2 Fundamental Solid-State Principles

8/14/2019 2 Fundamental Solid-State Principles

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Fundamental Solid-State PrinciplesSemiconductors


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A semiconductor is a material that is neither agood conductor nor a good insulator, butrather, lies in between the two. In their purestform, semiconductors have few applications

in electronics.However, when the characteristics of a pure

semiconductor are altered through a processknown as doping, many useful electronic

devices can be developed.

Introduction


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1. atomic theory

2. doping

3. pn junction

4. bias

Outline


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Atom

Bohr model

1. Atomic theory


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Valence shelloutermost shell

Determines the conductivity of the atom

Can contain up to 8 e-

Atomic Theory


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1 valence e- perfect conductor

8 valence e- insulator

4 valence e- semiconductor

Conductivity decreases with an increase in thenumber of valence electrons.

Atomic theory


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Silicon (Si) - 14Germanium (Ge) - 32

Carbon (C) 6

Carbon is used primarily in the production ofresistors and potentiometers.

Silicon is commonly used than germanium

because it is more tolerant of heat.

Semiconductors


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The net charge of an atom is zero. Positive net charge Negative net charge

Fundamental laws Electrons travel in orbital shells

Each orbital shell relates to a specific energy range(Which orbital electrons have higher energy levels?) For an electron to jump from its shell to one having a

higher energy level, it must absorb enough energy tomake up the difference between the energy levels ofthe two shells.

If an electron absorbs enough energy to jump from one

shell to another, it will eventually give up the energyit absorbed and return to a lower-energy shell.

Charge and conduction


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Energy gap difference between energy levels of any two orbitalshells

For conductors, semiconductors, and insulators, the valence toconduction-band energy gaps are approximately 0.4, 1.1, and 1.8eV.

The higher the energy gap, the harder it is to cause conduction.(Why?)

Excited state

Energy gaps


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Covalent bonding a means of holding atoms together bysharing valence electrons.The atoms are held together, forming a solid substance.The atoms are electrically stable, because their valence shells

are complete.The completed valence shells cause the silicon to act as an

insulator. Thus, intrinsic (pure) silicon or germanium is avery poor conductor.

Covalent bonding


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At room temperature, a silicon crystal hasfewer free electrons than a germanium.Hence, silicon is a better insulator ( desirablecharacteristic).

How about carbon?

Why is silicon more often used thangermanium?


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Hole a gap in the covalent bond

Electron-hole pair a free electron and itsmatching valence band hole

Recombination when a free electron returnsto the valence shell

Lifetime the time from electron-hole pairgeneration to recombination

Conduction


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At room temperature, thermal energy(heat)causes the constant creation of electron-holepairs, with their subsequent recombination.

A semiconductor always has some number offree electrons even when no voltage isapplied to the element.

temperature, free electrons-273 degree Celsius absolute zero!!!Conductivity in a semiconductor varies

directly with temperature.

Conduction versus Temperature


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1.What are the 3 particles that make up the atom?2.What is the relationship between the number of

valence electrons and the conductivity of agiven element?

3.How many valence e- are there in a conductor? Aninsulator? A semiconductor?

4.What 3 semiconductor elements are mostcommonly used in electronics?

5.What are the relationships between electrons and

orbital shells ?6.What is an energy gap? What are the energy gap

values for insulators, semiconductors, andconductors?

7.What forms of energy are given off by an electronthat is falling into the valence band from the

SECTION REVIEW


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8. What is covalent bonding?9. What are the effects of covalent bonding on

intrinsic semiconductor materials?10. What is an electron-hole pair?11. What is recombination?12. How long is the typical lifetime of an

electron-hole pair?13. What is the relationship between

temperature and conductivity?14. Why arent electron-hole pairs generated in

a semiconductor when its temperature dropsto absolute zero?

SECTION REVIEW


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Doping process of adding impurity atoms tointrinsic silicon or germanium to improve theconductivity of the semiconductor.

Extrinsic impure

Two element types used for dopingTrivalent (acceptor atoms)Pentavalent (donor atoms)

P-type material

N-type material

2. Doping


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Trivalent impurities Pentavalent impurities

Aluminum (Al) Phosphorus (P)

Gallium (Ga) Arsenic (As)

Boron (B) Antimony (Sb)Indium (In) Bismuth (Bi)

Doping elements


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Pentavalent + intrinsic = n-type

N-type materials are still electrically neutral!

Majority carriers: conduction band/free electrons

Minority carriers: valence band holes

N-type vs P-type materials

oTrivalent + intrinsic = p-typeoMajority carriers : hole

oMinority carriers: free electrons

oP-type materials are still electrically neutral!

N-type materials

P-type materials


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1.What is doping? Why is it necessary?2.What is an impurity element?

3.What are trivalent and pentavalent elements?

4.Despite their respective characteristics, n-typeand p-type materials are still electricallyneutral. Why?

5.In what ways are n-type and p-type materials

similar? In what ways are they different?

Section review


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n-type + p-type = useful pn junction

Diffusion results:One net + charge in the n-type material

One net charge in the p-type material

1-3 The PN Junction


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At a large-scale picture:Each electron that diffuses across thejunction leaves one positively chargedbond in the n-type material and producesone negatively charged bond in the p-typematerial.

Both conduction-band electrons (freeelectrons) and valence-band holes areneeded for conduction through thematerials.When an electron diffuses across a

junction, the n-type material lost aconduction-band electron. When theelectrons fall into a hole in the p-typematerial, the material has lost avalence band- hole.

Both bonds have been depleted ofcharge carriers.

Depletion layer


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1.What is the overall charge on an n-typecovalent bond that has just given up aconduction-band electron?

2.What is the overall charge on an p-type

covalent bond that has just accepted anextra valence-band electron?

3.Describe the forming of the depletion layer.

4.What is barrier potential? What causes it?

Section review


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A PN junction is useful because we can controlthe width of the depletion layer.

By controlling the depletion layer width, we

control the resistance, thus we can control theamount of current that can pass through adevice.dep. layer width, resistance, junction current dep. layer width, resistance, junction current

Bias potential applied to a pn junction toobtain a desirable mode of operation.

1-4 Bias


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Two types:Forward biasReverse bias

Forward bias potential used to reduce the

resistance of the pn junctionReverse bias potential used to increase the

resistance of the pn junction

Bias


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When the applied voltage causes the n-type material tobe more negative than the p-type material.

Allows current to pass

Majority carriers in the n-type and p-type materials are

pushed toward the junction.Minority carriers in the n-type and p-type materials are

drawn away from the junction.

Forward Bias


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Bulk resistancecombined resistance of the n-type and p-type

materials in a FB pn junctionTypically in the range of 5 ohms or less

Forward voltage (VF

)

Voltage across a FB pn junction0.7 v for silicon0.3 v for germanium

Two ways of forward biasing a pn junction:

By applying a potential to the n-material thatdrives it more negative than the p-material

By applying a potential to the p-material thatdrives it more positive than the n-material

Forward bias


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When the applied potential causes the n-type material to bemore positive than the p-type material.Depletion layer becomes wider, junction current reduced to

near zeroMajority carriers in the n-type and p-type materials are

drawn away from the junction

Minority carriers in the n-type and p-type materials arepushed toward the junctionDiffusion current

Majority carrier current during the time the depletionlayer is growing

Reverse bias


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Two ways to reverse bias a junction:By applying a potential to the n-type material

that drives it morepositive than the p-typematerial

By applying a potential to the p-type materialthat drives it more negative than the n-typematerial

Reverse bias


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Bias type Junctionpolarities

Junctionresistance

Forward N-type is more(-) than p-type

Extremely low

reverse P-type is more

(-) than n-type

Extremely high

Bottom line


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Silicon is more tolerant of heat and moreabundant than germanium. It is also a betterinsulator than germanium (at roomtemperature)

Germanium oxide is water soluble, makinggermanium more difficult to process thansilicon.

When heated, silicon readily produces silicondioxide, which is critical to the production ofintegrated circuits. Germanium lacks anycomparable property.

Final Note


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1.What are the resistance and currentcharacteristics of a pn junction when itsdepletion layer is at its maximum width? Itsminimum width?

2.What purpose is served by the use of bias?3.What effect does forward bias have on

depletion layer width of a pn junction?

4.What is the junction resistance of a FB pnjunction?

5.What are the approximate values of VF for FB Si

and Ge pn junctions?

Section review


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6.List the commonly used methods for forwardbiasing a pn junction?

7.What effect does reverse bias have on thedepletion layer width of a pn junction?

8.What is the junction resistance of a reverse-biased pn junction?

9.List the commonly used methods for reverserbiasing a pn junction.

10.Why is silicon more commonly used than thegermanium in the production of solid-statecomponents?

11.

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Section review

2 Fundamental Solid-State Principles

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