

three p orbitals per shell with 6 electronsfive d orbitals per shell with 10 electrons


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Orbitals for copper

shell s p d

first 1s2

second 2s2 2p6

third 3s2 3p6 3d10

fourth 4s1

Filled

One electronin 4s state


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Electrons

Although Bohrs model is notcompletely correct, it does showthe basic idea:The inner electron shells in copper

are full

The outermost shell electrons (4sand 3d) are loosely bound andmove around easily


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Conduction

The process of conduction incopper is due to the valence

electrons freely moving aboutfrom one atom to the next

Its as if we have a sea ofelectrons available to move atthe first hint of an electric field


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Apply an electric field with abattery, for example

Battery 9 V

No current since switch is open


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Apply an electric field with abattery, for example

Battery 9 V

Current flows when switch is closed

I

electron flow


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Battery

Inside the battery is anelectrochemical reaction thatproduces an excess of electrons at

the negative () terminal and adeficiency of electrons at the positive(+) terminal

When the battery is connected in thecircuit, electrons flow around fromto +


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Diode

A diode is a solid-statesemiconductor device that allowscurrent flow in one direction (onlyonly when V>0)

V

I


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Diode

The semiconductor diode consists of amaterial, often silicon, that conductspoorly

Silicon (Si) has four valence electrons In the periodic table of the elements, Si is in

column IV

A crystal of Si is an array of Si atoms in a 3-D

array held together with covalent bonds There are few free electrons, so pure Si does

not conduct very well


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Si Si

Si Si

Si Si

Si Si

Si Si

Si Si

Si Si

Si Si

ValenceElectron

CovalentBond

Silicon atoms in a lattice


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Part of the periodic tableIII

+3

IV

+4

V

+55B

6C

7N

13Al

14Si

15P

31

Ga

32

Ge

33

As49In

50Sn

51Sb


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Doping

By adding a small amount of boron (Bvalence +3) to the silicon, we makep-type Si having a deficiency of electrons

By adding a small amount ofphosphorus (P valence +5) to silicon,we make n-type Si having an excess of electrons


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Si Si

Si Si

Si Si

B Si

Si Si

Si Si

Si Si

Si Si

Hole(missingelectron)

p type Si (doped with B)

Free positive charge due to hole


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Si Si

Si Si

Si Si

P Si

Si Si

Si Si

Si Si

Si Si

n-type Si (doped with P)

Extraelectron

Free negative charge due to electron


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Diode

A p-n junction diode

p n

Charges diffuse

p nDepletion region(bound charges)


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Diode

Depletion region creates a barrierto flow of charge

If a battery forward biases thediode, the barrier is reduced (diodeturns on)

If a battery reverse biases thediode, the barrier is increased(diode turns off)


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p n

RVs

Vd

Id

Vd

Id


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Id

Vd~0.7 VmA to pArange


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Light-Emitting Diode

The LED is a solid-statesemiconductor device that emits

light when Vd>0

Vd

Id


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LED

Vd

Id

Forward

biasregion

Reverse

biasregion

~1 V


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LED in a circuit

390 W

9 V

I = 0 A


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LED in a circuit

390 W

9 V

Icurrent-limitingresistor

photons


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LED in a circuit

390 W

9 V

I = 0 A

diode turns offsince it isreversed biased

lesson 1 electron theory

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