Electronic Principles

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  • 1. ELECTRONIC PRINCIPLES (BEE 2113) By: NABIAH BT ZINAL Department of Electronic Engineering Faculty of Electrical and Electronic Engineering Kolej Universiti Teknologi Tun Hussein Onn 1
  • 2. CHAPTER 1: SEMICONDUCTOR PN JUNCTION THEORY 2
  • 3. Lecture Contents Atomic theory and energy band Insulator and semiconductor materials Formation of PN junction Distribution of holes and electrons Forward-biased and reversed-biased PN junction IV characteristics of forward-biased and reversed-biased PN junction 3
  • 4. 1.1 Atomic Theory The atom has 3 basic particles: i. Proton • positive charge • Same magnitude but different pole with electron ii. Electron • negative charge • Same magnitude but different pole with hole iii. Neutron • neutral Protons and neutrons form the nucleus  Electrons appear in fixed orbits around the nucleus.  4
  • 5. Cont… • For each atom; No. of proton in nucleus = no. of electron ATOM IS NEUTRAL • If an atom losses 1 valence electron - +ve • If an atom gains 1 valence electron - -ve 1.1.1 Bohr Model 5
  • 6. Cont… • The orbital paths or shells Orbital shells are identified using K L M K through M. • The innermost shell- K shell. • The outermost atom- valence shell. • Valence shell – determines the The orbital shells for an atom conductivity of atom. • The conductivity of atom depends on the number of electron in valence shell (valence electrons). 6
  • 7. Atomic Structure 7
  • 8. 1.1.2 Atomic structures The Periodic Table 8
  • 9. Cont…  Element in periodic table are arranged according to atomic number.  The atomic number of an element = the number of protons (which also equals the number of electrons) in the nucleus of a neutral atom. The Atomic Structure  Atomic number, often represented by the symbol Z. 9
  • 10. Cont..  Shells are divided into sub shells : i. s – max 2 electrons ii. p – max 6 electrons iii. d – max 10 electrons iv. f – max 14 electrons Example: 10 The structure for nickel atom
  • 11. 1.2 Energy Band • Electron energy level in valence shell is changing depend on the atomic force. • Electron energy level always stated as energy band. • In any material, there are 2 energy band; i. Valence band – the outermost shell that determines the conductivity ii. Energy band – the band outside the valence shell. • The 2 bands are separated by one energy gap called – forbidden gap. 11
  • 12. Cont… Example: • The valence band contains with electrons. • The electrons can move to the conduction band if it have enough energy ( eg: light or heat) • When the electron absorbs Energy band in Silicon Atom enough energy to jump from valence band to the conduction band, the electron is said to be in excited state. 12
  • 13. CONDUCTOR INSULATOR SEMICONDUCTOR The energy band gap for conductor, insulator and semiconductor 13
  • 14. 1.3 Insulator, Semiconductor and Conductor The concept of energy bands is particularly important in classifying materials as conductors, semiconductors, and insulators Energy Diagram for Three Types of Material 14
  • 15. Cont… • Insulator - very wide energy gap. The wider this gap, the greater the amount of energy required to move the electron from the valence band to the conduction band. • Therefore, an insulator requires a large amount of energy to obtain a small amount of current. • The insulator "insulates" because of the wide forbidden band or energy gap. 15
  • 16. Cont… • Semiconductor - has a smaller forbidden band and requires less energy to move an electron from the valence band to the conduction band. • Therefore, for a certain amount of applied voltage, more current will flow in the semiconductor than in the insulator. • Conductor - no forbidden band or energy gap and the valence and conduction bands overlap. • With no energy gap, it takes a small amount of energy to move electrons into the conduction band; consequently, conductors pass electrons very easily. 16
  • 17. Cont.. • The valence shell determines the ability of material to conduct current. • The number of valence electron in valence shell: 1 e – perfect conductor ( < 4e) (Easy to drift or move to other atom) 8 e – insulator 4 e – semiconductor Note: conductivity decreases with an increase in the number of valence electrons 17
  • 18. 1.3.1 Conductor • • • Most of the conductors used in electronics are metals like copper, aluminum and steel. Conductors are materials that obey Ohm's law and have very low resistance. They can also carry electric currents from place to place without dissipating a lot of power. 18
  • 19. 1.3.2 Insulator • i.e: glass, most polymers (plastics), rubber and wood. • Materials which will refuse to carry an electric current. • Useful for jobs like coating electric wires to prevent them from 'shorting together' or giving a shock. • Silk and cotton are also good insulators (when they're dry!!) • Modern insulators like PVC (Polyvinylchloride) are much better and safer. • Insulators are also very useful to fill the 'gap' in between the metal plates of a capacitor. 19
  • 20. 1.3.3 Semiconductor • • • • • • Special class of elements having a conductivity between that of a good conductor (like cooper) and that of an insulator (like plastic). Most of the transistors, diodes, integrated circuits, etc. used in modern electronics are built using a range of semiconductors. The basic property of a semiconductor is given away by its name - it 'conducts a little bit'. A semiconductor will carry electric current, but not as easily as a normal conductor. The semiconductor atoms complete their valence shells by sharing valence electrons with other atoms – covalent bonding. For low temperature, semiconductor material will act as an insulator. 20
  • 21. Cont… • In room temperature, the stability of atom is threatened. Some of the electrons free from its bonding and jump to forbidden gap. • When the temperature increases, more valence electrons (free electron) jump to conduction band and increase the conductivity. • When the covalent bonding break, the hole is created by free electrons in valence bands. • The thermal energy (heat) causes the constant creation of electron – hole pairs. • Recombination occurs when the free electrons loss their energy and fall down to valence band (fill the hole). 21
  • 22. 1.4 Types of Semiconductor • Semiconductors are mainly classified into two categories: i. Intrinsic ii. Extrinsic Intrinsic - chemically very pure and possesses poor conductivity. - It has equal numbers of negative carriers (electrons) and positive carriers (holes). - Impurities do not affect its electrical behavior. 22
  • 23. Cont… Extrinsic - improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping process, which alters the electrical properties of the semiconductor and improves its conductivity. - Introducing impurities into the semiconductor materials (doping process) can control their conductivity. 23
  • 24. 1.4.1 Intrinsic Semiconductor o The pure semiconductor material without impurities atoms. o Example: Silicon and Germanium. The Silicon bonding 24
  • 25. 1.4.2 Extrinsic Semiconductor • Adding impurities atom into intrinsic semiconductor = extrinsic semiconductor. • The process of adding specific types of atoms to a semiconductor to favorably alter electric characteristics - Doping • 2 types of extrinsic (impure) semiconductor; N-type – P-type – 25
  • 26. Cont… • When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N-TYPE. • An impurity that reduces the number of free electrons, causing more holes, creates a POSITIVE or P-TYPE semiconductor. 26
  • 27. N– type material - Diffused impurities with 5 valence electrons are called donor atoms. Antimony (Sb) impurity in n-type material 27
  • 28. P-type material -The diffused impurities with 3 valence electrons are called acceptor atoms. Boron (B) impurity in p-type material 28
  • 29. 1.5 PN Junction Formation • A PN junction is fabricated from a single slice of semiconductor. • One side doped with acceptor impurity atoms – p region • One side doped with donor impurity atoms – n region • The interface separating the n and p regions is referred as the metallurgical junction. The PN junction 29
  • 30. Majority and minority carriers a) n-type material b) p-type material 30
  • 31. Diffusion Process • In trying to neutralize charges; - free electrons in n-type diffuse across junction to p-type - free holes in p-type diffuse to n-type - electrons & holes close to junction recombine. The movement of holes and electrons in diffusion process. 31
  • 32. P E-field force on holes N Depletion region E-field force on electrons E-field A depletion region formation due to electrons and holes movement in diffusion process and electric field. 32
  • 33. Forward biased narrows the depletion region and produces a voltage drop across the PN junction equal to the barrier potential. 33
  • 34. Reverse biased condition in PN junction. 34
  • 35. The IV characteristics in forward biased and reverse biased. 35