applied electronics 1
- 1. Chapter one Basic semiconductor theory Introduction
- 2. Depending on their electrical properties, materials can be classified into three groups. These are: Conductors- Allows passage of electricity (easily conduct electrical current) Ex-silver, gold, copper, Al, …..,etc. are best conductors Insulators-resist passage of electricity (not conduct electrical current at normal conditions) Ex-oil, glass, plastic(rubber) Semiconductors- partially allows passage of electricity Ex-Germanium, Silicon & Gallium Arsenide
- 3. Semiconductor materials : Semiconductors are a special class of elements having a conductivity between that of a good conductor and an insulators. In general, semiconductor materials fall into one of two classes. These are: - single crystal – have repetitive crystal structure Fig.1 covalent bonding of silicon atom
- 4. compound crystals- constructed of two or more semiconductor materials of different atomic structure. Fig.2 covalent bonding of GaAs crystal
- 5. For the discovery of the diode and transistors germanium was used exclusively. Because of 1. Relatively easy to find 2. Available in large quantity 3. Easy to refine (to obtain in very high level of purity) Disadvantage of using Germanium 1. Heavy leakage current (sensitive to change in temperature) 2. Fabrication of germanium wafer is complex.
- 6. Silicon – is a semiconductor material of choice. Because of: 1. It improved temperature sensitivity(decreased leakage current) 2. Most abundant material on earth 3. Easy to design 4. Easily fabricated Disadvantage of using silicon Its refining process for manufacturing silicon of very high level of purity is difficult. Gallium arsenide For the issue of speed for computers and communication systems gallium arsenide is preferable one. Speed of GaAs=5*silicon Draw back of using gallium arsenide 1. Difficult to manufacture at high level of purity 2. More expensive
- 7. Atomic structure Atom • is the smallest particle of an element that retains the characteristics of that element. • Each of the known 109 elements has atoms that are different from the atoms of all other elements. • This gives each element a unique atomic structure. According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons. • The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. • The basic particles of negative charge are called electrons.
- 8. • Therefore, Fundamental components of an atom are electron, proton and neutron. • Protons and neutrons form the nucleus and electrons appear in fixed orbit around the nucleus. • atom is electrically neutral. • Number of electrons and protons are equal. Continued
- 9. ion - Is an atom that gain or loose an electron. (+ve ion-when atom loose electron & -ve ion when it gain electron) -If a valence electron acquires a sufficient amount of energy, it can actually escape from the outer shell and the atom's influence. -The departure of a valence electron leaves a previously neutral atom with an excess of positive charge (more protons than electrons). -The process of losing a valence electron is known as ionization. and the resulting positively charged atom is called a positive ion. Ionization energy- energy required by an electron to come out of the atom and the material completely.
- 10. Fig.3 Atomic structure of (a) silicon; (b) germanium; and (c) gallium and arsenic.
- 11. Covalent bonding and intrinsic materials covalent bonding is a bonding of atoms strengthened by sharing of electrons. when external energy (light energy or heat energy) is applied, the bond breaks and electrons are released from the bond. The hole or free electron is said to be created. Free electron is an electron do not bonded but within the medium. Hole is absence of an electron (vacant site).
- 12. An increase in temperature of semiconductor can result in a substantial increase in the number of free electrons in the material. Semiconductor materials have a negative temperature coefficient. ↑temp =↑in number of free carrier. (but, reverse for conductors) = ↓in resistance of semiconductor.
- 13. Energy band the electrons of an atom can exist only within prescribed energy bands. Each shell around the nucleus corresponds to a certain energy band and is separated from adjacent shells by energy gaps, in which no electrons can exist. Figure below shows the energy band diagram for an unexcited (no external energy such as heat) atom in a pure silicon crystal. This condition occurs only at a temperature of absolute 0 Kelvin.
- 14. Fig4. Energy band diagram for an unexcited atom in a pure (intrinsic) silicon crystal. There are no electrons in the conduction band.
- 15. Fig5.Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free electrons.
- 16. o Valence electrons The outer most electrons of an atom and have least binding energy. o Valence band The band of energy occupied by the valence electrons. It may be completely filled or partially filled with electrons but never empty. o Conduction band The higher permitted energy band. May either be empty or partially filled with electrons. o Conduction electrons Are electrons can move freely in the conduction band. o Forbidden energy gap(Eg) The gap between valence band and conduction band.
- 17. If valence electron happen to absorb enough energy, it jumps across the forbidden energy gap and enters the conduction band. When an electron is ejected from the valence band, hole is left behind. Conduction band has nothing to do with hole flow. Hole flow experiences more opposition than electron flow in the conduction band.
- 18. Energy bands of materials Fig6. Energy diagrams for the three types of materials.
- 19. Insulators Their valence electrons bound very tightly to their parent atoms. No free charge carriers available within them under normal condition. Full valence bands. Empty conduction bands. Large energy gap (Eg). At ordinary temperature, the probability of electrons to cross the band gap is slight. Conductors There is no physical distinction between the two bands. (VB & CB overlaps). There is no structure to establish holes. The total current in such conductors is simply a flow of electrons.
- 20. Semiconductors At normal condition, empty conduction band and almost filled valence band with a very narrow energy gap(Eg). With increase in temperature, width of energy gap decreases and some of the electrons are liberated into the conduction band.
- 21. semiconductors Intrinsic sc Extrinsic sc N-type sc P-type sc
- 22. Intrinsic semiconductor An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. Ex- pure Germanium and Silicon Fig. pure silicon crystal.
- 23. Process of adding impurities to a pure semiconductor is called doping. A semiconductor material that has been subjected to the doping process is called an extrinsic material. Conducting property of intrinsic semiconductors can be improved in the following ways. 1. By changing the temperature of the environment. 2. By illumination of medium-(light energy) 3. By applying magnetic field-(magnetic energy) 4. By addition of impurities.
- 24. N-type extrinsic semiconductor An N-type semiconductor is obtained when a pentavalent atom like Antimony, Arsenic and Phosphorus are added to pure Germanium or silicon crystals. Diffused impurities with five valence electrons are called donor atoms. Fig. Diffused donor atom(Sb) to pure silicon crystal.
- 25. Concentration of electrons in conduction band, exceeds the concentration of holes in the valence band. Fig. Concentration of donor ions in N-type semiconductor. In an n-type material the electron is called the majority carrier and the hole is minority carrier.
- 26. P-type material The p-type material is formed by doping a pure Germanium or silicon crystal with trivalent atoms like Boron, Gallium and Indium. Fig. Diffused acceptor atom(B) to pure silicon crystal.
- 27. The three valence electrons of Boron atom form covalent bonds with four surrounding silicon atom but , one bond is left incomplete and give rise to a hole. The diffused impurities with three valence electrons are called acceptor atoms. Concentration of holes in valence band, exceeds the concentration of electrons in the conduction band. In a p-type material the hole is majority carrier and electron is the minority carrier. Fig. Concentration of donor ions in N-type semiconductor.