Lecture 1 introduction to diodes edited
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Transcript of Lecture 1 introduction to diodes edited
Chapter 1
Semiconductor Diodes
1
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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