This document provides an introduction to semiconductor diodes and electronic devices. It discusses how electronic devices can perform functions like rectification, amplification, generation, and conversion between electricity and light. Semiconductors are materials with conductivity between conductors and insulators. The three most commonly used semiconductors are germanium, silicon, and gallium arsenide. The document describes the atomic structure of semiconductors and how covalent bonding results in some electrons becoming free to move within the material. Intrinsic and extrinsic semiconductors are discussed, along with how doping with impurities can create an excess or shortage of electrons to enhance conductivity.

1. Chapter 1
Semiconductor Diodes
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2. 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 Radar
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3. Continued...
 Integrated Circuit (IC) is a combination of several Electronic
Devices
 Intel Pentium®4 processor has more than 42 million transistor
and a host of other components
 1 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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4. Semiconductor Materials
 Semiconductors 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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5. 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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6. Elements
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7. 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) Silicon
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8. Some Basic Concepts
 The electrons at the valence shell are known as Valence
Electrons
 Atoms with -
 Four Valence Electrons –Tetravalent
 Three Valence Electrons- Trivalent
 Five 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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9. Covalent Bonding
 When 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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10. Covalent Bonding
Covalent Bond in the Silicon Atom
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11. 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
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12. 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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13. Effect of Temperature on Material
 Conductor: 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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14. Energy Level
 There 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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15. Energy Level
Conduction and Valence bands of an Insulator, Semiconductor and Conductor
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16. Extrinsic Materials
 The 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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17. n-Type Material
 n-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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18. 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 atomsBoron impurity in p-type
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