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Introduction to Semiconductor Materials and devices

  1. INTRODUCTION TO SEMICONDUCTOR MATERIALS AND DEVICES Presented By S.ARUNBHAARAT 6017f17de69b11e9b0fe59835bdf11f2(App SASTRA Deemed To be University B.Tech ECE 1
  2. ACKNOWLEDGEMENT In preparation of my assignment, I had to take the help and guidance of some respected persons. As the completion of this assignment gave me much pleasure, I would like to show my gratitude DR.AJAY SEMALTY, Course Instructor, from ACADEMIC WRITING under UGC SWAYAM ONLINE COURSE for giving me a good guidelines for assignment throughout numerous consultations. I would also like to expand my gratitude to all those who have directly and indirectly guided me in writing this assignment. 2
  3. DISCOVERY Vacuum Tubes- First Generation Contained Heavy Coverage of tubes in Solid state Era. Many Engineers joined together and discovered First Transistor on 1950 Miniaturisation limited by three factors 1.Quality 2. Network design 3. Manufacturing Equipment. 3
  5. WHY SEMICONDUCTOR SELECTED?? Semiconductors have a unique atomic structure that allows their conductivity to be controlled by stimulation with electric currents, electromagnetic fields, or even light. Therefore ,Semiconductors and semiconductor devices are the building blocks of all the electronic devices. Semiconductor materials generally have conductivity between metals and insulators. Types of semiconductors (i) Elemental semiconductors: Si and Ge (ii) Compound semiconductors :  Inorganic: CdS, GaAs, CdSe, InP, etc.  Organic: Anthracene, doped pthalocyanines, etc.  Organic polymers: Polypyrrole, polyaniline, polythiophene, etc. 5
  6. BAND THEORY Isolated atoms of materials have well defined energy levels over which electrons are distributed (in different orbits) In crystalline form, when the atoms are closely spaced their outer energy levels join together to form two separate bands called Valance band and Conduction band The energy gap between the valance band and the conduction band is known as Forbidden energy gap( Eg) or Energy band gap or simply Energy gap The conductivity of materials depends on this energy gap. 6
  8. ENERGY BAND STRUCTURE Metals SemiconductorNon-Metals 8
  9. INTRINSIC SEMICONDUCTORS An intrinsic semiconductor behaves like an insulator at T = 0 K It is the thermal energy at higher temperatures (T > 0K), which excites some electrons from the valence band to the conduction band 9
  10. EXTRINSIC SEMICONDUCTOR To improve the conductivity the pure Semiconductors are added with small amounts of Trivalent (Gallium, indium) or Pentavalent (Arsenic, Phosphorous) impurities Doping: Adding small amounts of trivalent or pentavalent impurities to pure semiconductors to improve their conductivity is known as doping The Two types of extrinsic semiconductors are: n-Type semiconductor & p-Type semiconductor 10
  11. N-TYPE SEMICONDUCTOR Pure semiconductors doped with small amounts of pentavalent impurities are known as n-type semiconductors The four valance electrons of each impurity atom form covalent bond with the neighboring semiconductor atoms. The fifth unpaired electron will remain at an energy level much higher than the bound electrons This energy level is known as the donor energy level The donor energy level is very close to conduction band( 0.01 to 0.05 eV). This electrons can easily jump in to conduction band to become free electrons by absorbing thermal energy 11
  13. P-TYPE SEMICONDUCTORS In this case the three valance electrons of each trivalent impurity atom forms covalent bonds with the neighboring semiconductor atom the fourth neighboring atom will have an unoccupied energy state. This energy state is called acceptor energy level Accepter energy level is very close to valance band ( 0.01 to 0.05 ev) Electrons from valance band can easily jump in to Acceptor energy level This creates Holes in the valance band the number holes the valance band is greater than the free electrons in the conduction band. nh > ne Majority current carriers in p-type semiconductors are holes Again ne x nh = ni 2 13
  15. P-N JUNCTION (SEMICONDUCTOR DIODE) p-n junction is the basic building block of many semiconductor devices like diodes, transistor, etc P-type & n-Type semiconductors are fused together to make p-n Junction or Diode (a). Diffusion and Drift In n-type- semiconductor concentration of electrons (number of electrons per unit volume) is more than the concentration of holes In a p-type semiconductor, concentration of holes is more than the concentration of free electrons During the formation of p-n junction holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p) 15
  16. P-N JUNCTION (SEMICONDUCTOR DIODE) This motion of charge carries gives rise to diffusion current across the junction Due to this diffusion of current carriers, a layer of negative charge (negative space-charge region) on the p-side and a layer of positive charge (or positive space-charge region) on n-side of the junction are developed. This region at the junction free of current carriers is known as Depletion layer 16
  17. P-N JUNCTION (SEMICONDUCTOR DIODE) This diffusion of majority current leads to a layer of +ve charge on n side and –ve charge on p side This charge accumulation on either side near the junction produces an electric field with in the depletion layer from n side to p side, known as Barrier electric field (Eb) Due to this electric field electrons move from p side to n side and holes from n side to p side This motion of charges due to barrier electric field is known as Drift That is drift current is opposite to diffusion current At equilibrium state, drift current = diffusion current. 17
  18. P-N JUNCTION (SEMICONDUCTOR DIODE) Due to the barrier, electric field n side will be at a higher potential compared to the p side. This potential difference across the depletion layer is called barrier potential ( V0) 18
  20. P-N JUNCTION DIODE UNDER FORWARD BIAS P side is connected to positive & n side is connected to negative External potential V appears across the depletion layer Due to the affect of external potential free electrons and holes accumulate in the depletion layer Thickness of depletion layer decreases Free electrons diffuse towards the positive end and holes diffuse to the negative end setting up a diffusion current( of the range of a few mA) The current through the diode remains small at the beginning As external voltage increases, at a particular voltage the current starts increase rapidly 20
  21. P-N JUNCTION ( DIODE) FORWARD BIAS His voltage is known as Thresh hold voltage or Cut in Voltage ( 0.2 V for Ge & 0.7 Volt for Si). Beyond thresh hold voltage the diode acts as a good conductor and V-I relationship is more or less linear 21
  22. REVERSE BIAS CHARACTERISTICS P side is connected to negative & n side to positive of the external power The barrier potential increases as the external voltage. Depletion layer thickens (widens). The diode remains non-conducting except for a small drift current (of the order of a few microampere) On further increasing, the external voltage the diode breaks down with a sudden increase in current. This damages the diode. This voltage is known as break down voltage A semiconductor Diode(p-n Junction) allows current only in one direction( Forward Bias). This property of Diode is used in the construction of a rectifier 22
  23. REVERSE BIAS CHARACTERISTICS A semiconductor Diode(p-n Junction) allows current only in one direction( Forward Bias). This property of Diode is used in the construction of a rectifier 23
  24. TIPS TO LEARN PHYSICS NCERT BOOK was only Reference and Text Book NCERT Worked Out Examples NCERT Book Back Exercise (Except Additional Exercise) Previous Yr. Paper Reference books: 1. XAM Idea (Board Type) 2. S.L..Arora (Board & Entrance exam) 24
  25. REFERENCE Textbook: 1.NCERT Physics Class 12 Volume 2 2. Electronic Devices and Circuit Theory-By ROBERT BOYLESTAD & LOUIS NASHELSKY Website: 1. 2. 25
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