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Chapter 1 – Semiconductor Fundamentals

Electronic Materials• The goal of electronic materials is to generate

and control the flow of an electrical current.• Electronic materials include:

1. Conductors: have low resistance which allows electrical current flow

2. Insulators: have high resistance which suppresses electrical current flow (no current flow)

3. Semiconductors: can allow or suppress electrical current flow

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Conductors

• Good conductors have low resistance so electrons flow through them with ease.

• Best element conductors include:– Copper, silver, gold, aluminum, & nickel

• Alloys are also good conductors:– Brass & steel

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Conductor Atomic Structure• The atomic structure of

good conductors usually includes only one electron in their outer shell. – It is called a valence electron. – It is easily striped from the

atom, producing current flow.

Copper Atom

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Insulators

• Insulators have a high resistance so current does not flow in them.

• Good insulators include:– Glass, ceramic, plastics, & wood

• The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.

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Insulator Atomic Structure

• The atomic structure of insulator usually includes seven electron in their outer shell. – It is very pack thus current

can’t flow across the materials

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Semiconductors

• Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between.

• Common elements such as carbon, silicon, and germanium are semiconductors.

• Silicon is the best and most widely used semiconductor.

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Semiconductor Valence Orbit

• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

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Electron OrbitsOrbit

NumberMaximum Electrons

1 223456

Valence Orbit

2

72

32

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Orbits closest to the nucleus fill first

Electricity at the Atomic Level

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50

8

9

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How many electrons are in the valence orbit?

Pictorial Representation

Copper

Cu 29

Is copper a conductor?

Why?

Example: Copper

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How many electrons are in the valence orbit?

Is Sulfur a conductor or insulator?

Why?

Pictorial Representation

Sulfur

S 16

Example: Sulfur

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Exercise

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Using periodic table, determine the types of material for:i) Calciumii) Bismuthiii) Cobaltiv) Antimony Justify your selection

SEMICONDUCTOR MATERIAL• Semiconductors are solid materials, either non –

metallic compounds, which allow electrons to pass through them so that they conduct electricity in much the same way as a metal.

• Atoms with fewer than 4 valence electrons are good conductors.

• Atoms with more than 4 valence electrons are an insulator.

• Atoms with four valence electrons are semiconductors.

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Exercise

Material

Parameters

Conductor Insulator Semiconductor

Resistance Value

Conductivity Level

Valence Electron

Examples

Fill in the blanks.

Cont.• 2 types of Semiconductor Material

INTRINSIC SEMICONDUCTOR- A pure semiconductor - Pure semiconductors, by themselves, are not particularly useful- No free electrons are available since all the covalent bonds are complete. - Behaves like a insulator. - Resistance of a semiconductor decreases with increase in temperature.

EXTRINSIC SEMICONDUCTOR- The impure semiconductor- Impurity semiconductor

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DOPING• process of adding impurity to a semiconductor to make it

extrinsic semiconductor• increase the conductivity of a semiconductor

• Intrinsic Semiconductor + IMPURITY = Extrinsic Semiconductor

• IMPURITY = DOPING AGENT• 2 types of Doping Agent

- Pentavalent atom having five valence electrons (arsenic, antimony, phosphorous). These atoms are called “donor atoms”.- Trivalent atoms having three valence electrons (gallium, aluminum, boron). These atoms are called “acceptor atoms”.

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N-type and P-type Material• Intrinsic Semiconductor + Pentavelent Atom

= N-type Material

• Intrinsic Semiconductor + Trivalent Atom = P-type Semiconductor

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Majority and Minority CarrierP-type- holes are majority carriers - they are positively charged materials (++++)N-type: - electrons are majority charge carriers - they are negatively charged materials (-----)

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P-N junction

Materials

P-type material N-type material

Material doped with acceptor

Material has high POSITIVE charge (holes) concentration

Majority Carrier – POSITIVE (immoveable)Minority Carrier – NEGATIVE(free)

Material doped with donor

Material has high NEGATIVE charge (electrons) concentration

Majority Carrier – NEGATIVE(immoveable)Minority Carrier – POSITIVE(free)

P-N junction formation• Happen when both materials are in close contact.

• Electrons and holes (minority carrier) will move to p-n junction because different types of charge will attract to each other

• This will forms a dipole charge layer called depletion layer• Thus, there is a built-in voltage at the layer

P-N junction current-voltage characteristics

• According to Current Continuity Law (CCL) the current can only flow if all the charges move forming a closed loop with a proper polarity of supply.

P-N junction current-voltage characteristics

From the figure, negative terminal are connected to the P-type material and positive terminal are connected to N-type material.

What happen? - increase the minority carrier - width of the DL will increase and the majority carrier of the material will demolish which will increase the built in voltage

P-N junction current-voltage characteristics

From the figure, negative terminal are connected to the N-type material and positive terminal are connected to P-type material.

What happen? - increase the majority carrier - width of the DL will decrease and at certain extend of time it will break and the built in voltage are decrease to zero thus all the charge in both material can move freely and create a closed loop path (CCL) for the current to pass through.

P-N junction current-voltage characteristics

• What can you relate on the previous slides?

FWD biased & RV biased