Semiconductor Material & Devices

1. Semiconductor Material & Devices An Overview of the Subject Usman Ali Khan

2. Contents • Subject Information • The study of Electronics • History • Semiconductor Materials • Atomic Structure

3. Subject Information • Code: EE120 • Text Book: Electronic Devices & Circuits by Theodore F. Bogart 6th ed. • Electronic Devices & Circuits by David A Bell 4th ed. • Electronic Devices & Circuits by Floyd • Electronic Devices & Circuits by Manzar Saeed • Basics of Electronic Device by NIIT

4. Marks distribution • Total Marks: 150 • Theory: 100 • Practical: 50 Session Marks: 20 • Assignments: 05 • Quiz: 05 • Project + Presentations: 05 • Attendance: 05

5. IntroductionSemiconductor Devices • Building blocks of useful electronic devices • Semiconductor devices include: • Diodes • PN junction • Light Emitting Diode (LED) • Zener Diode • Tunnel Diode • Varactor Diode • Laser Diode • Photo Diode

6. Transistors • Bipolar Junction Transistor (BJT) • NPN BJT • PNP BJT • Junction Field Effect Transistor (JFET) • Amplifier Fundamentals • Small Signal Transistor Amplifier • Integrated Circuits (ICs) • Analog ICs • Digital ICs

7. Basic Atomic Theory Every chemical element is composed of atoms All atoms within a single element have same structure Every element is unique because the structure of its atoms is unique Atom is composed of three basic particles: • Protons (+ive charge) • Neutrons • Electrons (-ive charge) Nucleus

8. P=14 N=14 + Silicon Atom Orbits or Shells K, L, M,N Draw the atomic structure of Ge (32) Valence Shell Ne( Electrons in nth orbit) = 2n2

9. Sub-shells

10. Free Electrons When electrons get enough energy (e.g. from heating), they leave their parent atoms and become free electrons. Flow of free electrons is called current. Therefore more free electrons and more current. + Valence electrons have more tendency to become free electrons because of less attraction force between nucleus and valence shell Free electrons in (i) conductors (ii) Insulators & (iii) Semiconductors

11. Flow of Free Electrons (Current) Material containing free electrons Force of attraction Force of repulsion - - + - - Excess of electrons Lack of electrons

12. * * * * * * * * * * * * * * + + + * * * * * * * * * * * * * * + + + * * * * * * * * + + + + * * * * * * * * * * * * * * * * * * * * * * + + + + + * * * * + + Silicon Crystal (Covalent Bonding) For stability there should be 8 electrons in valence shell Si Crystal

13. Current in Semiconductors Hole Current Usman Ali Khan

14. Contents • Basics • Electron Energy • Energy Bands • Temperature & Resistance • Holes & Hole Current

15. Basics: • Rupturing of covalent bond • The unit of energy is electronvolt(eV) • Energy acquired by one electron if it is accelerated through potential difference of one volt • 1 eV = 1.602 x 10-19 J • Valence Electron energy considerably large and need a few amount of energy to release • Electrons in inner shell possess little energy and need a large amount of energy to release • Electrons can lose energy in the form of heat and light • Free electrons can alco lose and fall into valence shell

16. Important Quantities

17. Important Relations • V = IR (Ohm's Law) • I = Q/t • W = QV • R = ρl/A • G = 1/R • σ = 1/ ρ • Charge on electron = e = 1.602 X 10-19 C • Electron energy = 1 eV = 1.602 X 10-19 J

18. + + + + Rupturing of Covalent Bonds Electron Freed (Conduction Band) Hole created Covalent bond ruptured + Energy is supplied in the form of heat to rupture covalent band Valence band

19. E2 P=14 N=14 E1 E3 Electron Energy Electrons closer to nucleus are more tightly bound and need more energy to become free Therefore: E1 > E2 > E3 If free electron loses energy and falls back to valence band, this process is called “Annihilation” or “Recombination” Lost energy emits as light

20. Energy Bands:Quantum theory explain these bands as • Conduction Band : • Free electrons accommodate there • Valence Band : • Electrons having lesser energy accommodate there • Forbidden band: • The region between valence and conduction band • No electrons can stay at this energy levet

21. Conduction Band (Free Electrons) Forbidden Band Valence Band (Electrons in Valence Shell) Energy Bands eV Energy gap is the energy required to rupture covalent bond Energy Gap

22. Conduction Band Forbidden Band Valence Band Energy Bands for Different Materials Conduction Band ≤0.01eV Forbidden Band Valence Band Conductors Insulators Conduction Band Conduction Band 0.67eV Forbidden Band Forbidden Band 1.1eV Valence Band Valence Band Germanium Silicon Temperature dependent

23. Temperature & Resistance Temp. Coeff. = α α = -ive α = +ive I I R R T T Conductors Semiconductors

24. + + + + + + + + + + + + Holes & Hole Current Hole Movement + Electron Movement

25. Hole Current Vs Electron Current The movement of holes and electrons is in opposite directions There are no holes in pure conductors, they are only created in semiconductors There are two currents in semiconductors: • Hole current (Band ? Charge ?) • Free electron current (Band ? Charge ?) The total current in semiconductor materials is the sum of hole current and electron current Number of holes = ?

26. - + - - + + + + - - + - Charge Carriers • Holes are called positive charge carriers • Free electrons are called negative charge carriers • For pure (Intrinsic) semiconductors: Number of positive charge carriers = Number of negative charge carriers • Is there any way to make charge carriers unequal? • Let hole density be pi (holes/m3) and electron density be ni (electrons/m3) where i denotes intrinsic semiconductor, then: ni = pi Intrinsic Semiconductor

27. Charge Carriers at Room Temperature

28. Thank You