Introduction to Semiconductor Devices Presented by K.Pandiaraj Ece Department Kalasalingam University Previous Class Topics

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ECE201 – ELECTRON DEVICES INTRODUCTION TO SEMICONDUCTOR DEVICES PRESENTED BY K.PANDIARAJ ECE DEPARTMENT KALASALINGAM UNIVERSITY PREVIOUS CLASS TOPICS

• Atom • Electron • Proton • Neutron • Orbit levels of electrons in an atom • Valence electrons • Free electrons • Energy band diagram • Valance band, Conduction band, Forbidden band • Insulators • Conductors • Semiconductors INTRODUCTION TO SEMICONDUCTOR • The material which has electrical conductivity between that of a conductor and that of an insulator is called as semiconductor. • silicon, germanium and graphite are some examples of semiconductors. • semiconductors are the foundation of modern electronics, including transistors, light-emitting diodes, solar cells etc. INTRODUCTION TO SEMICONDUCTOR

• It has a forbidden gap of about 1 electron volt (ev). • The semiconductor behaves as an insulator at low temperature. • However, at room temperature some of the electrons in valence band gains enough energy in the form of heat and moves in to conduction band. when the valence electrons moves in to conduction band they becomes free electrons. • The conduction band electrons are responsible for electrical conductivity. • When the temperature is goes on increasing, the number of valence band electrons moving in to conduction band is also increases. this shows that electrical conductivity of the semiconductor increases with increase in temperature. • In semiconductors, electric current is carried by two types of charge carriers they are electrons and holes. HOLE

• The absence of electron in a particular place in an atom is called as hole. • Hole is a electric charge carrier which has positive charge. the electric charge of hole is equal to electric charge of electron but have opposite polarity. • When a small amount of external energy is applied, then the electrons in the valence band moves in to conduction band and leaves a vacancy in valence band. This vacancy is called as hole. INTRINSIC SEMICONDUCTOR

INTRODUCTION

• Pure semiconductors are called intrinsic semiconductors. • Silicon and Germanium are the most common examples of intrinsic semiconductors. Both these semiconductors are most frequently used in the manufacturing of transistors, diodes and other electronic components. • Intrinsic semiconductor is also called as undoped semiconductor. • In intrinsic semiconductor the number of electrons in the conduction band is equal to the number of holes in the valence band. Therefore the overall electric charge of a atom is neutral. ATOMIC STRUCTURE OF SILICON AND GERMANIUM

• The atomic number of silicon is 14 i.e. 14 protons. • Two electrons in first orbit, eight electrons in second orbit and 4 electrons in the outermost orbit). • The atomic number of germanium is 32 i.e. 32 protons. • 2 electrons in first orbit, 8 electrons in second orbit, 18 electrons in third orbit and 4 electrons in the outermost orbit. COVALENT BONDING IN SILICON

• The outermost shell of atom is capable to hold up to eight electrons. • The atom which has eight electrons in the outermost orbit is said to be completely filled and most stable. • But the outermost orbit of silicon has only four electrons. Silicon atom needs four more electrons to become most stable. • Silicon atom forms four covalent bonds with the four neighboring atoms. In covalent bonding each valence electron is shared by two atoms. COVALENT BONDING IN GERMANIUM

• The outermost orbit of germanium has only four electrons. Germanium atom needs four more electrons to become most stable. • Germanium atom forms four covalent bonds with the four neighboring atoms. In covalent bonding each valence electron is shared by two atoms.

• The outermost shell of silicon and germanium is completely filled and valence electrons are tightly bound to the nucleus of atom because of sharing electrons with neighboring atoms. • In intrinsic semiconductors free electrons are not present at absolute zero temperature. Therefore intrinsic semiconductor behaves as perfect insulator. ELECTRON AND HOLE CURRENT • In conductors current is caused by only motion of electrons but in semiconductors current is caused by both electrons in conduction band and holes in valence band. • Current that is caused by electron motion is called electron current and current that is caused by hole motion is called hole current. Electron is a negative charge carrier whereas hole is a positive charge carrier. • When the valence electron moves from valence band to the conduction band a vacancy is created in the valence band where electron left. Such vacancy is called hole. CONDUCTION IN INTRINSIC SEMICONDUCTOR • An intrinsic semiconductor is connected to a battery. • Here, positive terminal of battery is connected to one side and negative terminal of the battery is connected to other side. • As we know like charges repel each other and opposite charges attract each other. • In the similar way negative charge carriers (electrons) are attracted towards the positive terminal of battery and positive charge carriers (holes) attracted towards the negative terminal of battery. CONDUCTION IN INTRINSIC SEMICONDUCTOR CONT’D

• In intrinsic semiconductor the number of free electrons in conduction band is equal to the number of holes in valence band. The current caused by electrons and holes is equal in magnitude. • The total current in intrinsic semiconductor is the sum of hole and electron current. • Total current = Electron current + Hole current

• I = IHOLE+ IELECTRON CONVENTIONAL CURRENT

• The electric current that flows from positive terminal of battery to the negative terminal of battery is called conventional current. • The conventional current direction is in the same direction of flow of holes but opposite to the direction of flow of free electrons. INTRINSIC CARRIER CONCENTRATION • In intrinsic semiconductor, when the valence Electrons broke the covalent bond and jumps into the conduction band, two types of charge carriers gets generated. They are free electrons and holes. • The number of electrons per unit volume in the conduction band or the number of holes per unit volume in the valence band is called intrinsic carrier concentration. • The number of electrons per unit volume in the conduction band is called electron-carrier concentration and the number of holes per unit volume in the valence band is called as hole-carrier concentration. • In an intrinsic semiconductor, the number of electrons generated in the conduction band is equal to the number of holes generated in the valence band. Hence the electron-carrier concentration is equal to the hole-carrier concentration. • It can be written as, ni = n = p Where, n = electron-carrier concentration P = hole-carrier concentration

And ni = intrinsic carrier concentration INTRINSIC CARRIER CONCENTRATION CONT’D

• The hole concentration in the valence band is given as

• The electron concentration in the conduction band is given as

• Where KB is the boltzmann constant • T is the absolute temperature of intrinsic semiconductor

• Nc is the effective density of states in conduction band.

• Nv is the effective density of states in valence band. FERMI LEVEL IN INTRINSIC SEMICONDUCTOR

• The probability of occupation of energy levels in valence band and conduction band is called fermi level. • At absolute zero temperature intrinsic semiconductor acts as perfect insulator. However as the temperature increases free electrons and holes gets generated. • In intrinsic or pure semiconductor, the number of holes in valence band is equal to the number of electrons in the conduction band. Hence, the probability of occupation of energy levels in conduction band and valence band are equal. • Therefore, the fermi level for the intrinsic semiconductor lies in the middle of forbidden band. FERMI LEVEL IN INTRINSIC SEMICONDUCTOR

• The fermi level for intrinsic semiconductor is given as,

• Where EF is the fermi level EC is the conduction band EV is the valence band

• Therefore, the Fermi level in an intrinsic semiconductor lies in the middle of the forbidden gap. REVIEW QUESTIONS

• How are covalent bonds formed? • What is meant by the term intrinsic? • Effectively, how many valance electrons are there in each atom within a silicon crystal? • Are free electrons in the valance band or in the conduction ban? • Which electrons are responsible for current in a material? • What is hole?