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

Chapter 18 -

Intrinsic Elemental vs. Compound Semiconductors

Elemental Semiconductors

Chapter 18 -

Conductivity in 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

Chapter 18 -

Intrinsic Semiconductor Conductivity Increases with T

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

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.

Chapter 18 -

What Elements Make Si an n-type Extrinsic Semiconductor?

• Majority carriers are electrons• Minority carriers are electron holes

Donor energy level

Chapter 18 -

How Does T Affect an n-type Semiconductor?

e e e e

Chapter 18 -

What Elements Make Si a p-type Extrinsic Semiconductor?

• Majority carriers are electron holes• Minority carriers are electrons

Chapter 18 -

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

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

Chapter 18 -

Summary of Band Gaps in Intrinsic vs Extrinsic Semic.

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

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- +

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

Chapter 18 -17

p - n Rectifying Junction: Avalanche Diode

Fig. 18.22, Callister & Rethwisch 8e.

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

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.

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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