SEMICONDUCTORS & DEVICES
#1

Presented By
KUMARASWAMY.A

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SEMICONDUCTORS & DEVICES
PART 1
MATERIALS
1.1:INTRODUCTION

All of us know how importance of using materials like copper , aluminium etc. in electrical applications. This is because copper aluminum are good conductors. Similarly some materials like glass, wood, paper etc also find wide applications in electrical and electronic applications, these are called insulators. There is also another category of materials whose ability to carry current called conductivity is lies between the insulators and the conductors.
To understand the fundamental concept of semiconductor one must apply modern physics to solid materials. More specifically, we are interested in semiconductors crystals. Crystals are solid materials consisting no of atoms, which are in a highly ordered structure called a lattice. Such a structure yields a periodic potential throughout the results in some remarkable properties.
Two properties of crystals particular interest, since they are needed to calculate the current in a semiconductor. First, we need to know how many fixed and mobile charges are present in the material. Second we need to understand the transport of the mobile carriers through the semiconductors.
In this we need to know the about atomic structure of semiconductor and concept of energy bands, energy band gaps.
We know about atoms, consisting of energy shells. Each energy shell associated with energy levels. An electron very close to the nucleus in the first shell is very much tightly bounded to the nucleus. Greater the distance of from the nucleus the greater is the energy hence the energy level of outer most shell is high due to the high energy the valence electrons in the outer most shell can be easily extracted out and hence such electrons take part in chemical reaction and bonding the atoms together.
In solids atoms are brought close together in such a case outer cell electrons are shared by more than one atom.
1.2:TYPES OF MATERIAL
Materials can be categorized into conductors, semi conductors and insulators by their ability to conduct electricity.
It is popular belief that insulators do not contain electricity because their valance electrons are not free to wonder throughout the material.
Now the valance electrons posses’ highest energy levels. When such electrons farm the covalent bonds due to coupling between the valance electrons, the energy levels associated with the valance electron merge into each other this merging form an energy band.
Nature of conductivity of materials can be explained by its energy bandout of all energy bands three bands are most important these are
1.3:TYPES OF BANDS
1.3.1:Valance band:
Energy band formed by due to the merging of energy levels associated with valance electrons the electrons jin the last cell is called valance band. Metals contain a band that is partly empty and partly filled regardless of temperature. Therefore they have very high conductivity. The lowermost, almost fully occupied band in an insulator or semiconductor, is called the valence band by analogy with the valence electrons of individual atoms.
1.3.2: Conduction band:
The uppermost, almost unoccupied band is called the conduction band because only when electrons are excited to the conduction band can current flow in these materials.
The energy band formed due to merging of energy levels associated with the free electrons is called conduction band.
1.3.3: Forbidden Energy gap:
The energy gap which separates the both bands i.e.valence band and conduction band. The difference between insulators and semiconductors is only that the forbidden band gap between the valence band and conduction band is larger in an insulator, so that fewer electrons are found there and the electrical conductivity is lower
Because one of the main mechanisms for electrons to be excited to the conduction band is due to thermal energy, the conductivity of semiconductors is strongly dependent on the temperature of the material.
PART 2
SEMICONDUCTORS
2.1.WHAT IS A SEMICONDUCTOR?

A semiconductor is a material that has an electrical conductivity between that of a conductor and an insulator, Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Semiconductor devices include the various types of transistor, solar cells, ma An external electrical field may change a semiconductor's resistivity. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively-charged "holes" in the electron structure of the material.
The ease with which electrons in a semiconductor can be excited from the valence band to the conduction band depends on the band gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing line (roughly 4 eV) between semiconductors and insulators.
Electrons excited to the conduction band also leave behind electron holes, or unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to electrical current.
freeing the electron does not imply destruction of the crystal structure.
Holes: electron absence as a charge carrier
2.2: Types of semiconductors
2.2.1: Intrinsic semiconductors:

The semiconductors in its natural or pure form are called the intrinsic semiconductors.
2.2.2: Semiconductor doping
Semiconductor doping is the process that changes an intrinsic semiconductor to an extrinsic semiconductor. During doping, impurity atoms are introduced to an intrinsic semiconductor. Impurity atoms are atoms of a different element than the atoms of the intrinsic semiconductor. Impurity atoms act as either donors or acceptors to the intrinsic semiconductor, changing the electron and hole concentrations of the semiconductor. Impurity atoms are classified as donor or acceptor atoms based on the effect they have on the intrinsic semiconductor. Donor impurity atoms have more valence electrons than the atoms they replace in the intrinsic semiconductor lattice. Donor impurities "donate" their extra valence electrons to a semiconductor's conduction band, providing excess electrons to the intrinsic semiconductor. Excess electrons increase the electron carrier concentration (n0) of the semiconductor, making it n-type.
Acceptor impurity atoms have less valence electrons than the atoms they replace in the intrinsic semiconductor. They "accept" electrons from the semiconductor's valence band. This provides excess holes to the intrinsic semiconductor. Excess holes increase the hole carrier concentration (p0) of the semiconductor, creating a p-type semiconductor.
2.2.3:Extrinsic semiconductor
An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic (pure) semiconductor.
The two types of extrinsic semiconductor
2.2.3.1:N-type semiconductors
N-type semiconductors are pure semiconducting materials (intrinsic semiconductors), which are doped with atoms capable of providing extra conduction electrons to the host material. This creates an excess of negative (n-type) electron charge carriers.
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