Electron and Hole Current

When a voltage is applied across a piece of intrinsic silicon, as shown in Figure 1 to 15, the thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attracted toward the positive end. This movement of free electrons is one type of current in a semiconductive material and is called electron current.

Figure 1 to 15 summary: The figure is an illustration of electron-hole pairs in silicon crystal and electron current in intrinsic silicon. It shows the generation of electron-hole pairs due to heat energy and the recombination of electrons with holes. The figure also depicts electron current in intrinsic silicon, produced by the movement of thermally generated free electrons. The generation and recombination processes are ongoing, and the application of voltage results in electron movement, ultimately leading to current flow.

Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

Another type of current occurs in the valence band, where the holes created by the free electrons exist. Electrons remaining in the valence band are still attached to their atoms and are not free to move randomly in the crystal structure as are the free electrons. However, a valence electron can move into a nearby hole with little change in its energy level, thus leaving another hole where it came from. Effectively the hole has moved from one place to another in the crystal structure, as illustrated in Figure 1 to 16. Although current in the valence band is produced by valence electrons, it is called hole current to distinguish it from electron current in the conduction band.

Figure 1 to 16 summary: This is a schematic diagram. It illustrates hole current in intrinsic silicon. The diagram depicts the movement of valence electrons and the resulting movement of holes in a silicon lattice. As valence electrons move to fill holes, the holes themselves appear to move in the opposite direction. The movement of holes is from right to left while the electron movement is from left to right.

As you have seen, conduction in semiconductors is considered to be either the movement of free electrons in the conduction band or the movement of holes in the valence band, which is actually the movement of valence electrons to nearby atoms, creating hole current in the opposite direction.

It is interesting to contrast the two types of charge movement in a semiconductor with the charge movement in a metallic conductor, such as copper. Copper atoms form a different type of crystal in which the atoms are not covalently bonded to each other but consist of a "sea" of positive ion cores, which are atoms stripped of their valence electrons. The valence electrons are attracted to the positive ions, keeping the positive ions together and forming the metallic bond. The valence electrons do not belong to a given atom, but to the crystal as a whole. Since the valence electrons in copper are free to move, the application of a voltage results in current. There is only one type of current—the movement of free electrons—because there are no "holes" in the metallic crystal structure.

Section 1 to 3 Checkup

1. Are free electrons in the valence band or in the conduction band?

2. Which electrons are responsible for electron current in silicon?

3. What is a hole?

4. At what energy level does hole current occur?

1 to 4 N-TYPE and P-TYPE Semiconductors

Semiconductor materials do not conduct current well and are of limited value in their intrinsic state. This is because of the limited number of free electrons in the conduction band and holes in the valence band. Intrinsic silicon (or germanium) must be modified by increasing the number of free electrons or holes to increase its conductivity and make it useful in electronic devices. This is done by adding impurities to the intrinsic material. Two types of extrinsic (impure) semiconductive materials, n -type and p -type, are the key building blocks for most types of electronic devices.

After completing this section, you should be able to

Describe the properties of n -type and p -type semiconductors

- Define doping.

- Explain how n -type semiconductors are formed.

- Describe a majority carrier and minority carrier in n -type material.

- Explain how p -type semiconductors are formed.

- Describe a majority carrier and minority carrier in p -type material.

Since semiconductors are generally poor conductors, their conductivity can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductive material. This process, called doping, increases the number of current carriers (electrons or holes). The two categories of impurities are n -type and p -type.

N-Type Semiconductor

To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb).

As illustrated in Figure 1 to 17, each pentavalent atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms. Four of the antimony atom's valence electrons are used to form the covalent bonds with silicon atoms, leaving one extra electron. This extra electron becomes a conduction electron because it is not involved in bonding. Because the pentavalent atom gives up an electron, it is often called a donor atom. The number of conduction electrons can be carefully controlled by the number of impurity atoms added to the silicon. A conduction electron created by this doping process does not leave a hole in the valence band because it is in excess of the number required to fill the valence band.

Figure 1 to 17 summary: The figure is an illustration of a pentavalent impurity atom in a silicon crystal structure. The illustration depicts an antimony atom surrounded by silicon atoms. The antimony atom has an extra electron that does not bond with the silicon atoms, becoming a free electron. The extra electron from the antimony atom contributes to the conductivity of the silicon material, making it an n-type semiconductor.

Majority and Minority Carriers Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an n -type semiconductor (the n stands for the negative charge on an electron). The electrons are called the majority carriers in n -type material. Although the majority of current carriers in n -type material are electrons, there are also a few holes that are created when electron-hole pairs are thermally generated. These holes are not produced by the addition of the pentavalent impurity atoms. Holes in an n -type material are called minority carriers.

P-Type Semiconductor

To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga). As illustrated in Figure 1 to 18, each trivalent atom (boron, in this case) forms covalent bonds with four adjacent silicon atoms. All three of the boron atom's valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added. Because the trivalent atom can take an electron, it is often referred to as an acceptor atom. The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. A hole created by this doping process is not accompanied by a conduction (free) electron.

Figure 1 to 18 summary: The figure is a schematic diagram. It illustrates a trivalent impurity atom within a silicon crystal structure, specifically depicting a boron atom at the center. The diagram shows the arrangement of atoms and their electron sharing, highlighting the presence of a hole resulting from the boron atom's bonding with silicon atoms. The inclusion of a trivalent impurity like boron in a silicon crystal leads to the creation of a hole, influencing the electrical properties of the semiconductor material. This process is fundamental to the creation of p-type semiconductors, where holes serve as the majority charge carriers.

Majority and Minority Carriers Since most of the current carriers are holes, silicon (or germanium) doped with trivalent atoms is called a p -type semiconductor. The holes are the majority carriers in p -type material. Although the majority of current carriers in p -type material are holes, there are also a few conduction-band electrons that are created when electron-hole pairs are thermally generated. These conduction-band electrons are not produced by the addition of the trivalent impurity atoms. Conduction-band electrons in p -type material are the minority carriers.

Section 1 to 4 Checkup

1. Define doping.

2. What is the difference between a pentavalent atom and a trivalent atom?

3. What are other names for the pentavalent and trivalent atoms?

4. How is an n -type semiconductor formed?

5. How is a p -type semiconductor formed?

6. What is the majority carrier in an n -type semiconductor?

7. What is the majority carrier in a p -type semiconductor?

8. By what process are the majority carriers produced?

9. By what process are the minority carriers produced?

10. What is the difference between intrinsic and extrinsic semiconductors?

1 to 5 The PN Junction

When you take a block of silicon and dope part of it with a trivalent impurity and the other part with a pentavalent impurity, a boundary called the pn junction is formed between the resulting p -type and n -type portions. The pn junction is the basis for diodes, certain transistors, solar cells, and other devices, as you will learn later.

After completing this section, you should be able to

Describe how a pn junction is formed

- Discuss diffusion across a pn junction.

- Explain the formation of the depletion region.

- Define barrier potential and discuss its significance barrier potential in silicon and germanium.

- State the values of.

- Discuss energy diagrams.

- Define energy hill.

A p -type material consists of silicon atoms and trivalent impurity atoms such as boron. The boron atom adds a hole when it bonds with the silicon atoms. However, since the number of protons and the number of electrons are equal throughout the material, there is no net charge in the material and so it is neutral.

An n -type silicon material consists of silicon atoms and pentavalent impurity atoms such as antimony. As you have seen, an impurity atom releases an electron when it bonds with four silicon atoms. Since there is still an equal number of protons and electrons (including the free electrons) throughout the material, there is no net charge in the material and so it is neutral.

If a piece of intrinsic silicon is doped so that part is n -type and the other part is p -type, a pn junction forms at the boundary between the two regions and a diode is created, as indicated in Figure 1 to 19(a). The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers). The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).

Figure 1 to 19(a) summary: The figure is a schematic representation of a semiconductor device. It illustrates the basic silicon structure at the start of junction formation, showing only the majority and minority carriers. Free electrons in the n region begin to diffuse across the junction and fall into holes near the junction in the p region. Initially, there are more holes in the p region and more free electrons in the n region. As electrons diffuse from the n region to the p region, they combine with holes near the junction, reducing the number of free electrons in the n region and the number of holes in the p region near the junction.

Formation of the Depletion Region

The free electrons in the n region are randomly drifting in all directions. At the instant of the pn junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction, as shown in Figure 1 to 19(b).

Figure 1 to 19(b) summary: The figure is a schematic diagram. It illustrates the formation of a depletion region and barrier potential at a p-n junction. The diagram shows a p-region and an n-region in contact, with a depletion region forming at the junction. The depletion region is characterized by a lack of free charge carriers and the presence of a barrier potential. The formation of the depletion region and barrier potential is a fundamental aspect of p-n junction behavior, influencing its electrical characteristics and enabling its use in various electronic devices. The barrier potential opposes the flow of majority carriers across the junction, while the depletion region is devoid of mobile charge carriers.

Before the pn junction is formed, recall that there are as many electrons as protons in the n -type material, making the material neutral in terms of net charge. The same is true for the p -type material.

When the pn junction is formed, the n region loses free electrons as they diffuse across the junction. This creates a layer of positive charges (pentavalent ions) near the junction. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. This creates a layer of negative charges (trivalent ions) near the junction. These two layers of positive and negative charges form the depletion region, as shown in Figure 1 to 19(b). The term depletion refers to the fact that the region near the pn junction is depleted of charge carriers (electrons and holes) due to diffusion across the junction. Keep in mind that the depletion region is formed very quickly and is very thin compared to the n region and p region.

After the initial surge of free electrons across the pn junction, the depletion region has expanded to a point where equilibrium is established and there is no further diffusion of

History Note

After the invention of the light bulb, Edison continued to experiment and in 1883 found that he could detect electrons flowing through the vacuum from the lighted filament to a metal plate mounted inside the bulb. This discovery became known as the Edison effect.

An English physicist, John Fleming, took up where Edison left off and found that the Edison effect could also be used to detect radio waves and convert them to electrical signals. He went on to develop a two-element vacuum tube called the Fleming valve, later known as the diode. It was a device that allowed current in only one direction. Modern pn junction devices are an outgrowth of this.

History Note

Russell Ohl, working at Bell Labs in 1940, stumbled on the semiconductor pn junction. Ohl was working with a silicon sample that had an accidental crack down its middle. He was using an ohmmeter to test the electrical resistance of the sample when he noted that when the sample was exposed to light, the current between the two sides of the crack made a significant jump. This discovery was fundamental to the work of the team that invented the transistor in 1947. electrons across the junction. This occurs as follows: As electrons continue to diffuse across the junction, more and more positive and negative charges are created near the junction as the depletion region is formed. A point is reached where the total negative charge in the depletion region repels any further diffusion of electrons (negatively charged particles) into the p region (like charges repel) and the diffusion stops. In other words, the depletion region acts as a barrier to the further movement of electrons across the junction.

Barrier Potential Any time there is a positive charge and a negative charge near each other, there is a force acting on the charges as described by Coulomb's law. In the depletion region there are many positive charges and many negative charges on opposite sides of the pn junction. The forces between the opposite charges form an electric field, as illustrated in Figure 1 to 19(b) by the blue arrows between the positive charges and the negative charges. This electric field is a barrier to the free electrons in the n region, and energy must be expended to move an electron through the electric field. That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region.

The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts. Stated another way, a certain amount of voltage equal to the barrier potential and with the proper polarity must be applied across a pn junction before electrons will begin to flow across the junction. You will learn more about this when we discuss biasing in Chapter 2.

The barrier potential of a pn junction depends on several factors, including the type of semiconductive material, the amount of doping, and the temperature. The typical barrier potential is approximately 0.7V for silicon and 0.3V for germanium at 25 sup . Because germanium devices are not widely used, silicon will be assumed throughout the rest of the book.

Energy Diagrams of the PN Junction and Depletion Region

The valence and conduction bands in an n -type material are at slightly lower energy levels than the valence and conduction bands in a p -type material. Recall that p -type material has trivalent impurities and n -type material has pentavalent impurities. The trivalent impurities exert lower forces on the outer-shell electrons than the pentavalent impurities. The lower forces in p -type materials mean that the electron orbits are slightly larger and hence have greater energy than the electron orbits in the n -type materials.

An energy diagram for a pn junction at the instant of formation is shown in Figure 1 to 20(a). As you can see, the valence and conduction bands in the n region are at lower energy levels than those in the p region, but there is a significant amount of overlapping.

Figure 1 to 20(a) summary: This is an energy diagram. It illustrates the formation of the pn junction. The diagram shows the energy levels of the conduction and valence bands in the p-region, the pn junction, and the n-region. The majority and minority carriers are also indicated in each region. Initially, at the instant of junction formation, there is a difference in the energy levels between the p and n regions. The majority carriers in each region are more abundant than the minority carriers. At equilibrium, the energy levels align, and a depletion region forms at the junction.

The free electrons in the n region that occupy the upper part of the conduction band in terms of their energy can easily diffuse across the junction (they do not have to gain additional energy) and temporarily become free electrons in the lower part of the p -region conduction band. After crossing the junction, the electrons quickly lose energy and fall into the holes in the p -region valence band as indicated in Figure 1 to 20(a).

As the diffusion continues, the depletion region begins to form and the energy level of the n region conduction band decreases. The decrease in the energy level of the conduction band in the n region is due to the loss of the higher-energy electrons that have diffused across the junction to the p region. Soon, there are no electrons left in the n region conduction band with enough energy to get across the junction to the p region conduction band, as indicated by the alignment of the top of the n region conduction band and the bottom of the p region conduction band in Figure 1 to 20(b). At this point, the junction is at equilibrium; and the depletion region is complete because diffusion has ceased. There is an energy gradient across the depletion region which acts as an "energy hill" that an n region electron must climb to get to the p region.

Notice that as the energy level of the n region conduction band has shifted downward, the energy level of the valence band has also shifted downward. It still takes the same amount of energy for a valence electron to become a free electron. In other words, the energy gap between the valence band and the conduction band remains the same.

Figure 1 to 20(b) summary: The figure is an energy band diagram. It illustrates the energy levels within a semiconductor material at equilibrium, specifically focusing on the p-region, n-region, and the pn junction with the depletion region. The figure depicts the conduction and valence bands in each region, along with the distribution of charge carriers (electrons and holes). The energy band diagram shows a shift in the energy levels at the pn junction, creating a potential barrier. The concentration of electrons is significantly higher in the n-region compared to the p-region, while the concentration of holes is greater in the p-region than in the n-region. This difference in carrier concentration and the formation of a potential barrier are fundamental to the behavior of a pn junction diode.

Section 1 to 5 Checkup

1. What is a pn junction?

2. Explain diffusion.

3. Describe the depletion region.

4. Explain what the barrier potential is and how it is created.

5. What is the typical value of the barrier potential for a silicon diode?

6. What is the typical value of the barrier potential for a germanium diode?

Summary

Section 1 to 1

- According to the classical Bohr model, the atom is viewed as having a planetary-type structure with electrons orbiting at various distances around the central nucleus.

- According to the quantum model, electrons do not exist in precise circular orbits as particles as in the Bohr model. The electrons can be waves or particles and precise location at any time is uncertain.

- The nucleus of an atom consists of protons and neutrons. The protons have a positive charge and the neutrons are uncharged. The number of protons is the atomic number of the atom.

- Electrons have a negative charge and orbit around the nucleus at distances that depend on their energy level. An atom has discrete bands of energy called shells in which the electrons orbit. Atomic structure allows a certain maximum number of electrons in each shell. In their natural state, all atoms are neutral because they have an equal number of protons and electrons.

- The outermost shell or band of an atom is called the valence band, and electrons that orbit in this band are called valence electrons. These electrons have the highest energy of all those in the atom. If a valence electron acquires enough energy from an outside source, it can jump out of the valence band and break away from its atom.

Section 1 to 2

- Insulating materials have very few free electrons and do not conduct current under normal circumstances.

- Materials that are conductors have a large number of free electrons and conduct current very well.

- Semiconductive materials fall in between conductors and insulators in their ability to conduct current.

- Semiconductor atoms have four valence electrons. Silicon is the most widely used semiconductive material.

- Semiconductor atoms bond together in a symmetrical pattern to form a solid material called a crystal. The bonds that hold the type of crystal used in semiconductors are called covalent bonds.

- The valence electrons that manage to escape from their parent atom are called conduction electrons or free electrons. They have more energy than the electrons in the valence band and are free to drift throughout the material.

- When an electron breaks away to become free, it leaves a hole in the valence band creating what is called an electron-hole pair. These electron-hole pairs are thermally produced because the electron has acquired enough energy from external heat to break away from its atom.

- A free electron will eventually lose energy and fall back into a hole. This is called recombination. Electron-hole pairs are continuously being thermally generated so there are always free electrons in the material.

- When a voltage is applied across the semiconductor, the thermally produced free electrons move toward the positive end and form the current. This is one type of current and is called electron current.

- Another type of current is the hole current. This occurs as valence electrons move from hole to hole creating, in effect, a movement of holes in the opposite direction.

- An n -type semiconductive material is created by adding impurity atoms that have five valence electrons. These impurities are pentavalent atoms. A p -type semiconductor is created by adding impurity atoms with only three valence electrons. These impurities are trivalent atoms.

- The process of adding pentavalent or trivalent impurities to a semiconductor is called doping.

- The majority carriers in an n -type semiconductor are free electrons acquired by the doping process, and the minority carriers are holes produced by thermally generated electron-hole pairs.

- The majority carriers in a p -type semiconductor are holes acquired by the doping process, and the minority carriers are free electrons produced by thermally generated electron-hole pairs.

- A pn junction is formed when part of a material is doped n -type and part of it is doped p -type. A depletion region forms starting at the junction that is devoid of any majority carriers. The depletion region is formed by ionization.

- The barrier potential is typically 0.7V for a silicon diode and 0.3V for germanium.

Key terms and other bold terms are defined in the end-of-book glossary.

Atom The smallest particle of an element that possesses the unique characteristics of that element.

Barrier potential The amount of energy required to produce full conduction across the p n junction in forward bias.

Conductor A material that easily conducts electrical current.

Crystal A solid material in which the atoms are arranged in a symmetrical pattern.

Doping The process of imparting impurities to an intrinsic semiconductive material in order to control its conduction characteristics.

Electron The basic particle of negative electrical charge.

Free electron An electron that has acquired enough energy to break away from the valence band of the parent atom; also called a conduction electron.

Hole The absence of an electron in the valence band of an atom.

Insulator A material that does not normally conduct current.

Ionization The removal or addition of an electron from or to a neutral atom so that the resulting atom (called an ion) has a net positive or negative charge.

Metallic bond A type of chemical bond found in metal solids in which fixed positive ion cores are held together in a lattice by mobile electrons.

Orbital Subshell in the quantum model of an atom.

PN junction The boundary between two different types of semiconductive materials.

Proton The basic particle of positive charge.

Semiconductor A material that lies between conductors and insulators in its conductive properties. Silicon, germanium, and carbon are examples.

Shell An energy band in which electrons orbit the nucleus of an atom.

Silicon A semiconductive material.

Valence Related to the outer shell of an atom.

Key Formula

1 to 1 N sub e equals 2 times n squared Maximum number of electrons in any shell

True/False Quiz

Answers can be found at www.pearsonglobaleditions.com/Floyd.

1. Each element has a unique atomic structure.

2. A proton is a negatively charged particle.

3. A hydrogen atom has two protons and two neutrons.

4. In their normal (or neutral) state, all atoms of a given element have the same number of electrons as protons.

5. Valence electrons contribute to chemical reactions.

6. Most metals are bad conductors.

7. An insulator is a material that does not conduct electrical current under normal conditions.

8. A pn junction is formed from silicon when both p - and n -type materials are used on opposite sides of a crystal.

9. Doping increases the number of current carriers.

10. A boron atom removes a hole when it bonds with silicon atoms.

11. An intrinsic crystal is one that has no impurities.

Self-Test

Answers can be found at www.pearsonglobaleditions.com/Floyd.

Section 1 to 1 1. Every known element has

(a) the same type of atoms (b) the same number of atoms.

(c) a unique type of atom (d) several different types of atoms.

2. An atom consists of

(a) one nucleus and only one electron (b) one nucleus and one or more electrons.

(c) protons, electrons, and neutrons (d) answers (b) and (c).

3. The nucleus of an atom is made up of

(a) protons and neutrons (b) electrons.

(c) electrons and protons (d) electrons and neutrons.

4. Valence electrons are

(a) in the closest orbit to the nucleus (b) in the most distant orbit from the nucleus.

(c) in various orbits around the nucleus (d) not associated with a particular atom.

5. The correct explanation for the notation 2s to the 2 2p to the 1 is

(a) 5 electrons in shell 2: 2 in orbital s , 3 in orbital p.

(b) 3 electrons in shell 2: 2 in orbital s , 1 in orbital p.

(c) 3 electrons in shell 1: 2 in orbital s , 1 in orbital p.

(d) 2 electrons in shell 1 and 1 electron in shell 2.

6. is an example of an insulator.

(a) Copper (b) Gold (c) Mica (d) Boron

7. The difference between an insulator and a semiconductor is

(a) a wider energy gap between the valence band and the conduction band.

(b) the number of free electrons.

(c) the atomic structure.

(d) answers (a), (b), and (c).

8. ______ _ is an example of single element semiconductor.

(a) Silver (b) Gold (c) Mica (d) Arsenic

9. In a semiconductor crystal, the atoms are held together by

(a) the interaction of valence electrons (b) forces of attraction.

(c) covalent bonds (d) answers (a), (b), and (c).

10. The atomic number of gold is

(a) 79 (b) 29 (c) 4 (d) 32

11. The atomic number of copper is

(a) 8 (b) 29 (c) 4 (d) 32

12. The valence shell in a carbon atom has electrons.

(a) 0 (b) 1 (c) 3 (d) 4

13. Each atom in a silicon crystal has

(a) four valence electrons.

(b) four conduction electrons.

(c) eight valence electrons, four of its own and four shared.

(d) no valence electrons because all are shared with other atoms.

Section 1 to 3 14. Electron-hole pairs are produced by

(a) recombination (b) thermal energy (c) ionization (d) doping

15. Recombination is when

(a) an electron falls into a hole.

(b) a positive and a negative ion bond together.

(c) a valence electron becomes a conduction electron.

(d) a crystal is formed.

16. The current in a semiconductor is produced by

(a) electrons only (b) holes only (c) negative ions (d) both electrons and holes

Section 1 to 4 17. In an intrinsic semiconductor,

(a) there are no free electrons.

(b) the free electrons are thermally produced.

(c) there are only holes.

(d) there are as many electrons as there are holes.

(e) answers (b) and (d).

18. A p -type semiconductor has impurity atoms with ____ _ valence electrons.

(a) 3 (b) 5 (c) 0 (d) 1

19. A trivalent impurity is added to silicon to create

(a) germanium (b) a p -type semiconductor.

(c) an n -type semiconductor (d) a depletion region.

20. The purpose of a pentavalent impurity is to

(a) reduce the conductivity of silicon (b) increase the number of holes.

(c) increase the number of free electrons (d) create minority carriers.

21. ____ _ is an example of an element with five valence electrons. (a)Arsenic (b) Boron (c) Gallium (d) Silicon

22. Holes in an n -type semiconductor are

(a) minority carriers that are thermally produced.

(b) minority carriers that are produced by doping.

(c) majority carriers that are thermally produced.

(d) majority carriers that are produced by doping.

Section 1 to 5 23. A pn junction is formed by

(a) the recombination of electrons and holes.

(b) ionization.

(c) the boundary of a p -type and an n -type material.

(d) the collision of a proton and a neutron.

24. The barrier potential for a silicon diode at 25 sup is approximately

(a) 0.7V

(b) 0.3V

(c) 0.1V

(d) 0.8V

25. The depletion region consists of

(a) nothing but minority carriers

(b) positive and negative ions

(c) no majority carriers

(d) answers (b) and (c)

Problems

Answers to all odd-numbered problems are at the end of the book.

Basic Problems

Section 1 to 1 The Atom

1. What is the most important difference between the Bohr model and the quantum model of the atom?

2. What is a free electron?

3. What is the number of protons and electrons in a neutral germanium atom?

4. What is the maximum total number of electrons that can exist in the first four shells of an atom?

Section 1 to 2 Materials Used in Electronic Devices

5. For each of the energy diagrams in Figure 1 to 21, determine the class of material based on relative comparisons.

6. Why is silicon more widely used as a semiconductor than germanium?

7. How many covalent bonds does a single atom form in a germanium crystal?

Figure 1 to 21 summary: The figure is an energy diagram. It illustrates the energy band structure in three different materials. The diagram depicts the valence band, conduction band, and the energy gap between them for each material. Based on the energy gap size, the materials can be classified differently. One material has a smaller energy gap, another has a larger energy gap, and the last one has an even larger energy gap.

Figure 1 to 21(b) summary: This figure is an energy band diagram. It illustrates the relationship between the valence band, conduction band, and band gap of a material. The diagram shows the valence band at a lower energy level and the conduction band at a higher energy level, separated by the band gap. The band gap represents the energy difference that electrons must overcome to move from the valence band to the conduction band. The material depicted has a certain band gap, indicating it is likely a semiconductor or an insulator, as conductors have minimal or no band gap.

Figure c summary: The figure is an energy band diagram. It depicts the relative positions of the valence band and conduction band with an area of overlap. The overlap between the valence and conduction bands suggests a material with metallic properties, where electrons can easily move between bands.

Section 1 to 3 Current in Semiconductors

8. What happens when a conduction-band electron in a silicon crystal loses some energy?

9. Name the two energy bands at which current is produced in silicon.

10. How is hole current generated in a semiconductor?

11. How is an electrical current generated in a metallic conductor?

Section 1 to 4 N-Type and P -Type Semiconductors

12. How does the addition of pentavalent impurity alter the atomic structure of intrinsic silicon?

13. What is antimony? What is boron?

Section 1 to 5 The PN Junction

14. How is the depletion region in a pn junction created?

15. Because of its barrier potential, can a diode be used as a voltage source? Explain.

Diodes and Applications

Chapter Outline

- 2 to 1 Diode Operation.

- 2 to 2 Voltage-Current (V-I) Characteristic of a Diode.

- 2 to 3 Diode Approximations.

- 2 to 4 Half-Wave Rectifiers.

- 2 to 5 Full-Wave Rectifiers.

- 2 to 6 Power Supply Filters and Regulators.

- 2 to 7 Diode Limiters and Clampers.

- 2 to 8 Voltage Multipliers.

- 2 to 9 The Diode Datasheet.

- 2 to 10 Troubleshooting.

Device Application

Chapter Objectives

- Use a diode in common applications.

- Analyze the voltage-current (V - I) characteristic of a diode.

- Explain how the three diode approximations differ.

- Explain and analyze the operation of half-wave rectifiers.

- Explain and analyze the operation of full-wave rectifiers.

- Explain and analyze power supply filters and regulators.

- Explain and analyze the operation of diode limiters and clampers.

- Explain and analyze the operation of diode voltage multipliers.

- if and only if Interpret and use diode datasheets.

- Troubleshoot diodes and power supply circuits.

Key Terms

- Diode.

- Bias.

- Forward bias.

- Reverse bias.

- V-I characteristic.

- DC power supply.

- Rectifier.

- Filter.

- Regulator.

- Half-wave rectifier.

- Peak inverse voltage (PIV).

- Full-wave rectifier.

- Ripple voltage.

- Line regulation.

- if and only if Load regulation.

- Limiter.

- Clamper.

- Troubleshooting.

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Study aids, Multisim files, and LT Spice files for this chapter are available at pearsonglobaleditions.com URL

Introduction

In Chapter 1, you learned that many semiconductor devices are based on the pn junction. In this chapter, the operation and characteristics of the diode are covered. Also, three diode models representing three levels of approximation are presented and testing is discussed. The importance of the diode in electronic circuits cannot be overemphasized. Its ability to conduct current in one direction while blocking current in the other direction is essential to the operation of many types of circuits. One circuit in particular is the ac rectifier, which is covered in this chapter. Other important applications are circuits such as diode limiters, diode clampers, and diode voltage multipliers. Datasheets are discussed for specific diodes.

Device Application Preview

You have the responsibility for the final design and testing of a power supply circuit that your company plans to use in several of its products. You will apply your knowledge of diode circuits to the Device Application at the end of the chapter.

2 to 1 Diode Operation

A modern diode is a two-terminal semiconductor device formed by two doped regions of silicon separated by a pn junction. In this chapter, the most common category of diode, known as the general-purpose diode, is covered. Other descriptors, such as rectifier diode or signal diode, are used depending on the particular application for which the diode was designed. You will learn how to use a voltage to cause the diode to conduct current in one direction and block it in the other direction. This process is called biasing.

After completing this section, you should be able to

- Recognize the electrical symbol for a diode and several diode package configurations.

- Apply forward bias to a diode.

- Define forward bias and state the required conditions Discuss the effect of forward bias on the depletion region Explain how the barrier potential affects the forward bias.

- Reverse-bias a diode.

- Define reverse bias and state the required conditions Discuss reverse current and reverse breakdown.

The Diode

As mentioned, a diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between. The p region is called the anode and is connected to a conductive terminal. The n region is called the cathode and is connected to a second conductive terminal. The basic diode structure and schematic symbol are shown in Figure 2 to 1.

Figure 2 to 1 summary: This is a diagram that illustrates the basic structure of a diode. The diagram depicts a semiconductor device composed of two regions: a p-region and an n-region, forming a p-n junction. The region between the p and n material is labeled as the depletion region. The p-region is connected to the anode, while the n-region is connected to the cathode. The diode allows current to flow more easily in one direction than the other, which is a fundamental property of diodes.

Typical Diode Packages Several common physical configurations of through-hole mounted diodes are illustrated in Figure 2 to 2(a). The anode (A) and cathode (K) are indicated on a diode in several ways, depending on the type of package. The cathode is usually marked by a band, a tab, or some other feature. On those packages where one lead is connected to the case, the case is the cathode.

Surface-Mount Diode Packages Figure 2 to 2(b) shows typical diode packages for surface mounting on a printed circuit board. The SOD and SOT packages have gull-wing shaped leads. The SMA package has L-shaped leads that bend under the package. The SOD and SMA types have a band on one end to indicate the cathode. The SOT type is a three-terminal package in which there are either one or two diodes. In a single-diode SOT package, pin 1 is usually the anode and pin 3 is the cathode. In a dual-diode SOT package, pin 3 is the common terminal and can be either the anode or the cathode. Always check the datasheet for the particular diode to verify the pin configurations.

Typical diode packages with terminal identification. The letter K is used for cathode to avoid confusion with certain electrical quantities that are represented by C . Case type numbers are indicated for each diode.

Forward Bias To bias a diode, you apply a dc voltage across it. Forward bias is the condition that allows current through the pn junction. Figure 2 to 3 shows a dc voltage source connected by conductive material (contacts and wire) across a diode in the direction to produce forward bias. This external bias voltage is designated as V sub BIAS . The resistor limits the forward current to a value that will not damage the diode. Notice that the negative side of V sub BIAS is connected to the n region of the diode and the positive side is connected to the p region. This is one requirement for forward bias. A second requirement is that the bias voltage, V sub BIAS , must be greater than the barrier potential (V sub B) .

Figure 2 to 3 summary: The figure is a schematic diagram. It illustrates a diode connected for forward bias. The diagram shows a p-n junction diode connected in a circuit with a limiting resistor and a voltage source. The positive terminal of the voltage source is connected to the p-region of the diode, and the negative terminal is connected to the n-region. The diode is forward-biased, allowing current to flow through the circuit. This forward bias condition allows the diode to conduct electricity more readily. The resistor limits the current to prevent damage to the diode.

A diode connected for forward bias.

A fundamental picture of what happens when a diode is forward-biased is shown in Figure 2 to 4. Because like charges repel, the negative side of the bias-voltage source "pushes" the free electrons, which are the majority carriers in the n region, toward the pn junction. This flow of free electrons is called electron current. The negative side of the source also provides a continuous flow of electrons through the external connection (conductor) and into the n region as shown.

Figure 2 to 4 summary: This figure is a diagram of a forward biased diode. The diagram displays the flow of majority carriers and the voltage due to the barrier potential across the depletion region. The diagram shows that the positive side attracts the valence electrons toward the left end of the region, and the holes in the p region provide the medium for these valence electrons to move through the p region. The effective flow of holes is the hole current. The continuous availability of holes effectively moving toward the pn junction to combine with the continuous stream of electrons is the same across the junction into the p region.

The bias-voltage source imparts sufficient energy to the free electrons for them to overcome the barrier potential of the depletion region and move on through into the p region. Once in the p region, these conduction electrons have lost enough energy to immediately combine with holes in the valence band.

Now, the electrons are in the valence band in the p region, simply because they have lost too much energy overcoming the barrier potential to remain in the conduction 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, as you can see in Figure 2 to 4. This effective flow of holes is the hole current. You can also view the hole current as being created by the flow of valence electrons through the p region, with the holes providing the only means for these electrons to flow.

As the electrons flow out of the p region through the external connection (conductor) 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. Recall that the conduction band in a conductor overlaps the valence band so that it takes much less energy for an electron to be a free electron in a conductor than in a semiconductor and that metallic conductors do not have holes in their structure. There is a continuous availability of holes effectively moving toward the pn junction to combine with the continuous stream of electrons as they come across the junction into the p region.

The Effect of Forward Bias on the Depletion Region As electrons from the n side are pushed into the depletion region, they combine with holes on the p side, effectively reducing the depletion region. This process during forward bias causes the depletion region to narrow, as indicated in Figure 2 to 5(b).

The depletion region narrows and a voltage drop is produced across the pn junction when the diode is forward-biased.

The Effect of the Barrier Potential During Forward Bias Recall that the electric field between the positive and negative sides of the junction creates an "energy hill" that prevents free electrons from diffusing across the junction at equilibrium. This creates the barrier potential, which in silicon is approximately 0.7V .

When forward bias is applied, the free electrons are provided with enough energy from the bias-voltage source to overcome the barrier potential and effectively "climb the energy hill" and cross the depletion region. The energy per charge that the electrons require in order to cross the depletion region is equal to the barrier potential. In other words, the electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the pn junction equal to the barrier potential (0.7V) , as indicated in Figure 2 to 5(b). An additional small voltage drop occurs across the p and n regions due to the internal resistance of the material. For doped semiconductive material, this resistance, called the dynamic resistance, is very small and can usually be neglected. This is discussed in more detail in Section 2 to 2.

Figure 2 to 1(b) summary: The figure is a schematic diagram. It depicts the symbol of a diode. The diode has two terminals: the anode and the cathode. The anode is on one side and the cathode is on the other side. The diode symbol indicates the direction of conventional current flow, suggesting that current flows more easily from the anode to the cathode than in the opposite direction.

Figure 2 to 2 summary: This figure is a collection of component diagrams. The figure illustrates various diode package types along with their terminal identification. Each diode is labeled with a letter K or A to indicate the cathode and anode, respectively, and a code to indicate the case style. The figure provides a visual reference for identifying different diode packages and their terminals. Different diodes have different case styles. The terminal identification is important for correct circuit implementation.

Figure 2 to 2(b) summary: The figure shows an image of an electronic component. The component is an SOD-323 diode package. The diode package has a rectangular body with metallic terminals extending from two sides. The SOD-323 package represents a type of diode commonly used in electronic circuits.

Figure 2 to 5(a) summary: The figure is a schematic illustration. It depicts a semiconductor device, specifically a p-n junction diode at equilibrium with no bias applied. The illustration shows the p-type and n-type semiconductor materials joined together, with a depletion region forming at their interface. The diode is in a state of equilibrium, meaning there is no external voltage applied. The depletion region is formed due to the diffusion of charge carriers across the junction, resulting in a region devoid of mobile carriers. The figure illustrates the basic structure of a p-n junction diode before any external voltage is applied, showing the formation of the depletion region.

Figure 2 to 5 summary: The figure is a schematic diagram. It illustrates a pn junction diode under forward bias. The diagram depicts the depletion region narrowing and a voltage drop being produced across the pn junction when the diode is forward-biased. The voltage drop is equal to the barrier potential. The forward bias causes the depletion region to become smaller, which facilitates current flow through the diode.

Reverse Bias

Reverse bias is the condition that essentially prevents current through the diode. Figure 2 to 6 shows a dc voltage source connected across a diode in the direction to produce reverse bias. This external bias voltage is designated as V sub BIAS just as it was for forward bias. Notice that the positive side of V sub BIAS is connected to the n region of the diode and the negative side is connected to the p region. Also note that the depletion region is shown much wider than in forward bias or equilibrium.

Figure 2 to 6 summary: The figure is a schematic diagram illustrating a diode connected for reverse bias. The diagram shows a diode with a p-region and an n-region connected in a circuit with a voltage source and a resistor. The positive side of the voltage source is connected to the n-region of the diode, and the negative side is connected to the p-region. From the figure, it is evident that a diode connected in reverse bias does not allow current to flow through it. The resistor is included to limit current in forward bias, but it is not relevant in reverse bias due to the absence of current.

An illustration of what happens when a diode is reverse-biased is shown in Figure 2 to 7. Because unlike charges attract, the positive side of the bias-voltage source "pulls" the free electrons, which are the majority carriers in the n region, away from the pn junction. As the electrons flow toward the positive side of the voltage source, additional holes are created at the depletion region. This results in a widening of the depletion region and fewer majority carriers.

Figure 2 to 7 summary: The figure is an illustration of a diode during the short transition time immediately after reverse bias voltage is applied. The illustration depicts a P-N junction diode with a depletion region. The P region contains holes, and the N region contains electrons. Under reverse bias, mobile charge carriers are swept away from the junction, resulting in a wider depletion region with fewer majority carriers. The depletion region has positive and negative ions.

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 charge. This results in a widening of the depletion region and a depletion of majority carriers. The flow of valence electrons can be viewed as holes being "pulled" toward the positive side.

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 electric field increases in strength until the potential across the depletion region equals the bias voltage, V sub BIAS. At this point, the transition current essentially ceases except for a very small reverse current that can usually be neglected.

Reverse Current The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs. The small number of free minority electrons in the p region are "pushed" toward the pn junction by the 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. Reverse current is illustrated in Figure 2 to 8.

Figure 2 to 8 summary: The figure is an illustration of a diode under reverse bias. It depicts the p-region, n-region, and the depletion region. The illustration shows that the reverse current in a reverse-biased diode is due to the minority carriers from thermally generated electron-hole pairs. The flow of minority carriers from the p-region to the n-region constitutes a small reverse current. This current is significantly smaller compared to the forward current in a forward-biased diode.

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.

This is what happens. The high reverse-bias voltage imparts energy to the free minority electrons so that as they speed through the p region, they collide with atoms with enough energy to knock valence electrons into the conduction band. The newly created conduction electrons are also high in energy and repeat the process. If one electron knocks only two others out of their valence orbit during its travel through the p region, the numbers quickly multiply. As these high-energy electrons go through the depletion region, they have enough energy to go through the n region as conduction electrons, rather than combining with holes.

The multiplication of conduction electrons just discussed is known as the avalanche effect, and reverse current can increase dramatically if steps are not taken to limit the current. When the reverse current is not limited, the resulting heating will permanently damage the diode. Most diodes are not operated in reverse breakdown, but if the current is limited (by adding a series-limiting resistor for example), there is no permanent damage to the diode.

Section 2 to 1 Check-Up Answers can be found at www.pearsonggloba Editions.com/ Floyd.

1. Describe forward bias of a diode.

2. Explain how to forward-bias a diode.

3. Describe reverse bias of a diode.

4. Explain how to reverse-bias a diode.

5. Compare the depletion regions in forward bias and reverse bias.

6. Which bias condition produces majority carrier current?

7. How is reverse current in a diode produced?

8. When does reverse breakdown occur in a diode?

9. Define avalanche effect as applied to diodes.

2 to 2 Voltage-Current Characteristic of a Diode

As you have learned, forward bias produces current through a diode and reverse bias essentially prevents current, except for a negligible reverse current. Reverse bias prevents current as long as the reverse-bias voltage does not equal or exceed the breakdown voltage of the junction. In this section, we will examine the relationship between the voltage and the current in a diode on a graphical basis.

After completing this section, you should be able to

- Analyze the voltage-current (V - I) characteristic of a diode.

- Explain the V - I characteristic for forward bias.

Graph the V - I curve for forward bias Describe how the barrier potential affects the V - I curve Define dynamic resistance

- Explain the V - I characteristic for reverse bias.

- Graph the V - I curve for reverse bias.

- Discuss the complete V - I characteristic curve.

- Describe the effects of temperature on the diode characteristic.

V-I Characteristic for Forward Bias

When a forward-bias voltage is applied across a diode, there is current. This current is called the forward current and is designated I sub F. Figure 2 to 9 illustrates what happens as the forward-bias voltage is increased positively from 0V. The resistor is used to limit the forward current to a value that will not overheat the diode and cause damage.

Figure 2 to 9 summary: The figure illustrates a circuit diagram designed to measure the forward voltage and forward current of a diode under varying bias voltages. The circuit includes a diode, a resistor, a bias voltage source, a voltmeter to measure the forward voltage across the diode, and an ammeter to measure the forward current through the diode. As the bias voltage is increased, the forward voltage across the diode increases, leading to a corresponding increase in the forward current. The forward voltage reaches and remains nearly constant at approximately 0.7 V. Forward current continues to increase as the bias voltage is increased.

With 0V across the diode, there is no forward current. As you gradually increase the forward-bias voltage, the forward current and the voltage across the diode gradually increase, as shown in Figure 2 to 9 (a). A portion of the forward-bias voltage is dropped across the limiting resistor. When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7V (barrier potential), the forward current begins to increase rapidly, as illustrated in Figure 2 to 9 (b).

As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases only gradually above 0.7V . This small increase in the diode voltage above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semiconductive material.

Graphing the V-I Curve If you plot the results of the type of measurements shown in Figure 2 to 9 on a graph, you get the V-I characteristic curve for a forward-biased diode, as shown in Figure 2 to 10 (a). The diode forward voltage (V sub F) increases to the right along the horizontal axis, and the forward current (I sub F) increases upward along the vertical axis.

Figure 2 to 10 summary: This is a curve that shows the relationship of voltage and current in a forward-biased diode. The figure illustrates the voltage-current characteristic curve for forward bias. The forward current increases very little until the forward voltage across the pn junction reaches approximately a certain voltage at the knee of the curve. After this point, the forward voltage remains nearly constant at approximately this voltage, but the forward current increases rapidly. The dynamic resistance decreases as you move up the curve, as indicated by the decrease in the value of voltage change over current change.

As you can see in Figure 2 to 10(a), the forward current increases very little until the forward voltage across the pn junction reaches approximately 0.7V at the knee of the curve. After this point, the forward voltage remains nearly constant at approximately 0.7V , but I sub F increases rapidly. As previously mentioned, there is a slight increase in V sub F above 0.7V as the current increases due mainly to the voltage drop across the dynamic resistance. The I sub F scale is typically in mA, as indicated.

Three points A, B, and C are shown on the curve in Figure 2 to 10 (a). Point A corresponds to a zero-bias condition. Point B corresponds to Figure 2 to 10 (a) where the forward voltage is less than the barrier potential of 0.7V. Point C corresponds to Figure 2 to 10 (a) where the forward voltage approximately equals the barrier potential. As the external bias voltage and forward current continue to increase above the knee, the forward voltage will increase slightly above 0.7V. In reality, the forward voltage can be as much as approximately 1V, depending on the forward current.

You have reached the end of the main document. Additional summarized content follows

Figure 2 to 9(a) summary: The figure is a circuit diagram. The diagram illustrates a circuit with a diode, a resistor, a voltage source, and two measurement devices. The circuit is designed to measure the forward voltage and forward current across the diode. As the bias voltage increases, the forward voltage across the diode remains relatively constant, while the forward current through the diode increases.

Figure 2 to 10(a) summary: The figure is a curve that represents the voltage-current characteristic for forward bias. The curve illustrates the relationship between forward voltage and forward current in a forward-biased diode. Initially, as the forward voltage increases, the forward current increases very gradually until the knee of the curve is reached. After this point, the forward current increases much more rapidly with further increases in forward voltage. The forward voltage remains nearly constant at approximately a certain value, while the forward current increases significantly above the knee. The forward voltage will increase slightly above a certain value, as the ready, the forward voltage can be as much as approximately a certain value, depending on the forward current.