Chapter 1

Semiconductor Diodes

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Introduction Electronic Device:

An Electronic Device is that in which current flows through a vacuum or gas or semiconductor.

Electronic devices are capable of performing the following functions :

Rectification: The conversion of a.c into d.c Amplification: The process of raising the strength of a weak

signal Generation: Conversion of d.c power into a.c power-Oscillation Photo-electricity: Conversion of light into electricity

Used for Burglar alarms , sound recording for motion pictures etc Conversion of electricity into light:

used in television and Radar2

Continued...Integrated Circuit (IC) is a combination of several

Electronic DevicesIntel Pentium®4 processor has more than 42

million transistor and a host of other components1 Billion transistors will soon be placed on a silver

of silicon smaller than a fingernail !!!This is being possible through

Miniaturization Further Miniaturization is limited by three

factors – Quality of the semiconductor material Network Design Technique Limits of manufacturing and processing equipment

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Semiconductor MaterialsSemiconductors are a special class of elements

having a conductivity between that of a good conductor and that of insulator.

There are two classes of Semiconductors- Single Crystal (Si, Ge)- Repetitive Crystal Structure Compound (GaAs, GaN, CdS)- Constructed of two or

more materials of different atomic structures

The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, GaAs.

Earlier – Germanium (Ge)Now – Silicon (Si)Future- Gallium Arsenide (GaAs)

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Continued...Ge- Sensitive to changes in temperature

------Hence devices suffer from low levels of reliability

Si- Improved temperature sensitivity and easily available ------Electronics became more sensitive to “Speed” issues

GaAs Transistors have speeds of operation up to 5 times that of Si

GaAs –Expensive, Difficult to manufacture at high level of purity

Si-Still the leading material for electronic components and ICs.

GaAs will soon begin to challenge Si !!!

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Elements

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Bohr Model

atom is composed of three basic particles: the electron, the proton, and the neutron.

Neutrons and Protons form the Nucleus

Electrons appear in fixed orbits around the Nucleus

Atomic Structure: (a) Germanium (b) Silicon7

Some Basic ConceptsThe electrons at the valence shell are known as

Valence Electrons

Atoms with -Four Valence Electrons –TetravalentThree Valence Electrons- TrivalentFive Valence Electrons- Pentavalent

Valence: Potential required to remove any one of the electrons at the outermost shell from the atomic structure is lower than that required for any other electron in the structure.

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Covalent BondingWhen two Silicon or Germanium atoms are

placed close to one another , the valence electrons are shared between the two atoms , forming a Covalent Bond.

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Covalent Bonding

Covalent Bond in the Silicon Atom10

Free Electrons Covalent bond results in a stronger bond between the

valence electrons and their parent atom

The valence electrons can still absorb sufficient kinetic energy from external natural sources to break the covalent bond

The External Sources--------- Thermal Energy from the surrounding medium Light energy in the form of Photons

At room Temperature there are approx. 15 Billion free electrons

1 cm3 of intrinsic silicon material 11

Intrinsic Materials Intrinsic materials are those semiconductors that have been

carefully refined to reduce the impurities to a very low level—essentially as pure as can be made available through modern technology.

Free Electrons-Due only to External natural Causes-Intrinsic Carriers

Ability of the free carriers to move throughout the material-Relative Mobility

Ge is used in High Speed Radio Frequency applications (Because of Higher Relative Mobility than Si)

Semiconductor

Intrinsic Carriers (Per Cubic cm)

GaAs

1.7 x 106

Si 1.5 x 106

Ge 2.5 x 10 13

Semiconductor

Relative Mobility (µn)

Si 1500 Ge 3900 GaAs 8500

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Effect of Temperature on MaterialConductor: Resistance increase with increase in

temperature and have a Positive Temperature Coefficient.

The number of carriers in a conductor do not increase significantly with temperature, but their vibration pattern about a relatively fixed location make it increasingly difficult for electrons to pass through.

Semiconductor: Resistance decreases with increase in temperature and have a Negative Temperature Coefficient.

As the temperature rises , an increasing number of valence electrons absorb sufficient thermal energy to break the covalent bond and contribute to the number of free carriers which increase the conductivity index and result in a lower resistance level.

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Energy LevelThere are specific energy levels associated

with each shell and orbiting electron

The further an electron is from the nucleus, the higher is the energy state

Free electrons has a higher energy state than any electron in the atomic structure

Only specific energy levels can exist for the electrons in the atomic structure

This results in a series of gaps where no electrons (carriers) are permitted

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Energy Level

Conduction and Valence bands of an Insulator, Semiconductor and Conductor15

Extrinsic MaterialsThe characteristics of semiconductor materials can be altered

significantly by the addition of certain impurity atoms into the relatively pure semiconductor material.

These impurities, although only added to perhaps 1 part in 10 million, can alter the band structure sufficiently to totally change the electrical properties of the material

This addition of foreign atoms to the semiconductor is known as doping

A semiconductor material that has been subjected to the doping process is called an Extrinsic material.

There are two types of Extrinsic Materials- n-type p-type

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n-Type Materialn-type material is created by introducing

impurity elements having five valence electrons (Antimony, arsenic, phosphorus)

An additional fifth valence electron is introduced ,which is unassociated with any particular covalent bond

Material is still charge neutral, but very little energy is required to free the electron for conduction since it’s not participating in any bond

Diffused impurities with five valence electrons are called donor atomsAntimony Impurity in n-type

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p-Type Material p-type material is created by

introducing impurity elements having three valence electrons (Boron, Gallium, Indium)

There is now an incomplete bond pair, creating a vacancy for an electron

Little energy is required to move a near by electron into the vacancy (hole)

As the ‘hole’ propagates, charge is moved across the semiconductor

Diffused impurities with three valence electrons are called acceptor atoms

Boron impurity in p-type

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