ele232 - chapter 1- semiconductor [compatibility mode]-4

7/26/2019 ELE232 - Chapter 1- Semiconductor [Compatibility Mode]-4

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ELE 232 ELECTRONICS

Prepared By:

Norsabrina Sihab

Faculty of Electrical Engineering,

Universiti Teknologi MARA

Pulau Pinang

Tel : 04-3823355

Email : [email protected]

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Chapter : SEMICONDUCTOR MATERIAL

Norsabrina Sihab

Faculty of Electrical Engineering,

Universiti Teknologi MARA

Pulau Pinang

Tel : 04-3823355

Email : [email protected]

Chapter 1 Semiconductor Material

Updated Nov 2013LE232 Electronics 1Norsabrina Sihab

Learning Outcome

At the end of this chapter, students able to:

Discuss the basic structure of atoms, energy band, covalent

bonds, conduction in semiconductor, free electrons andholes as carrier.

Describe intrinsic, doping and extrinsic semiconductor

Describe the properties of n-type and p-type semiconductors

3Chapter 1 Semiconductor Material


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Introduction

Electronics devices are complex component which mostly used inelectronics systems:

Communication (TV, radio)

Digital system (PC, calculator) Industrial system (robotic, process control)

Medical system (x-ray, ECG)

Instrumentation (oscilloscope)

Since 1940s, electronics system constructed using solid-statecomponents.

Solid state components are made from semiconductor elements,neither conductor nor insulator, has useful characteristics as an

amplifier or rectifier.


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Conductor, Semiconductor & Insulator

Figure 1.3 - Energy diagrams for the three types of materials(insulator, semiconductor, and conductor)

Energy gap > The difference in energy between the valence bandand the conduction band. This is the amount of energy required for avalence electrons to jump from valence band to conduction band.

Once at conduction band, electrons is free to move throughout thematerial and is not tied to any given atom. For example, it absorb anamount of energy 1.8eV-0.7eV=1.1eV (for Si material).*1eV=1.6X10-9J.



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Current in Semiconductors

Energy Level -> Electrons orbit the nucleus of an atom at certaindistance from the nucleus. Electrons near the nucleus have less

energy. Each distance from nucleus corresponds to a certainenergy level.

Conduction band

e-

(valence e-)

e- (free electron)

Figure 1.5 Energy Level



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Free Electrons > When an electron acquires enough energy (fromheat energy), it can leave the valence band and become a free electronwhich its exist in conduction band.

Conduction band > Band outside valence band which level ofenergy of an electron is high enough and capable of being influence byan external force.

Figure 1.6 Creation of EHPs



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Covalent Bond -> Is a method by which atoms complete theirvalence shells by sharing valence e- with other atoms. It strongbonding between atoms. Eg. Si atom has 4 valence e- and it create

8 shared valence e-

of each atom. When Si atoms combined bycovalent bonding it form a solid material.

Figure 1.7 - Covalent bonding of the silicon atom.


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Conduction Versus Temperature > At room temperature pure Sihas no free e-. Semiconductor has no free electrons when no

voltage applied. As temperature increase, electrons will absorbenough energy to break their covalent bonds and number of freeelectrons will increase. As temperature decrease, less thermalenergy to release the e- from their covalence band and number ofelectrons will decrease. Conductivity of semiconductor temperature. When circuit is warm up, current will increase.



N-type & P-type Semiconductor

Intrinsic Semiconductor > Semiconductor which has a very lowlevel of impurities. Intrinsic Si & Ge poor conductor (relatively

large energy gap)Extrinsic Semiconductor > Semiconductor that has been subjectedto a doping process. Not longer as pure/intrinsic material.

Doping - Is a process of adding impurities atoms to the intrinsic Sior Ge to improve the conductivity of a semiconductor.

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Two types of impurities:

1. Trivalent > To increase the number of holes in intrinsicsemiconductor. It has 3 valence electrons. Known as acceptor

(accept electrons). Eg. Aluminum (Al), Gallium (Ga), Boron(B), Indium (In). Trivalent doped with Si/Ge is called a p-typesemiconductor.

2. Pentavalent > To increase the number of conduction bandelectrons in intrinsic semiconductor. It has 5 valenceelectrons. Known as donor (donate electrons). Eg.Phosphorus (P), Arsenic (As), Antimony (Sb), Bismuth (Bi).Pentavalent doped with Si/Ge is called a n-type

semiconductor.

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Two types of extrinsic semiconductor (material that are subjectedto doping process):

1. n-type > negative charge of electrons. Created by adding

impurity element with 5 valence electrons into pure Si or Ge.Electrons are majority carriers. Holes created by EHP areminority carrier.

2. p-type > positive charge of hole. Holes are majority carrier.Electrons are minority carrier.


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Pentavalent impurity atom in a silicon crystalstructure. An antimony (Sb) impurity atom isshown in the center. The extra electron from theSb atom becomes a free electron.

Trivalent impurity atom in a siliconcrystal structure. A boron (B) impurityatom is shown in the center.

Figure 1.10 Trivalent and pentavalent impurities



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Figure 1.11 N-type semiconductor



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Figure 1.12 P-type semiconductor



p-n Junction

P-N junction > formed by p-type region jointed with n-typeregion.

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Figure 1.13 - The basic diode structure at the instant of junction formationshowing only the majority and minority carriers.

C S C S


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Exercise

29Chapter 1 Semiconductor Material


Exercise

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Exercise

1. Define the following terms:

a) Free electron

b) Intrinsic material

c) Ionization

2. Define covalent bonding and sketch a diagram showing thecovalent bonding of the silicon atom.

3. Explain the concepts of electron hole pair (EHP) and Lifetime ofEHP.

4. Draw the energy diagrams of conductor, semiconductor andinsulator.

5. Describe the difference between n-type and p-typesemiconductor materials.

6. Describe the differences between majority and minoritycarriers.

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ele232 - chapter 1- semiconductor [compatibility mode]-4

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