ELECTRONIC DEVICES MENJANA MINDA KREATIF DAN INOVATIF

Transcription

1 INTRODUCTION TO ELECTRONIC DEVICES MENJANA MINDA KREATIF DAN INOVATIF

2 Introduction What is Electronics? Electronic Devices? Electronic Systems? introduction

3 Electronics: The branch of physics that deals with the emission and effects of electrons ; and the use of electronic devices. Science of the motion of charges in a gas, vacuum or semiconductor.

4 Electronic devices: An electronic building block packaged in a discrete form with two or more connecting leads or metallic pads. Components are connected together to create an electronic circuit with a particular function. E.g.: an amplifier, radio receiver, or oscillator. Active components are sometimes called devices.

5 Electronic systems: Composed of subsystems or electronic circuits, which may include amplifiers signal sources, power supplies etc E.g.: Laptop, DVD players, ipod, PDA, mobile phones

6 What we are learning? Electronic Devices: Diodes Transistor (BJT) Transistor (FET)

7 CHAPTER 1 Semiconductor Material

8 Semiconductor Material 1.1 Atomic Structure Materials Classification 1.3 Energy Band 1.4 Covalent Bonds Conduction in Semiconductors 1.6 p type and n type semiconductors

9 11Atomic 1.1 structure All matters on earth made of atoms (made up of elements or combination of elements). All atoms consist of electrons, protons, and neutrons. An atom is the smallest particle of an element that retains the characteristics of that element.

10 Bohr s Atomic Structuret According to Bohr, atoms have a planetary structure that consists of a central nucleus, surrounded d by orbiting electrons. Nucleus contains protons and neutrons. Electrons (-) orbits the nucleus Nucleus : Protons (+) Neutrons (neutral)

11 Cont.. Each type of atom has a certain number of electrons and protons that distinguishes it from atoms of other elements. H: 1 proton, 1 He: 2 proton, 2 electron neutron, 2 electron

12 Helium Atom Each electron has its own orbit which corresponds to different energy levels. Similar energy levels (orbits) are grouped into energy bands called shells.

13 Electron Shells and Orbits Each electron travels on its own orbit. The different orbit corresponds to different energy level. In an atom, orbits are grouped into energy bands known as shells. Each shell has a fixed max no. of electrons at allowed energy levelsl

14 The Energy Band Concept The number of electrons in shell 1-4 can be calculated as: N e = 2n 2 Electron orbits the nucleus at certain distances The outermost t shell is called the valence shell. Electrons on this shell are called valence electrons

15 Valence Electrons The outermost shell is called the valence shell and electrons at this layer are called valence electrons. Valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties. Maximum number of valence electron is 8. An atom is stable if it has 8 valence electrons; eg: Neon (10), Argon (18), Krypton (36).

16 Cont.. The number of valence electrons determines the ability of material to conduct current. The less complete a shell is filled to capacity (max 8) the more conductive the material is. If an atom of a material carries 1 to 3 valence electrons, the material is a conductor because the atom has more tendency to lose its valence electrons which become free electrons. (Copper, Aluminium)

17 Cont If an atom of a material carries 5 to 8 valence electron, the material is an insulator because the atom has more tendency to gain free electrons to complete its shell. (Argon, Neon) If an atom of a material carries 4 valence electrons, the material is a semiconductor because it is not easy for the atom to lose or gain any electrons. (Silicon, Germanium)

18 Cont A Silicon atom has 4 electrons in its valence ring. This makes it a semiconductor. A Copper atom has only 1electron in its valence ring. This makes it a good conductor.

19 Semiconductor Material 1.1 Atomic Structure Materials Classification Insulators, Conductors, and Semiconductor 1.3 Energy Band Covalent Bonds 1.5 Conduction in Semiconductors 1.6 p type and n type semiconductors

20 Materials Classification Semiconductors, conductors and insulators How are they different? What is the difference between silicon and germanium semiconductor? Why is Si more widely used compared to Ge?

21 Cont.. Conduction band and valence electrons determines the electrical properties of a material.

22 Conductor A conductor is a material that easily conducts electrical current. The best conductors are (with one valence electron) eg: copper, silver, gold and aluminium. In a conductor, the valence band and the conductor band overlaps ( 0.01 ev). Only a little energy or voltage is needed for the electron to jump into conduction band.

23 Insulator An insulator is a material that does not conduct electrical current under normal circumstances. Energy gap in an insulator is very wide ( 6eV). Valence electron requires a large electric field to gain enough energy to jump into conduction band.

24 Semiconductor A semiconductor is between a conductor and insulator. A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. Most common semiconductor are silicon, germanium and carbon. A semiconductor has a conductivity level between the extreme of an insulator and a conductor.

25 Cont.. Compare silicon and germanium. Which one is better and why?

26 Semiconductor Material 1.1 Atomic Structure 1.2 Materials Classification 1.3 Energy Band - Energy Level - Valence Band, Conduction Band, Energy Gap 1.4 Covalent Bonds 1.5 Conduction in Semiconductors 1. 6 p type and n type semiconductors

27 13Energy 1.3 Band ENERGY in an electron is of two types - kinetic (energy of motion) and potential (energy of position). Each material has its own set of permissible energy levels for the electrons in its atomic structure. Energy level in an atom is measured in ELECTRON VOLT (ev): 1 ev = x J Electrons that orbits within an energy level will have similar amount of energy.

28 Energy Level More energy Less energy Energy increases as the distance from the nucleus increases.

29 Energy Gap When an electron acquires sufficient additional energy, it can leave the valence shell and become a free electron and exists in the conduction band. The energy difference between the valence band and conduction band is called the energy gap. Energy gap: the amount of energy that a valence electron must have to jump into the conduction band. Once in conduction band, the electron is free to move throughout the material and is not tied to any atom.

30 Cont.. Energy diagrams for the three types of materials:

31 Semiconductor Material 1.1 Atomic Structure Materials Classification 1.3 Energy Band 1.4 Covalent Bonds 1.5 Conduction in Semiconductors 1.6 p type and n type semiconductors

32 Covalent Bonds In a pure silicon or germanium crystal, the four valence electrons of one atom form a bonding arrangement with four adjoining atoms. This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding is a method by which atoms complete their valence shell by sharing valence electrons with other atoms

33 Cont.. The atoms are electrically stable because their valence shells are complete.

34 Cont.. Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way in their intrinsic i i or pure state. t Covalent Bonds in a silicon crystal

35 Review 1. In the atomic structure of a semiconductor, within which energy band do free electrons exist? Within which energy band do valence electrons exist? 2. Why is current established more easily in a semiconductor than in an insulator?

36 Previously.. Diodes and transistors are all made of semiconductor material. Bohr model: electrons circle the nucleus. (planetary-type) Atomic structure of a material determines its ability to conduct or insulate. The orbit paths (energy band) of the electrons surrounding the nucleus are called shells. Each shell has a defined number of electrons it will hold. The outer shell is called the valence shell. The valence shell determines the ability of material to conduct current. Energy gap: difference in energy levels between the conduction and valence bands. (forbidden band) Covalent bonding is a bonding of two or more atoms by the interaction of their valence electrons to form a crystal structure.

37 Semiconductor Material 1.1 Atomic Structure Materials Classification 1.3 Energy Band 1.4 Covalent Bonds Conduction in Semiconductors 1.6 p type and n type semiconductors

38 15C 1.5 Conduction in Semiconductors How is current produced in a semiconductor?

39 Conduction in Semiconductors Unexcited (no external energy) silicon atom. (0 K)

40 Electron-Hole Pair Excited silicon atom. (Room temperature) t

41 Conduction Electrons and Holes When an intrinsic silicon crystal gains sufficient heat (thermal energy), some valence electrons could break their covalent bonds to jump the gap into conduction band, becoming free electrons. Free electrons are also called conduction electrons, (negative charge). This vacancy in the valence band is called a hole (positive charge). For every electron raised to the conduction band, there is 1 hole in the valence band creating electron-hole pair. When a conduction electron loses energy and falls back into a When a conduction electron loses energy and falls back into a hole, this is called recombination.

42 Cont.. There is an equal number of holes in the valence band created when these electrons jump into the conduction band.

43 Cont.. Free electrons tend to search for positive holes, while holes tends to attract electrons to reform covalent bonds. (Opposites attract) With the existence of holes and electrons, current can be produced when a voltage is applied across the terminals.

44 Current In Semiconductor There are 2 types of current flow: Electron current Hole current

45 Electron Current When voltage is applied across a piece of intrinsic silicon, free electrons are easily attracted towards the positive end. This movement caused the electron current.

46 Hole Current Holes created when valence electrons escaped. Valence electrons remaining at valence band are still attached to their atoms. They could only move into a nearby hole with little change in its energy level, thus leaving another hole behind. The hole appears to move from one place to the next in the crystal structure. t This movement of hole is called hole current.

47 Cont... When a valence electron moves left to right to fill a hole while leaving another hole behind, the hole has effectively moved from right to left. Gray arrows indicate effective movement of a hole. FIGURE 1 13 Hole current in intrinsic silicon.

48 Cont... Electron current - + Hole current The direction of hole current is normally used as the The direction of hole current is normally used as the conventional current direction.

49 Electron and Hole Current Electron Current Movement of free electrons in the conduction band. Electron posses the conduction band. Hole Current Movement of valence band electron in the valence band to fill up the hole left when an electron moves into the conduction band. Electron posses the valence band energy.

50 Review 1. At room temperature, an intrinsic semiconductor has some holes in it due to. 2. The merging g of a free electron and a hole is called. 3. electrons are responsible for current in a material. 4. Hole current occurs at the level.

51 Semiconductor Material 1.1 Atomic Structure Materials Classification 1.3 Energy Band 1.4 Covalent Bonds 1.5 Conduction in Semiconductors 1.6 p type and n type semiconductors -Doping -majority and minority carriers

52 Doping A process of adding impurities to intrinsic (pure) Silicon and Germanium to improve the conductivity of the semiconductor material. 2 types of semiconductor material that are subjected to doping process which are: N-type P-type

53 Cont.. 2 types of semiconductor material that are subjected to doping process which are: N-type P-type tpe 2 types of elements used doping are: Trivalent element with 3 valence electrons Pentavalent element with 5 valence electrons

54 N-type Semiconductor Pentavalent impurity atoms are added Arsenic (As), phosphorus (P),bismuth (Bi), Pentavalent also known as a donor atoms since they donate electrons. When a pentavalent atom is added to an intrinsic semiconductor, it ll readily donate it s 5 th electron, as a result becomes n-type extrinsic semiconductor.

55 N-type Semiconductor Each pentavalent atom forms covalent bond with 4 adjacent Si atom Since only 4 electrons are needed to form a covalent bond, leaving an extra electron becomes a free electron when each pentavalent atom is added d In n-type material a electrons eecto sae are majority carrier, and holes the minority carrier

56 P-type Semiconductor Trivalent ( with 3 valence electrons) impurity atoms are added Aluminium (Al), boron (B), indium (In),gallium (Ga) Trivalent also known as a acceptor atom since they accpet electrons. When a trivalent atom is added to an intrinsic semiconductor, it ll readily accept free electron, as a result becomes p-type extrinsic semiconductor.

57 P-type Semiconductor Each trivalent atom forms covalent bond with 4 adjacent Si atom. Since 4 electrons are needed to form a covalent bond, causes an existence of hole in the covalent bonding. It also causes a lack of valence electrons in the B atoms In p-type material holes are majority carrier, and electron the minority carrier

58 Review.. 1. What s the difference between a pentavalent atom and a trivalent t atom? 2. By what process are the majority carriers produced? 3. By what process are the minority carriers produced? 4. What s the difference between intrinsic and extrinsic semiconductors?