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EMT111 CHAPTER 1 Introduction to Semiconductor By En. Rosemizi B

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1 EMT111 CHAPTER 1 Introduction to Semiconductor By En. Rosemizi B
EMT111 CHAPTER 1 Introduction to Semiconductor By En. Rosemizi B. Abd Rahim

2 Introduction to Semiconductor - Chapter Outline :
1.1 Atomic Structures 1.2 Semiconductors, Conductors, and Insulators 1.3 Covalent Bonds 1.4 Conduction in Semiconductor 1.5 N-Type and P-Type Semiconductor 1.6 The Diode 1.7 Biasing the Diode 1.8 Voltage Current Characteristic of a Diode 1.9 Diode Models 1.10 Testing a Diode

3 Introduction to Semiconductor - Chapter Objectives :
Discuss basic operation of a diode Discuss the basic structure of atoms Discuss properties of insulators, conductors and semiconductors Discuss covalent bonding Describe the properties of both p and n type materials Discuss both forward and reverse biasing of a p-n junction

4 1.1 Atomic Structures History of Semiconductor

5 1.1 Atomic Structures Atomic number basic structure Electron shells
Valence electron Figure 1-1 Bohr Model Free electron ionization

6 1.1 Atomic Structures An atom is a smallest particle of an element.
contain 3 basic particles: Neutrons (uncharged) Protons (positive charge) Electrons (negative charge) Nucleus (core of atom) Figure 1-1 Bohr Model ATOM

7 1.1 Atomic Structures Bohr model of an atom
This model was proposed by Niels Bohr in 1915. electrons circle the nucleus. nucleus made of: i) +protons ii) Neutral:neutron Figure 1-1 Bohr Model

8 1.1 Atomic Structures Atomic Number Hydrogen = group 1
Element in periodic table are arranged according to atomic number Atomic number = number of protons in nucleus At balanced (neutral) atom, number of electron= number of protons Figure 1-1 Bohr Model Hydrogen = group 1 Helium = group 2

9 1.1 Atomic Structures Electron Shells and Orbits
The orbits are group into energy bands - shells Valence - the outermost shell , electrons in this shell – valence electrons Valence electrons contribute to chemical reactions and bonding within the structure of material and determine its properties. Diff. in energy level within a shell << diff. an energy between shells Energy increases as the distance from the nucleus increases. Figure 1-1 Bohr Model

10 1.1 Atomic Structures Valence Electrons
Electrons with the highest energy levels exist in the outermost shell. Electron in the valence shell called valence electrons. The term valence is used to indicate the potential required to removed any one of these electrons. Figure 1-1 Bohr Model

11 1.1 Atomic Structures Ionization
A process of an atom either losing or gaining an electron to become positive ion or negative ions. For example, Positive ion - in a neutral hydrogen atom, the valence electron acquires a sufficient amount of energy to jump out from the outmost shell, this will leave the atom with less number of electron and more a proton. (H become H+) Negative ion – a free electron fall into the outer shell of a neutral hydrogen atom. (H become H-) Figure 1-1 Bohr Model

12 1.1 Atomic Structures Number of electrons in each shell
Number of electrons (Ne) that can exist in each shell of an atom can be calculated by the formula: Ne=2n2 n is the number of shell Figure 1-1 Bohr Model

13 1.2 Semiconductors, Conductors and Insulators

14 1.2 Semiconductors, Conductors and Insulators
material that easily conducts electrical current. The best conductors are single-element material (copper, silver, gold, aluminum) One valence electron very loosely bound to the atom- free electron Insulators material does not conduct electric current valence electron are tightly bound to the atom – less free electron Semiconductors material between conductors and insulators in its ability to conduct electric current in its pure (intrinsic) state is neither a good conductor nor a good insulator most commonly use semiconductor ; silicon(Si), germanium(Ge), and carbon(C). contains four valence electrons

15 1.2 Semiconductors, Conductors and Insulators
Atom can be represented by the valence shell and a core A core consists of all the inner shell and the nucleus Carbon atom: -valence shell – 4 e -inner shell – 2 e Nucleus: -6 protons -6 neutrons Net charge = +4 +6 for the nucleus and -2 for the two inner-shell electrons

16 1.2 Semiconductors, Conductors and Insulators
Energy Level Each discrete shell (orbit) corresponds to a certain energy level. Electrons are bounded to their respective shells because of the attraction force between proton (+) and electron (-). The electrons in orbits further from nucleus are less tightly bound (loose) compare to the atom closer to the nucleus due to the attractive force. The distance between each shell to the nucleus depends on the energy of the respective electrons. Therefore, electrons in the highest energy level exist in the outermost shell of an atom. Energy level increases as distance from nucleus.

17 1.2 Semiconductors, Conductors and Insulators
Energy Bands

18 1.2 Semiconductors, Conductors and Insulators
Energy Bands Energy gap-the difference between the energy levels of any two orbital shells Band-another name for an orbital shell (valence shell=valence band) Conduction band –the band outside the valence shell

19 1.2 Semiconductors, Conductors and Insulators
Energy Bands at room temperature 25° eV (electron volt) – the energy absorbed by an electron when it is subjected to a 1V difference of potential

20 1.2 Semiconductors, Conductors and Insulators
Comparison of a Semiconductor Atom & Conductor Atom A Copper atom: only 1 valence electron a good conductor Electron conf.:2:8:18:1 A Silicon atom: 4 valence electrons a semiconductor Electron conf.: 2:8:4 Fig. 1-6 Copper and Silicon atoms 14 protons 14 nucleus 10 electrons in inner shell 29 protons 29 nucleus 28 electrons in inner shell

21 1.3 Covalent Bonding 1-3 Covalent Bonding
Covalent bonding – holding atoms together by sharing valence electrons sharing of valence electron produce the covalent bond To form Si crystal Fig. 1-8 Covalent bonding

22 1.3 Covalent Bonding The result of the bonding:
The atom are held together forming a solid substrate The atoms are all electrically stable, because their valence shells are complete The complete valence shells cause the silicon to act as an insulator-intrinsic (pure) silicon is a very poor conductor Fig. 1-8 Covalent bonding

23 1.3 Covalent Bonding Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way in their intrinsic or pure state. Fig. 1-9 Intrinsic Silicon Covalent bonds in a 3-D silicon crystal

24 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Fig. 1-9 Intrinsic Silicon FIGURE Energy band diagram for a pure (intrinsic) silicon crystal with unexcited atoms. There are no electrons in the conduction band.

25 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Absorbs enough energy (thermal energy) to jumps Fig. 1-9 Intrinsic Silicon a free electron and its matching valence band hole FIGURE Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free.

26 1.4 Conduction in Semiconductor (Conduction Electron and holes)
Fig. 1-9 Intrinsic Silicon FIGURE Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.

27 1.4 Conduction in Semiconductor (Electron and holes currents)
Electron current free electrons Fig. 1-9 Intrinsic Silicon Apply voltage FIGURE Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

28 1.4 Conduction in Semiconductor (Electron and holes currents)
movement of holes Fig. 1-9 Intrinsic Silicon FIGURE Hole current in intrinsic silicon.

29 1.5 N-types and P-types Semiconductors (Doping)
Doping -the process of creating N and P type materials -by adding impurity atoms to intrinsic Si or Ge to imporove the conductivity of the semiconductor -Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-) p-type material – a semiconductor that has added trivalent impurities n-type material – a semiconductor that has added pentavalent impurities Trivalent Impurities: Aluminum (Al) Gallium (Ga) Boron (B) Indium (In) Pentavalent Impurites: Phosphorus (P) Arsenic (As) Antimony (Sb) Bismuth (Bi) Fig & 16 Pentavalent and Trivalent

30 1.5 N-types and P-types Semiconductors
n -type semiconductor: Pentavalent impurities are added to Si or Ge, the result is an increase the free electrons. Example Pentavalent: Antimony(Sb), Phosphorus(Ph), Arsenic(As), Bismuth(Bi) Extra electrons becomes a conduction electrons because it is not attached to any atom - No. of conduction electrons can be controlled by the no. of impurity atoms Pentavalent atom gives up (donate) an electron - call a donor atom Current carries in n-type are electrons – majority carries Holes – minority carries Sb impurity atom Fig & 16 Pentavalent and Trivalent Pentavalent impurity atom in a Si crystal

31 1.5 N-types and P-types Semiconductors
P-type semiconductor: Trivalent impurities are added to Si or Ge to create a deficiency of electrons or hole charges Example Trivalent : Boron(B), Aluminium(Al), Gallium(Ga), Indium(In) The holes created by doping process The no. of holes can be controlled by the no. of trivalent impurity atoms - The trivalent atom is take (accept) an electron- acceptor atom - Current carries in p-type are holes – majority carries - electrons – minority carries B impurity atom Fig & 16 Pentavalent and Trivalent Trivalent impurity atom in a Si crystal

32 1.6 The Diode n-type material & p-type material become a diode (pn junction) when joined together p region- majority carriers - holes minority carriers - electron n region - majority carriers – electron minority carriers - holes - before the pn junction is formed -no net charge (neutral) Fig 1-18 a & b depletion region

33 1.6 The Diode (The Depletion Region)
Depletion region – the area around a pn junction that is depleted of free carriers due to diffusion across the junction Also known as depletion layer. When an n-type material is joined with a p-type material: A small amount of diffusion occurs across the junction. When e- diffuse into p-region, they give up their energy and fall into the holes in the valance band covalent bonds. Since the n-region have lost an electron, they have an overall +ve charge. Since the p-region have gained an electron, they have an overall –ve charge. The difference in charges on the two sides of the junction is called the barrier potential. (typically in the mV range) Fig 1-18 a & b depletion region

34 1.6 The Diode (The Depletion Region)
Barrier Potential: The buildup of –ve charge on the p-region of the junction and of +ve charge on the n-region of the junction-therefore difference of potential between the two sides of the junction is exist. The forces between the opposite charges form a “field of forces "called an electric field. This electric field is a barrier to the free electrons in the n-region need energy to move an e- through the electric field. The potential difference of electric field across the depletion region is the amount of voltage required to move e- through the electric field. [ unit: V ] Depend on: type of semicon. material, amount of doping, temperature. (e.g : 0.7V for Si and 0.3 V for Ge at 25°C) Fig 1-18 a & b depletion region

35 1.6 The Diode (Energy Diagram of the PN Junction and Depletion Region)
Energy level for n-type << p- type material (diff. in atomic characteristic : pentavalent & trivalent) After cross the junction, the e- lose energy & fall into the holes in p-region valence band. As the diffusion continues, the depletion region begins to form and the energy level of n-region conduction band decrease. Soon, no more electrons left in n-region conduction band with enough energy to cross the junction to p-region conduction band. Figure (b), the junction is at equilibrium state, the depletion region is complete diffusion has ceased (stop). Fig 1-18 a & b depletion region

36 1.7 Biasing The Diode (Bias)
At equilibrium state – no electron move through the pn-junction. Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer Two bias conditions : forward bias & reverse bias The relationship between the width of depletion layer & the junction current Depletion Layer Width Junction Resistance Junction Current Min Max Fig. 1-22b depletion region forward & Fig depletion region reverse

37 1.7 Biasing The Diode ( Forward Bias)
Flow of majority carries and the voltage across the depletion region Diode connection The negative side of the bias voltage push the free electrons in the n-region -> pn junction Also provide a continuous flow of electron through the external connection into n-region Bias voltage imparts energy to the free e- to move to p-region Electrons in p-region loss energy- positive side of bias voltage source attracts the e- left the p-region Holes in p-region act as medium or pathway for these e- to move through the p-region Voltage source or bias connections are + to the p material and – to the n material Bias must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon diodes) The depletion region narrows. R – limits the current to prevent damage for diode Fig. 1-22b depletion region forward & Fig depletion region reverse

38 1.7 Biasing The Diode ( The Effect of Forward Bias on the Depletion Region)
As more electrons flow into the depletion region, the no. of +ve ion is reduced. As more holes flow into the depletion region on the other side – the no. of –ve ions is reduced. Reduction in +ve & -ve ions – causes the depletion region to narrow Fig. 1-22b depletion region forward & Fig depletion region reverse

39 1.7 Biasing The Diode ( The Effect of the Barrier Potential during Forward Bias)
Electric field between in depletion region prevent free e- from diffusing at equilibrium state -> barrier potential When apply forward bias – free e- enough energy to cross the depletion region Electron got the same energy = barrier potential to cross the depletion region An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected Fig. 1-22b depletion region forward & Fig depletion region reverse

40 1.7 Biasing The Diode ( Reverse Bias)
Shot transition time immediately after reverse bias voltage is applied Diode connection + side of bias pulls the free electrons in the n- region away from pn junction cause add. +ve ions are created , widening the depletion region In the p-region, e- from – side of the voltage source enter as valence electrons e- move from hole to hole toward the depletion region, then created add. –ve ions. As the depletion region widens, the availability of majority carriers decrease Condition that prevents current through the diode Voltage source or bias connections are – to the p material and + to the n material Current flow is negligible in most cases. The depletion region widens Fig. 1-22b depletion region forward & Fig depletion region reverse

41 1.7 Biasing The Diode ( Reverse Current)
extremely small current exist small number of free minority e- in p region are “pushed” toward the pn junction by the –ve bias voltage e- reach wide depletion region –combine with minority holes in n -region – create small hole current Fig. 1-22b depletion region forward & Fig depletion region reverse

42 1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias)
-When a forward bias voltage is applied – current called forward current, -In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small. -Increase forward bias voltage – current also increase Fig 1-26a measurements with meters FIGURE Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

43 1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias)
-With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly. -But the voltage across the diode increase only above 0.7 V. Fig. 1-26b measurements with meters FIGURE Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

44 dynamic resistance r’d decreases as you move up the curve
1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias) -Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode Increase to the right increase upward dynamic resistance r’d decreases as you move up the curve zero bias Fig. 1-26b measurements with meters

45 1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for Reverse bias)
Breakdown voltage not a normal operation of pn junction devices the value can be vary for typical Si Fig. 1-26b measurements with meters Reverse Current

46 1.8 Voltage-Current Characteristic of a Diode ( Complete V-I Characteristic curve)
Combine-Forward bias & Reverse bias  Complete V-I characteristic curve Fig. 1-26b measurements with meters

47 1.8 Voltage-Current Characteristic of a Diode ( Temperature effect on the diode V-I Characteristic)
Forward biased dioed : for a given value of For a given Barrier potential decrease as T increase Reverse current breakdown – small & can be neglected Fig. 1-26b measurements with meters

48 1.9 Diode Models ( Diode structure and symbol)
Directional of current cathode anod Fig ideal diode curve

49 1.9 Diode Models DIODE MODEL The Ideal Diode Model
The Practical Diode Model DIODE MODEL Fig ideal diode curve The Complete Diode Model

50 1.9 Diode Models ( The ideal Diode model)
Ideal model of diode- simple switch: Closed (on) switch -> FB Open (off) switch -> RB Assume Forward current, by Ohm’s law (1-2) Fig ideal diode curve

51 1.9 Diode Models ( The Practical Diode model)
Adds the barrier potential to the ideal switch model ‘ is neglected From figure (c): The forward current [by applying Kirchhoff’s voltage low to figure (a)] Ohm’s Law Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V Represent by produced across the pn junction Same as ideal diode model Fig ideal diode curve (1-3)

52 1.9 Diode Models ( The Complete Diode model)
Complete model of diode consists: Barrier potential Dynamic resistance, Internal reverse resistance, The forward voltage: The forward current: acts as closed switch in series with barrier potential and small acts as open switch in parallel with the large (1-4) Fig ideal diode curve (1-5)

53 1.9 Diode Models ( Example)
(1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current. 1.0kΩ Fig ideal diode curve 1.0kΩ 5V 10V

54 1.9 Diode Models ( Example)
Ideal Model: Practical Model: (c) Complete model: Fig ideal diode curve

55 1.9 Diode Models ( Typical Diodes)
Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in. Fig ideal diode curve

56 1.10 Testing A Diodes ( By Digital multimeter)
Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode. With the diode check function a good diode will show approximately .7 V or .3 V when forward biased. Fig 1-38 DMM check w/electrode labels When checking in reverse bias the full applied testing voltage will be seen on the display. K A A K

57 1.10 Testing A Diodes ( By Digital multimeter)
Defective Diode Fig 1-38 DMM check w/electrode labels

58 1.10 Testing A Diodes ( By Analog multimeter – ohm function )
Select OHMs range Good diode: Forward-bias: get low resistance reading (10 to 100 ohm) Reverse-bias: get high reading (0 or infinity) Fig 1-38 DMM check w/electrode labels

59 Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. P-materials are doped with trivalent impurities N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

60 Summary A diode conducts when forward biased and does not conduct when reverse biased When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage. There are three ways of analyzing a diode. These are ideal, practical, and complex. Typically we use a practical diode model.

61 Assignment – due : next week class
1. Describe the difference between: a) n-type and p-type semiconductor materials b) donor and acceptor impurities c) majority and minority carries 2. Predict the voltmeter reading in Figure 2.1. (assumed voltage across the diode is 0.7V, R1= 10kohm, V1 = 5V). Then, calculate current, I. voltmeter I Figure 2.1


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