EMT 111/4 ELECTRONIC DEVICES

1. EMT 111/4 ELECTRONIC DEVICES Prepared By: Miss Bibi Nadia Bt Taib H/Phone No: 013-9841724 Office No: 04-9798141

2. ASSESMENTS (1) Coursework: 50% (i) Theory Tests = 20% [Test 1 (10%) + Test 2 (10%)] (ii) Labs = 15% [Lab 1-3] (iv) Assignments/Quizzes = 15% (2) Final Exam: 50% Total: 100%

3. LIST OF EXPERIMENTS Lab 0: Introduction to Basic Laboratory Equipment Lab 1: Diode As Rectifier Lab 2: Current and Voltage Characteristic of BJT Lab 3: JFET Characteristics

4. Text Books • Thomas L. Floyd, “Electronic Devices”, 8th Edition, Pearson, 2008. • 2. Floyd,T.,”Electronic Devices”, 7th Edition, Prentice Hall, 2002. • 3. Boylestad, R.L, Nashelsky, L., ” Electronic Devices and Circuit Theory”, 8th Edition, Prentice Hall, 2002. • 4. Thomas L. Floyd , Electronic Devices (Conventional Current Version), 8/E, Prentice Hall, 2007. • 5. Ahmad Radzi Mat Isa, Yaacob Mat Daud, Roslinda Zainal, ” Elektronik Asas Peranti Semikonduktor", ISBN 983-52-0419-5, 2007.

5. CHAPTER 1: Semiconductor Material • Introduction • Atomic Structure • Insulator, Semiconductor, Conductor • Current in semiconductors • The p-n junction • The structure of the diode • Biasing a Diode • Diode I-V characteristic • Diode Models • Testing a Diode

6. Introduction • Radio • Television • Computer • Telephone Vacuum Tube 1890s Electronic Systems Vacuum Tubes Amplifier Rectifier • Large • Fragile • High power consumption Able to operate very well To increase the strength of ac signals To convert ac energy to dc energy

7. Historical • Diode , in 1939 was using Ge • Transistor, in 1947 was using Ge • In 1954 Si was used in Transistor because Si is less temperature sensitive and abundantly available. • High speed transistor was using GaAs in 1970 (which is 5 times faster compared to Si) • Si, Ge and GaAs are the semiconductor of choice

8. Periodic Table

9. The Atom • Atom is the smallest particle of an element that retains the characteristics of that element. • An atom consists of the protons and neutrons that make up the nucleus (core) at the center and electrons that orbit about the nucleus. • The nucleus carries almost the total mass of • the atom. • Neutrons are neutral and carry no charge. • Protonscarry positive charges. • The electrons carry negative charges. • The number of protons = the number of electrons in an atom, which makes it electrically neutral or balanced. Fig. 1.1: Bohr model of an atom Atomic Structure

10. L K M N Valence Shell Valence shell is the outermost shell in an atom that determines the conductivity of an atom. The electrons in valence shell are called valence electrons. - - - - - - - - - - - - 29 p - - - - - + Shells or orbital paths - - 29 n 1st shell (K): 2n2 = 2(1)2 = 2 electrons 2nd shell (L): 2n2 = 2(2)2 = 8 electrons 3rd shell (M): 2n2 = 2(3)2 = 18 electrons 4th shell (N): 1 electrons Total: 29 electrons n = the shell number - - - - - - - - - - Valence shell Valence electron Fig.1.2: Bohr model of copper atom (Cu)

11. Tabel 1.1: Electron contents of shells and subshells of the copper atom

12. Si - Ge - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Insulator, Semionductor, Conductor A semiconductors is a material that is between conductors and insulators in its ability to conduct electrical current. Play a significant role in the development of modern electronic device such as diodes, transistors, and integrated circuits. Class of semiconductor : - Single-crystal : Ge, Si & C - Compound : GaAs, CdS & GaAsP Fig. 1.3: Semiconductor atoms

13. Insulators • A material that does not conduct electrical current under normal conditions. • Valence electrons are tightly bound to the atoms → very few free electrons. • Most good insulators are compounds rather than single-elemet materials. • Ex. : rubber, plastics, glass, mica, and quartz. • Conductors • A material that easily conducts electrical current. • Valence electrons are very loosely bound to the atoms → many free electrons. • Characterized by atoms with only one valence electron. • Thebestconductors are single-element materials. • Ex. : copper, silver, gold and aluminum.

14. Energy Bands Valence shell of an atom represent a band of energy levels that contains valence electrons. When an electron have enough additional energy, it can leave the valence shell to become a free electron and exist in conduction band. The energy gap is the difference between the valence band and conduction band. This is the amount of energy that a valence electron must have to jump form valence band to conduction band. In conduction band, electron is free to move throughout the material and is not tied to any atom.

15. Fig. 1.4: Energy band diagrams for three different materials The amount of energy that the valence electrons must attain to be elevated to the next level (conduction band) is measured in electron volts(1 eV = 1.6 x 10-19 joules), which is the energy gap between valence band and conduction band.

16. Comparison of a Semiconductor Atom to a Conductor Atom • Core of Si atom has a net charge of +4 (14 protons – 10 electrons) and core Cu has a net charge of +1 (29 protons – 28 electrons). • The core includes everything except the valence electron. • A valence electron in Si atom feels an attractive force of +4 compared to Cu • atom which feels an attractive force of +1. • Force holding valence electrons to the atom in Si > in Cu. • The distance from its nucleus of Copper’s valence electron (in 4th shell) > • silicon’s valence electron (in 3rd shell).

17. Valence electrons Valence electrons Core (+4) Core (+1) (a) Silicon atom (a) Copper atom Fig.1.5: Diagrams of the silicon and copper atoms

18. - - - - - + - + - - + - - - - - - + - - - - - + - Covalent Bonds • Covalent bonding is the method by which atoms complete their valence shells by “sharing” valence electrons. • The results of this bonding are: • The atoms are held together, • forming a solid substance. • 2. The atoms are all electrically stable, because their valence shells are complete. • 3. The completed valence shells cause the atom to act as an insulator. Fig. 1.6: Covalent bonding in a semiconductor crystal

19. (a) Covalent bonding of Si crystal (b) Covalent bonding of GaAs crystal Fig. 1.6: Covalent bonding in (a) Si and (b) GaAs crystal This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding

20. Energy Energy Conduction band Conduction band E4 = 1.8eV Energy gap E3 = 0.7eV Valence band E2 E1 Second band (shell 2) E = energy level Valence band First band (shell 1) Nucleus Current in Semiconductor Fig. 1.7: Energy band diagram for an unexited atom in a pure (intrinsic) Si crystal.

21. Conduction Electrons and Holes • At intrinsic (pure) silicon at room temperature • has sufficient heat (thermal) energy for some • valence electrons to jump from valence band • to conduction band. • When an electron jumps to the conduction band, a vacancy is left in the valence band. For every electron raised to the conduction band by external energy, there is one hole left in valence band, creating an electron-hole pair. • Recombination occurs when a conduction-band electron loss its energy and falls back into a hole in the valence band. • The energy given up by the electron is in the form of light and/or heat. (a) (b) Fig.1.8: Creation of electron-hole pairs in a Si atom. (a) energy diagram, and (b) bonding diagram

22. At the temperature room, at any instant, a number of free electrons that are unattached to any atom drift randomly throughout the material. This condition occurs when no voltage is applied across a piece of intrinsic Si Fig.1.9: Free electrons are being generated continuously while some recombine with holes

23. Electron and Hole Current When a voltage is applied across the piece of intrinsic Si (Fig. 1.10), the thermally generated free electrons in the conduction band, which are free to move, are now easily attracted toward the positive end. The movement of free electrons in a semiconductive material is called electron current. At the same time, there are also an equal number of holes in the valence band created by electrons that jump into the conduction band. The hole has moved from place to another in crystal structure (Fig. 1.11). The current in valence band is produced by valence electrons is called hole current.

24. Fig.1.10: Free electrons are attracted toward the positive end Fig. 1.11: Hole current in intrinsic Si

25. The P-N Junction Table 1.2: Few terms and processes that are frequently referred to in p-n junction theory

26. Table 1-2: Few terms and processes that are frequently referred to in p-n junction theory

27. Energy Conduction band - - - Si P Si Si Si - Excess covalent bond electron - - - - - - - - - - - - - - - - - - - - - - - - - - - - Electrons (majority carriers) - - - - - Valence band - - - - - Holes (minority carriers) - - N-Type Semiconductor An n-type semiconductor is produced when the intrinsic semiconductor is doped with n-type impurity atoms that have five valence electrons (pentavalent), such as arsenic (As), antimony (Sb), Bismuth (B) and phosphorus (P). Pentavalent atom is called a donor atom. Fig. 1.12: N-type semiconductor. Four of P atom’s valence electrons are used to form the covalent bond with Si atoms, leaving one extra electron Fig. 1.13: Energy diagram (n-type semiconductor)

28. Energy - Conduction band - - Si B Si Si Si - - - - - - - - - - - - - Covalent bond hole - - Electrons (minority carriers) Valence band - - - - - - - - - - - Holes (majority carriers) - P-type Semiconductor A p-type semiconducotor is produced when the intrinsic semiconductor is doped with p-type impurity atoms that have three valance electrons (trivalent), such as aluminum, boron, and gallium. Trivalent atom is referred to as an acceptor atom. Fig. 1.14: P-type semiconductor. Three of B atom’ valence electrons are used in the covalent bonds, leaving one hole Fig. 1.15: Energy diagram (p-type semiconductor)

29. Formation of The P-N Junction If a piece of intrinsic silicon is doped so that part is n-type and other part is p-type, a p-n junction forms at the boundary between the two region and a diode is created Fig. 1.16: At instant junction formation, free electrons in the n region near the p-n junction begin to diffuse acrosss the junction and fall into holes near the junction in p region

30. Formation of Depletion region • When the p-n junction is formed – n region loses electron (diffuse across junction) and creates a layer of positive charges near the junction. • As electron moves across the junction, p region loses hole as the electron and holes combine. This create a layer of negative charges near the junction. These two layers form the depletion region. • Depletion refer to region near the p-n junction is depleted of charge carrier (electrons and holes) due to diffusion across junction. Fig. 1.17: For Every free electron that diffuse across the junction and combines with a hole, a positive charge is created in p region, forming a barrier potential. This action continues until the voltage of the barrier prevent further diffusion.

31. - - - - - - +4 +4 +4 +4 +3 +5 +4 +4 +4 +4 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - N-type Junction P-type Total (+) = 19 Total (-) = 20 Net charge = -1 Total (+) = 21 Total (-) = 20 Net charge = +1 Fig.1.18: Depletion layer charges

32. Each electron that diffuses across the junction leaves one positively charged bondin the n-typematerial and produces one negatively charged bondin the p-typematerial. • Both conduction-band electrons and valence-band holes are need for conduction through the materials. When an electron diffuses across the junction, the n-type material has lost a conduction-band electron. When the electron falls into a hole in the p-type material, that material has lost a valence-band hole. At this point, both bonds have been depleted of charge carriers.

33. Energy diagram of PN junction and Depletion Region

34. Bias • Biasis a potential applied to p-n junction to obtain certain operating conditions. • This potential is used to control the width of the depletion layer. • By controlling the width of the depletion layer, we are able to control the resistance • of the p-n junction and thus the amount of current that can pass through the • device. Table 1.3: The relationship between the width of depletion layer and the junction current

35. Forward Bias • Forward bias is a potential used to reduce the resistance of p-n junction. • A forward-biased p-n junction has minimum depletion layer width and junction • resistance. • There are two requirements to produce forward bias: • - The positive side of voltage source (denoted as bias voltage) is connected • to the p-type materialof the p-n junction semiconductor and the negative • side is connected to the n-type material. • - Bias voltage must be greater than the barrier potential. Barrier potensial is an • energy hill that is created by the electric field between the positive and • negative ions in the depletion region on either side of the junction. • The resistor limits the forward current to a value that will not damage the • device.

36. Fig.1.19: A p-n junction connected for forward bias

37. What happens when a p-n junction is forward- biased? • The negative side of the bias-voltage source “pushes” the free electrons (the • majority carriers in n-type material) toward the p-n junction because like • charges repel. • The negative terminal also provides a continuous flow of electrons into the n region. • The free electrons obtain sufficient energy from the bias-voltage to overcome the • barrier potential of the depletion region and move on through into the p region. • Once in the p region, this free electron have lost too much energy overcoming the • barrier potensial and thus, the free electrons can’t remain in the conduction • band for longer. They immediately combine with holes in valence band. • Since unlike charges attract, the positive side of the bias-voltage source attracts the valence electrons toward the left end of the p region. • The holes in the p region provide the medium or pathway for these valence electrons to move through the p region. • The valence electrons move from one hole to the next toward the left. • The holes, which are the majority carriers in the p region, effectively (not actually) move to the right toward the junction.

38. As the electrons flow out of the p region through the external connection and to the positive side of the bias-voltage source, they leave holes behind in the p region; at the same time, these electrons become conduction electrons in the metal conductor. • There is a continuous availability of holes effectively moving toward the p-n junction to combine with the continuous stream of electrons as they come across the junction into the p region. Fig.1.20: A forward-biased p-n junction showing the flow of majority carriers and the voltage due to the barrier potential across the depletion region

39. The Effect of Forward Bias on the Depletion Region • As more electrons flow into the depletion region, the number of positive ions is reduced. • As more holes effectively flow into the depletion region on the other side of the p-n junction, the number of negative ions is reduced. • This reduction in positive and negative ions during forward bias causes the depletion region to narrow.

40. The Effect of Barrier Potential During Forward Bias • When forward bias is applied, the free electrons have enough energy to overcome the barrier potentialand effectively “climb the energy hill” and cross the depletion region. • When the free electrons cross the depletion region, they give up an amount of energy equivalent to the barrier potential. • The energy loss results in a volatge drop across the p-n junction equal to the barrier potential. • An additional small voltage drop occurs across the p and n regions due to the internal resistance of the material. This resistance is called the dynamic resistance. • For doped semiconductive material, the dynamic resistance is very small and can usually be negleted.

41. Reverse Bias • Reverse bias is a potential that essentially “prevents” current through the diode. • A reverse-biased p-n junction has maximum depletion layer width and junction • resistance. • There are two requirements to produce forward bias: • - The positive side of voltage source (denoted as bias voltage) is connected • to the n-type materialof the p-n junction semiconductor and the negative • side is connected to the p-type material. • - The depletion region is much wider than in forward bias.

42. Fig.1.21: A p-n junction connected for reverse bias.

43. What happens when a p-n junction is reverse- biased? • The positive side of the bias-voltage source “pulls” the free electrons (the • majority carriers in n-type material) away from the p-n junction because unlike • charges attract. • As the electrons flow toward the positive side of the voltage source, additional positive • ions are created. This results in a widening of the depletion region and • a depletion of majority carriers. • In the p region, electrons from the negative side of the voltage source enter as valence • electrons and move from hole to hole toward the depletion region where they • create additional negative ions. This results in a widening of the depletion • region and depletion of majority carriers. • The initial flow of charge carriers is transitional and lasts for only a very short time after • the reverse-bias voltage is applied. • As the depletion region widens, the availability of majority carriers decreases. • As more of the n and p regions become depleted of majority carriers, the electrical field • between the positive and negative ions increases in strength until the potential • across the depletion region equals the bias voltage. At this point, the transition • current essentially ceases except for a very small reverse current.

44. Fig.1.22: The p-n junction during the short transition time immediately after reverse-bias voltage is applied

45. Reverse Current • There is the extremely small current exists in reverse bias after the transition • current dies out. It is caused by the minority carriers in the n and p region that • are produced by thermally generated electron-hole pairs. • The small number of free minority electronsin the p region are “pushed” toward • the p-n junction by negative bias voltage. • When these electrons reach the wide depletion region, they “fall down the energy • hill” and combine with the minority holes in the n region as valence electrons • and flow toward the positive bias voltage, creating a small hole current. • The conduction band in the p region is at a higher energy level than the • conduction band in the n region. Therefore, the minority electrons easily pass • through the depletion region because they require no additional energy.

46. Fig.1.23: The extremely small reverse current in a reverse-biased diode is due to the minority carriers from thermally generated electron-hole pairs

47. Reverse Breakdown • Normally, the reverse current is so small that it can be neglected. However, if the • external reverse-bias voltage is increased to a value called the breakdown • voltage, the reverse current will drastically increase. • The small number of free minority electronsin the p region are “pushed” toward • the p-n junction by negative bias voltage. • When these electrons reach the wide depletion region, they “fall down the energy • hill” and combine with the minority holes in the n region as valence electrons • and flow toward the positive bias voltage, creating a small hole current. • The conduction band in the p region is at a higher energy level than the • conduction band in the n region. Therefore, the minority electrons easily pass • through the depletion region because they require no additional energy.

48. VD Cathode (K) Anode (A) - + ID Diodes • Introduction • A diode is a two-electrode (two-terminal) • device that acts as an one-way conductor. • The p region is called the anodeand the • n region is called the cathode. • The arrow in the symbol points in the • direction of conventional current (opposite • to electron flow). Fig. 1.24: The symbol for the p-n junction diode

49. VF - + IF + VBias R - (a) Forward-biased diode VD - + - I ≈ 0 VBias R + (b) Reverse-biased diode Fig.1.25: Two different bias circuits • The most basic type of diode is the p-n • junction diode. • A diode is forward-biasedwhen the positive • terminal of the source is connected to the • anode through a current-limiting resistor and • the negative terminal is connected to the • cathode. • A diode is reverse-biasedwhen the negative • terminal of the source is connected to the • anode and the positive terminal is connected to • the cathode. • When forward biased, a p-n junction diode • conducts. When reverse biased, it effectively • blocks the flow of charge (current).

50. VA VA VA IF = 0 IF > 0 VF VF < 0 VF = 0 IF VK VK VK OFF ON The Ideal Diode Model The ideal diode behaves like a closed switch(ON) when forward biased, and like an open switch (OFF) when it is reverse biased. • The behavior of the ideal diode can be summarized as following: • If the diode is ON, current passes • from the anode to cathode. Therefore, • we can replace it with a short circuit. • If the diode is OFF, cathode voltage is • greater than anode voltage. Then, we • can replace it with an open circuit. Diode ON : IF > 0; VF = 0 → VA = VK Diode OFF: IF = 0; VF < 0 → VA < VK Fig.1.26: The behavior of the diode: (a) ideal diode, (b) short circuit and (c) open circuit This model is adequate for most troubleshooting when you are trying to determine wheter the diode is working properly.