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Chapter 18 - 1 ISSUES TO ADDRESS... How are electrical conductivity and resistivity characterized? What electronic structure distinguishes conductors, semiconductors, and insulators? How is conductivity in metals affected by impurities, temperature, and deformation? How is conductivity in semiconductors affected by impurities (doping) and temperature? Chapter 18: Electrical Properties II: Intrinsic & Extrinsic Semiconductors Class 21 – Chapter 18: Conductivity II – Intrinsic & Extrinsic Semic - Topic #7.2

Intrinsic & Extrinsic Semiconductors

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Page 1: Intrinsic & Extrinsic Semiconductors

Chapter 18 - 1

ISSUES TO ADDRESS...• How are electrical conductivity and resistivity characterized?

• What electronic structure distinguishesconductors, semiconductors, and insulators?

• How is conductivity in metals affected byimpurities, temperature, and deformation?

• How is conductivity in semiconductors affectedby impurities (doping) and temperature?

Chapter 18: Electrical Properties II: Intrinsic & Extrinsic Semiconductors

Class 21 – Chapter 18: Conductivity II – Intrinsic & Extrinsic Semic - Topic #7.2

Page 2: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Intrinsic Elemental vs. Compound Semiconductors

Elemental Semiconductors

Page 3: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Conductivity in Semiconductors

Page 4: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Conduction in Terms of Electron & Hole Migration

Adapted from Fig. 18.11, Callister 7e.

electric field electric field electric field

• Electrical Conductivity given by:

# electrons/m3 electron mobility

# holes/m 3

hole mobilityhe epen

• Concept of electrons and holes:

+-

electron holepair creation

+-

no applied applied

valence electron Si atom

applied

electron holepair migration

Page 5: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Intrinsic Semiconductor Conductivity Increases with T

Page 6: Intrinsic & Extrinsic Semiconductors

Chapter 18 - 6

Intrinsic Semiconductors: Conductivity vs Tand Also Plotted as ln Conductivity vs. 1/T

• Data for Pure Silicon:-- increases with T-- opposite to metals

materialSiGeGaPCdS

band gap (eV)1.110.672.252.40

Selected values from Table 18.3, Callister & Rethwisch 8e.

ni eEgap / kT

ni e e h

• Data for Pure Silicon:-- ln decreases vs. 1/T

Page 7: Intrinsic & Extrinsic Semiconductors

Chapter 18 - 7

• Intrinsic:-- case for pure Si-- # electrons = # holes (n = p)

• Extrinsic:-- electrical behavior is determined by presence of impurities

that introduce excess electrons or holes-- n ≠ p

Intrinsic vs Extrinsic Conduction

3+

• p-type Extrinsic: (p >> n)

no applied electric field

Boron atom

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+ hep

hole

• n-type Extrinsic: (n >> p)

no applied electric field

5+

4+ 4+ 4+ 4+

4+

4+4+4+4+

4+ 4+

Phosphorus atom

valence electron

Si atom

conductionelectron

een

Adapted from Figs. 18.12(a) & 18.14(a), Callister & Rethwisch 8e.

Page 8: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

What Elements Make Si an n-type Extrinsic Semiconductor?

• Majority carriers are electrons• Minority carriers are electron holes

Donor energy level

Page 9: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

How Does T Affect an n-type Semiconductor?

e e e e

Page 10: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

What Elements Make Si a p-type Extrinsic Semiconductor?

• Majority carriers are electron holes• Minority carriers are electrons

Page 11: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Activity - How Does T Affect a p-type Semiconductor?

Page 12: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Activity – Electrical Property Concept Questions1. What will increase the conductivity of polycrystalline silicon?

1. decreasing temperature 2. increasing the grain size3. decreasing the grain size

2. Will adding 1% As to silicon increase or decrease its conductivity? 1. Increase because the As will contribute electrons 2. Decrease because As scatters electrons3. Increase because arsenic is a better conductor than silicon 4. Decrease because arsenic is a worse conductor than silicon5. Decrease because As takes more thermal energy for the electrons to move

3. Electrical conductivity will decrease when : 1. a metal has impurities added2. a semiconductor has impurities added3. a metal has its temperature increased4. a semiconductor has its temperature increased5. a metal is annealed

4. The addition of boron (III) to silicon (IV): 1. provides donors2. gives electron hole minority carriers3. makes it n-type4. makes it p-type5. reduces the energy gap

Page 13: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Summary of Band Gaps in Intrinsic vs Extrinsic Semic.

Page 14: Intrinsic & Extrinsic Semiconductors

Chapter 18 -14

Extrinsic Semiconductors: Conductivity vs. Temperature

• Data for Doped Silicon:-- increases doping-- reason: imperfection sites

lower the activation energy toproduce mobile electrons.

• Comparison: intrinsic vsextrinsic conduction...

-- extrinsic doping level:1021/m3 of a n-type donorimpurity (such as P).

-- for T < 100 K: "freeze-out“,thermal energy insufficient toexcite electrons.

-- for 150 K < T < 450 K: "extrinsic"-- for T >> 450 K: "intrinsic"

Adapted from Fig. 18.17, Callister & Rethwisch8e. (Fig. 18.17 from S.M. Sze, Semiconductor Devices, Physics, and Technology, Bell Telephone Laboratories, Inc., 1985.)

Con

duct

ion

elec

tron

conc

entra

tion

(1021

/m3 )

T (K)6004002000

0

1

2

3

freez

e-ou

t

extri

nsic

intri

nsic

dopedundoped

Page 15: Intrinsic & Extrinsic Semiconductors

Chapter 18 -15

• Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current).

• Processing: diffuse P into one side of a B-doped crystal.

-- No applied potential:no net current flow.

-- Forward bias: carriersflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.

-- Reverse bias: carriersflow away from p-n junction;junction region depleted of carriers; little current flow.

Building Block for Devices: p-n Rectifying Junction

++

+ ++

- ---

-p-type n-type

+ -

++ +

++

--

--

-

p-type n-typeAdapted from Fig. 18.21 Callister & Rethwisch8e.

+++

+

+

---

--

p-type n-type- +

Page 16: Intrinsic & Extrinsic Semiconductors

Chapter 18 -16

p - n Rectifying Junction: AC to DC Conversion

Fig. 18.22, Callister & Rethwisch 8e. Fig. 18.23, Callister & Rethwisch 8e.

Silicon controlled rectifier

Page 17: Intrinsic & Extrinsic Semiconductors

Chapter 18 -17

p - n Rectifying Junction: Avalanche Diode

Fig. 18.22, Callister & Rethwisch 8e.

Page 18: Intrinsic & Extrinsic Semiconductors

Chapter 18 -18

• For intrinsic semiconductors, conductivity is increased by-- increasing temperature

• For extrinsic semiconductors conductivity is increased by -- doping by adding B (III) to Si (IV) (p-type)-- doping by adding P to Si (n-type)]

• p – n junctions can be used to make devices such as-- rectifiers to convert AC to DC current-- avalanche diodes for surge protectors

Summary

Page 19: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

HW# 21 Electrical Properties II: Intrinsic & Extrinsic Semiconductors

5. Explain how the avalanche diode in the surge protector plug-in outlet strip willprotect your computer from damage by the voltage surge.

Page 20: Intrinsic & Extrinsic Semiconductors

Chapter 18 -

Points of Reflection on Today’s Class

Please briefly describe your insights on the following points from today’s class.

• Point of Interest: Describe what you found most interesting in today’s class.How Interesting? (circle) Little Bit 1 2 3 4 5 Very Much

• Muddiest Point: Describe what was confusing or needed more detail.How Muddy? (circle) Little Bit 1 2 3 4 5 Very Much

• Learning Point: Describe what you learned about how you learn?

Letter + 4 digit number ______________ F MClass Topic: _______________________Date: ________________

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