Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)

Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)
EMT 111/4 ELECTRONIC DEVICE by Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP) Pn. Syahrul Ashikin Azmi (PPKSE)

Semiconductor Theory & P-N Junction
CHAPTER-1 Semiconductor Theory & P-N Junction 1.1 Introduction 1.2 Atomic Theory 1.3 Insulators, Conductors, and Semiconductors 1.4 Current in Semiconductors 1.5 P-N Junction 1.6 Bias 1.7 Summary

Able to operate very well
Amplifier 1.1 Introduction Electronic Systems Radio Television Computer Telephone Vacuum Tubes Rectifier Able to operate very well Large Fragile High power consumption To increase the strength of ac signals To convert ac energy to dc energy Vacuum Tube 1890s

1.1 Introduction Fig.1-1: Structure of a vacuum tube diode and triode

1.1 Introduction Single small chip
Transistor 1950s A Semiconductor Device Smaller More rugged Less power consumption Integrated circuits 1960s μicro-Processors 1980s Single small chip commonly used as an amplifier or an electrically controlled switch

1.1 Introduction Fig.1-2: Transistor and symbols BJT JFET
PNP NPN P-channel N-channel BJT JFET BJT = Bipolar Junction transistor JFET = Junction Field-Effect Transistor Fig.1-2: Transistor and symbols

1.1 Introduction (a) (b) Fig.1-3: (a) Integrated circuits and (b) microprocessor A microprocessor is a programmable digital electronic component that incorporates the functions of a central processing unit (CPU) on a single semiconducting integrated circuit (IC).

Fig. 1-4: Bohr model of an atom
1.2 Atomic Theory 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. Protons carry 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. 1-4: Bohr model of an atom

Shells or orbital paths
1.2 Atomic Theory 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. N - - - M - - - L - - - - K - 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): electrons Total: electrons n = the shell number - - - - - - - - - - Valence shell Valence electron Fig.1-5: Bohr model of copper atom (Cu)

1.2 Atomic Theory Tabel 1-1: Electron contents of shells and subshells of the copper atom Shell Subshells Capacity (2n2) Content K (n = 1) s 2 L (n = 2) p 6 M (n = 3) d 10 N (n = 4) 1 f 14

1.2 Atomic Theory 1.2.3 Energy Bands Energy Energy Conduction band
Valence band Conduction band Second band (shell 2) First band (shell 1) Nucleus Energy Conduction band Energy E3 = 0.7eV E1 E2 E4 = 1.8eV Energy gap E = energy level Valence band Fig. 1-6: Energy band diagram for an unexited (no external energy) atom in a pure (intrinsic) Si crystal.

1.2 Atomic Theory Each orbital shell around the nucleus corresponds to a certain energy band. A shell is separated from adjacent shells by energy gaps, in which no electron can exist (forbidden band). For an electron to jump from one orbital shell to another, it must absorb enough energy to overcome its energy gap between the shells.

Fig. 1-7: Energy band diagrams for three different materials
1.2 Atomic Theory Conduction Band Valence Band Energy Forbidden Band (a) Insulator (b) Semiconductor (c) Conductor Fig. 1-7: 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 joules), which is the energy gap between valence band and conduction band. For conductors, semiconductors, and insulators, the valence to conduction-band energy gaps are approximately 0.4, 1.1, and 1.8 eV, respectively

1.2 Atomic Theory 1.1.4 Covalent Bonding - + -
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. 1-8: Covalent bonding in a semiconductor crystal

1.2 Atomic Theory 1.1.5 Semiconductors - -
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 - Ge - Si Fig. 1-9: Semiconductor atoms

1.3 Insulators, Conductors, and Semiconductors
In terms of electrical properties All materials are made up of atoms that contribute to its ability to conduct electrical current Materials Insulators Conductors Semiconductors

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. The best conductors are single-element materials. Ex. : copper, silver, gold and aluminum.

A material that is between conductors and insulators in its ability to conduct electrical current. In its pure (intrinsic) state is neither a good conductor nor a good insulator. Characterized by atoms with four valence electron. Ex. : - Single-crystal : Ge, Si & C - Compound : GaAs, CdS & GaAsP

Comparison of a Semiconductor Atom to a Conductor Atom Core of Si atom has a net charge of +4 (14 protons – 10 electrons) and +1 (29 protons – 28 electrons) for Cu atom. 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).

Valence electrons Valence electrons Core (+4) Core (+1) (a) Silicon atom (a) Copper atom Fig.1-10: Diagrams of the silicon and copper atoms

1.4 Current in Semiconductors
Conduction Electrons and Holes When an electron jumps to the conduction band, a vacancy is left in the vallence band, this vacancy is called a hole and the electron is said to be in an excited state. Recombination occurs when a conduction-band electron after within a few microseconds of becoming a free, 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.1-11: Creation of electron-hole pairs in a Si atom. (a) energy diagram, and (b) bonding diagram

Electron and Hole Current 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 (as illustrated in Fig. 1-12). When a voltage is applied across the piece of intrinsic Si, as shown in Fig. 1-13, the thermally generated free electrons in the conduction band, which are free to move, are now easily attracted toward the positive end. 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 (Fig. 1-14). The movement of free electrons in a semiconductive material is called electron current. The movement of electrons in a valence band is called hole current.

Fig.1-13: Free electrons are attracted toward the positive end Fig.1-12: Free electrons are being generated continuously while some recombine with holes

Fig. 1-14: Hole current in intrinsic Si

1.5 The P-N Junction Table 1-2: Few terms and processes that are frequently referred to in p-n junction theory Terms/Processes Definitions Intrinsic semiconductor Doping N-type impurities (donor) P-type impurities (acceptors) Extrinsic semiconductor the pure semiconductor, in which the number of free electrons equals the number of holes in the crystal structure. the process of adding impurity atoms to the intrinsic semiconductor in order to alter the balance between holes and electrons (or to increase the conductivity of the semiconductor). the type of impurities that add (donate) electrons to intrinsic semiconductors, when combined. the type of impurities that produce holes (accept electrons) in intrinsic semiconductors, when combined. the impure semiconductor that has been doped with n-type or p-type impurity atoms, resulting in imbalance between the hole and electron densities.

1.5 The P-N Junction Table 1-2: Few terms and processes that are frequently referred to in p-n junction theory Terms/Processes Definitions Ionization Diffusion current the process of losing or gaining a valence electron. If a neutral atom loses a valence electron, it is no longer neutral and is called a positive ion. On the other hand, if a neutral atom gains a valence electron, it is called a negative ion. results when there is a non-uniform concentration of charge carriers (electrons or holes) in the semiconductor; that is, if there is a higher density of carriers in one region and lower density in another, carriers start migrating from the region of higher density to the region with lower density until a fairly uniform concentration is established in the semiconductor. The flow of these charge carriers during migration constitutes a current flow called diffusion current, and the carriers are said to diffuse from one region to another.

Excess covalent bond electron
1.5 The P-N Junction 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. - Si As Excess covalent bond electron - Electrons (majority carriers) Holes (minority carriers) Conduction band Valence band Energy Fig. 1-15: N-type semiconductor. Four of As atom’s valence electrons are used to form the covalent bond with Si atoms, leaving one extra electron Fig. 1-16: Energy diagram (n-type semiconductor)

1.5 The P-N Junction 1.5.2 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. - Si Al Covalent bond hole Electrons (minority carriers) Holes (majority carriers) Conduction band Valence band Energy - Fig. 1-14: P-type semiconductor. Three of Al atom’ valence electrons are used in the covalent bonds, leaving one hole Fig. 1-15: Energy diagram (p-type semiconductor)

1.5 The P-N Junction 1.5.3 Formation of The P-N Junction
The p-n junction is a fundamental component of many electronic devices and is formed by joining, through a certain manufacturing process, a block of p-type semiconductor to a block of n-type semiconductor. - N-type P-type - N-type P-type - Conduction band Valence band Energy - Conduction band Valence band Energy - Conduction band Valence band (a) (b) Fig. 1-16: N-type and P-type semconductors (a) before and (b) at the instant they are joined

1.5 The P-N Junction 1.5.4 Formation of The Depletion Region - - - - -
+ - + + Depletion layer - Energy - Energy Fig.1-17: When two materials are joined together, some of the free electrons in n-type material diffuse to p-type material across the juction

Fig.1-18: Depletion layer charges
1.5 The P-N Junction N-type Junction P-type - +4 +5 - +4 +3 Total (+) = 21 Total (-) = 20 Net charge = +1 Total (+) = 19 Total (-) = 20 Net charge = -1 Fig.1-18: Depletion layer charges

1.5 The P-N Junction Several things to remember: Each electron that diffuses across the junction leaves one positively charged bond in the n-type material and produces one negatively charged bond in the p-type material. 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.

1.6 Bias Depletion Layer Width Junction Resistance Junction Current
Bias is 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 1-1: The relationship between the width of depletion layer and the junction current Depletion Layer Width Junction Resistance Junction Current Minimum Maximum

1.6 Bias 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 material of 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.

Fig.1-19: A p-n junction connected for forward bias

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.

1.6 Bias 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

1.6 Bias 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.

1.6 Bias The Effect of Barrier Potential During Forward Bias
When forward bias is applied, the free electrons have enough energy to overcome the barrier potential and 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.

1.6 Bias 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 material of 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.

Fig.1-21: A p-n junction connected for reverse bias.

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.

1.6 Bias Fig.1-22: The p-n junction during the short transition time immediately after reverse-bias voltage is applied

1.6 Bias 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 electrons in 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.

1.6 Bias 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

1.6 Bias Reverse Breakdown the p-n junction by negative bias voltage.
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 electrons in 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.

1.7. Diodes VD - + ID 1.7.1 Introduction
A diode is a two-electrode (two-terminal) device that acts as an one-way conductor. The p region is called the anode and the n region is called the cathode. The arrow in the symbol points in the direction of conventional current (opposite to electron flow). Anode (A) Cathode (K) + - ID VD Fig. 1-24: The symbol for the p-n junction diode

1.7. Diodes junction diode. - IF I ≈ 0 VF
+ - VF R IF VBias (a) Forward-biased diode VD I ≈ 0 (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-biased when 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-biased when the negative 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).

1.7. Diodes 1.7.2 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. VA VK VF IF VF = 0 IF > 0 ON VF < 0 IF = 0 OFF Fig.1-26: The behavior of the diode: (a) ideal diode, (b) short circuit and (c) open circuit Diode ON : IF > 0; VF = 0 → VA = VK Diode OFF: IF = 0; VF < 0 → VA < VK This model is adequate for most troubleshooting when you are trying to determine wheter the diode is working properly.

1.7. Diodes Based on the characteristics of a switch, it can be stated that the ideal diode: 1. When reverse biased (open switch): a. The diode has infinite resistance. b. The diode does not pass current. c. The diode drops the applied voltage across its terminals. 2. When forward biased (closed switch): a. The diode has no resistance. b. The diode does not limit the circuit current. c. The diode has no voltage drop Reverse operating region Forward operating region VF VR I II III IV IF IR Fig.1-27: I-V characteristics of the ideal diode

1.7 Diodes Since the barrier potential and the forward dynamic resistance are neglected, the diode is assumed to have a zero voltage across it when forward-biased, as indicated by the portion of the curve on the positive vertical axis (Fig.1-27). The forward current is determined by the bias voltage and the limiting resistor using Ohm’s law: (1-1) Since the reverse current is neglected, its value is assumed to be zero, as indicated in Fig.1-27 by the portion of the curve on the negative horizontal axis. The reserve voltage equals the bias voltage.

1.7. Diodes Example 1-1: For the diode circuits in Fig. 2-2 (a) and (c), determine if the diode is ON or OFF. Find VR, IR, VF, and IF. +12 V R = 2K V1 V2 VR + - VF IR IF (a) (b) (c) (d) Figure 1-28

1.7. Diodes 1.7.3 The Practical Diode Model
The practical model includes the barrier potential. When the diode is forward-biased, it is equivalent to a closed switch in series with a small equivalent voltage source (VF) equals to the barrier potential (0.7) with the positive side toward the anode, as indicated in Fig.1-29(a). This equivalent voltage source represents the barrier potential that must be exceeded by the bias voltage before the diode will conduct and is not an active source of voltage. When the diode is reverse-biased, it is equivalent to an open switch just as in the ideal model, as shown in Fig.1-29(b). The barrier potential does not affect reverse bias. Since the barrier potential is included and the dynamic resistance is neglected, the diode is assumed to have a voltage across it when forward-bias, as indicated by the portion of the curve to the right of the origin (in Fig.1-29(c)).

1.7 Diodes Since the barrier potential is included and the forward dynamic resistance are neglected, the diode is assumed to have a voltage across it when forward-biased, as indicated by the portion of the curve to the right of the origin (Fig.1-29(c)). The forward current is determined as follows by first applying Kirchoff’s voltage law: (1-2) The diode is assumed to have zero reverse current, as indicated by the portion of the curve on the negative horizontal axis. This model is useful to troubleshoot in lower-voltage circuits and to design the basic diode circuits.

1.7 Diodes Fig.1-29: The practical model of a diode

1.7 Diodes 1.7.4 The Complete Diode Model
The complete model includes the barrier potential, the small forward dynamic resistance (r’d), and the large internal reverse resistance (r’R). When the diode is forward-biased, it acts as a closed switch in series with the equivalent barrier voltage (VB) and the small forward dynamic resistance (r’d), as indicated in Fig.1-30(a). When the diode is reverse-biased, it acts as an open switch in parallel with the large internal reverse resistance (r’R), as shown in Fig.1-30(b). The barrier potential does not affect reverse bias. Since the barrier potential and the dynamic resistance are included, the diode is assumed to have a voltage across it when forward-biased. This voltage (VF) consists of the barrier potential voltage plus the small voltage drop across the dynamic resistance, as indicated by the portion of the curve to the right of the origin. The curve slops because the voltage drop due to dynamic resistance increases as the current increases.

1.7 Diodes Fig.1-30: The complete model of a diode

1.7 Diodes For the complete model of a silicon diode, the following formulas apply: (1-3) (1-4) For troubleshooting work, it is unnecessary to use the complete model, because it involves complicated calculations. This model is generally suited to design problems using a computer for simulation.

1.7 Diodes Exercise 1-1: Determine the forward voltage and forward current for the diode in Fig.1-31(a) for each of the diode models. Also find the voltage across the limiting resistor in each case. Assume r’d = 10 Ω at the determined value of forward current. Determine the reverse voltage and reverse current for the diode in Fig.1-31(b) for each of the diode models. Also find the voltage across the limiting resistor in each case. Assume IR = 1μA. + - RLimit IF VBias (a) Forward-biased diode 10 V 1 kΩ RLimit IF 1 kΩ + VBias 10 V - (b) Reverse-biased diode Fig.1-31: Two different bias circuits

Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)

Similar presentations

Presentation on theme: "Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)

Similar presentations

Presentation on theme: "Dr. Naser alrawi (PPKMe) Pn. 'Aini Syuhada Md Zain (PPKKP)"— Presentation transcript:

Similar presentations

About project

Feedback