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I ntroductIon to
S emIconductorS
CHAPTER OUTLINE
1–1 The Atom
1–2 Materials Used in Electronic Devices
1–3 Current in Semiconductors
1–4 N -Type and P -Type Semiconductors
1–5 The PN Junction
CHAPTER OBJECTIVES
◆ ◆ (^) Describe the structure of an atom
◆ ◆ (^) Discuss insulators, conductors, and semiconductors and how they differ
◆ ◆ (^) Describe how current is produced in a semiconductor
◆ ◆ (^) Describe the properties of n -type and p -type semiconductors
◆ ◆ (^) Describe how a pn junction is formed
◆ ◆ Atom
◆ ◆ Proton
◆ ◆ Electron
◆ ◆ Shell
◆ ◆ Valence
◆ ◆ Ionization
◆ ◆ Free electron
◆ ◆ Orbital
◆ ◆ Insulator
◆ ◆ Conductor ◆ ◆ Semiconductor ◆ ◆ Silicon ◆ ◆ Crystal ◆ ◆ Hole ◆ ◆ Metallic bond ◆ ◆ Doping ◆ ◆ PN junction ◆ ◆ Barrier potential
VISIT THE WEBSITE Study aids for this chapter are available at https://www.pearsonhighered.com/careersresources/
INTROdUCTION Electronic devices such as diodes, transistors, and integrated circuits are made of a semiconductive material. To under- stand how these devices work, you should have a basic knowledge of the structure of atoms and the interaction of atomic particles. An important concept introduced in this chapter is that of the pn junction that is formed when two different types of semiconductive material are joined. The pn junction is fundamental to the operation of devices such as the solar cell, the diode, and certain types of transistors.
KEY TERMS
2 ◆^ Introduction to Semiconductors
The Bohr Model
An atom * is the smallest particle of an element that retains the characteristics of that ele- ment. Each of the known 118 elements has atoms that are different from the atoms of all other elements. This gives each element a unique atomic structure. According to the classi- cal Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons, as illustrated in Figure 1–1. The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. The basic particles of negative charge are called electrons. Each type of atom has a certain number of electrons and protons that distinguishes it from the atoms of all other elements. For example, the simplest atom is that of hydrogen, which has one proton and one electron, as shown in Figure 1–2(a). As another example, the helium atom, shown in Figure 1–2(b), has two protons and two neutrons in the nucleus and two electrons orbiting the nucleus.
Atomic Number
All elements are arranged in the periodic table of the elements in order according to their atomic number. The atomic number equals the number of protons in the nucleus, which is the same as the number of electrons in an electrically balanced (neutral) atom. For example, hydrogen has an atomic number of 1 and helium has an atomic number of 2. In their normal (or neutral) state, all atoms of a given element have the same number of electrons as protons; the positive charges cancel the negative charges, and the atom has a net charge of zero.
1–1 the A tom
All matter is composed of atoms; all atoms consist of electrons, protons, and neutrons except normal hydrogen, which does not have a neutron. Each element in the periodic table has a unique atomic structure, and all atoms for a given element have the same number of protons. At first, the atom was thought to be a tiny indivisible sphere. Later it was shown that the atom was not a single particle but was made up of a small, dense nucleus around which electrons orbit at great distances from the nucleus, similar to the way planets orbit the sun. Niels Bohr proposed that the electrons in an atom circle the nucleus in different obits, similar to the way planets orbit the sun in our solar system. The Bohr model is often referred to as the planetary model. Another view of the atom called the quantum model is considered a more accurate representation, but it is dif- ficult to visualize. For most practical purposes in electronics, the Bohr model suffices and is commonly used because it is easy to visualize. After completing this section, you should be able to ❑ (^) Describe the structure of an atom ◆ (^) Discuss the Bohr model of an atom ◆ (^) Define electron, proton, neutron, and nucleus ❑ (^) Define atomic number ❑ (^) Discuss electron shells and orbits ◆ (^) Explain energy levels ❑ (^) Define valence electron ❑ (^) Discuss ionization ◆ (^) Define free electron and ion ❑ (^) Discuss the basic concept of the quantum model of the atom
Niels Henrik David Bohr (October 7, 1885–November 18,
- was a Danish physicist, who made important contributions to understanding the structure of the atom and quantum mechanics by postulating the “planetary” model of the atom. He received the Nobel Prize in physics in 1922. Bohr drew upon the work or collaborated with scientists such as Dalton, Thomson, and Rutherford, among others and has been described as one of the most influential physicists of the 20th century.
H I S T O R Y N O T E
*All bold terms are in the end-of-book glossary. The bold terms in color are key terms and are also defined at the end of the chapter.
4 ◆^ Introduction to Semiconductors
1 H
Silicon Atomic number = 14
Helium Atomic number = 2
2 He
3 Li
4 Be
11 Na
12 Mg
71 Lu
58 Ce
60 Nd
61 Pm
62 Sm
63 Eu
59 Pr
57 La
64 Gd
65 Tb
67 Ho
68 Er
69 Tm
70 Yb
66 Dy
103 Lr
90 Th
89 Ac
92 U
93 Np
94 Pu
95 Am
91 Pa
96 Cm
97 Bk
99 Es
100 Fm
101 Md
102 No
98 Cf
19 K
20 Ca
21 Sc
22 Ti
23 V
24 Cr
25 Mn
26 Fe
27 Co
28 Ni
29 Cu
30 Zn
31 Ga
32 Ge
33 As
34 Se
35 Br
36 Kr
37 Rb
38 Sr
39 Y
40 Zr
41 Nb
42 Mo
43 Tc
44 Ru
45 Rh
46 Pd
47 Ag
48 Cd
49 In
50 Sn
51 Sb
52 Te
53 I
54 Xe
55 Cs
56 Ba
72 Hf
73 Ta
74 W
75 Re
76 Os
77 Ir
***** 78 Pt
79 Au
80 Hg
81 Tl
82 Pb
83 Bi
84 Po
85 At
86 Rn
5 B
6 C
7 N
8 O
9 F
10 Ne
13 Al
14 Si
15 P
16 S
17 Cl
18 Ar
87 Fr
88 Ra
104 Rf
105 Db
106 Sg
107 Bh
108 Hs
109 Mt
****** (^) 110 111 112 113 114 115 116 Ds Rg Cp Uut Uuq Uup Uuh
117 118 Uus Uuo
Shell 3 Shell 2 Shell 1
Nucleus 14p, 14n
▶ (^) FIGURE 1– Illustration of the Bohr model of the silicon atom.
▲ (^) FIGURE 1– The periodic table of the elements. Some tables also show atomic mass.
The Maximum Number of Electrons in Each Shell The maximum number of electrons ( Ne ) that can exist in each shell of an atom is a fact of nature and can be calculated by the formula, Equation 1–1 (^) N e 5 2 n^2 where n is the number of the shell. The maximum number of electrons that can exist in the innermost shell (shell 1) is Ne = 2 n^2 = 2(1)^2 = 2
The Atom ◆^5
The maximum number of electrons that can exist in shell 2 is
Ne = 2 n^2 = 2(2)^2 = 2(4) = 8
The maximum number of electrons that can exist in shell 3 is
Ne = 2 n^2 = 2(3)^2 = 2(9) = 18
The maximum number of electrons that can exist in shell 4 is
Ne = 2 n^2 = 2(4)^2 = 2(16) = 32
Valence Electrons
Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus. This is because the force of attraction between the positively charged nucleus and the negatively charged electron decreases with increasing distance from the nucleus. Electrons with the highest energy exist in the outermost shell of an atom and are relatively loosely bound to the atom. This outermost shell is known as the valence shell, and electrons in this shell are called valence electrons. These valence electrons contribute to chemical reactions and bonding within the structure of a material and determine its electrical properties. When a valence electron gains sufficient energy from an external source, it can break free from its atom. This is the basis for conduction in materials.
Ionization
When an atom absorbs energy, the valence electrons can easily jump to higher energy shells. If a valence electron acquires a sufficient amount of energy, called ionization energy, it can actually escape from the outer shell and the atom’s influence. The departure of a valence electron leaves a previously neutral atom with an excess of positive charge (more protons than electrons). The process of losing a valence electron is known as ionization , and the resulting positively charged atom is called a positive ion. For example, the chemical symbol for hydrogen is H. When a neutral hydrogen atom loses its valence electron and becomes a positive ion, it is designated H+^. The escaped valence electron is called a free electron. The reverse process can occur in certain atoms when a free electron collides with the atom and is captured, releasing energy. The atom that has acquired the extra electron is called a negative ion. The ionization process is not restricted to single atoms. In many chemical reac- tions, a group of atoms that are bonded together can lose or acquire one or more electrons. For some nonmetallic materials such as chlorine, a free electron can be captured by the neutral atom, forming a negative ion. In the case of chlorine, the ion is more stable than the neutral atom because it has a filled outer shell. The chlorine ion is designated as Cl-.
The Quantum Model
Although the Bohr model of an atom is widely used because of its simplicity and ease of visualization, it is not a complete model. The quantum model is considered to be more ac- curate. The quantum model is a statistical model and very difficult to understand or visualize. Like the Bohr model, the quantum model has a nucleus of protons and neutrons surrounded by electrons. Unlike the Bohr model, the electrons in the quantum model do not exist in precise circular orbits as particles. Three important principles underlie the quantum model: the wave-particle duality principle, the uncertainty principle, and the superposition principle.
◆ ◆ (^) Wave-particle duality. Just as light can be thought of as exhibiting both a wave and a particle (photon), electrons are thought to exhibit a wave-particle duality. The velocity of an orbiting electron is related to its wavelength, which interferes with neighboring electron wavelengths by amplifying or canceling each other. ◆ ◆ (^) Uncertainly principle. As you know, a wave is characterized by peaks and valleys; therefore, electrons acting as waves cannot be precisely identified in terms of their position. According to a principle ascribed to Heisenberg, it is impossible to deter- mine simultaneously both the position and velocity of an electron with any degree
Atoms are extremely small and cannot be seen even with the strongest optical microscopes; however, a scanning tunneling microscope can detect a single atom. The nucleus is so small and the electrons orbit at such distances that the atom is mostly empty space. To put it in perspective, if the proton in a hydrogen atom were the size of a golf ball, the electron orbit would be approximately one mile away. Protons and neutrons are approximately the same mass. The mass of an electron is 1> 1836 of a proton. Within protons and neutrons there are even smaller particles called quarks. Quarks are the subject of intense study by particle physicists as they help explain the existence of more than 100 subatomic particles.
F Y I
Materials Used in Electronic Devices ◆^7
In a three-dimensional representation of the quantum model of an atom, the s- orbitals are shaped like spheres with the nucleus in the center. For energy level 1, the sphere is a single sphere, but for energy levels 2 or more, each single s -orbital is composed of nested spherical shells. A p- orbital for shell 2 has the form of two ellipsoidal lobes with a point of tangency at the nucleus (sometimes referred to as a dumbbell shape.) The three p -orbitals in each energy level are oriented at right angles to each other. One is oriented on the x- axis, one on the y- axis, and one on the z- axis. For example, a view of the quantum model of a sodium atom (Na) that has 11 electrons as shown in Figure 1–5. The three axes are shown to give you a 3-D perspective.
2 p z orbital (2 electrons)
2 p x orbital (2 electrons)
2 p y orbital (2 electrons)
1 s orbital (2 electrons) 2 s orbital (2 electrons) 3 s orbital (1 electron)
Nucleus
x-axis
z-axis
y-axis
◀ (^) FIGURE 1– Three-dimensional quantum model of the sodium atom, showing the orbitals and number of electrons in each orbital.
- Describe the Bohr model of the atom.
- Define electron.
- What is the nucleus of an atom composed of? Define each component.
- Define atomic number.
- Discuss electron shells and orbits and their energy levels.
- What is a valence electron?
- What is a free electron?
- Discuss the difference between positive and negative ionization.
- Name three principles that distinguish the quantum model.
SECTION 1– CHECKUP Answers can be found at www .pearsonhighered.com/floyd.
1–2 mAterIAlS uSed In electronIc devIceS
In terms of their electrical properties, materials can be classified into three groups: conductors, semiconductors, and insulators. When atoms combine to form a solid, crys- talline material, they arrange themselves in a symmetrical pattern. The atoms within a semiconductor crystal structure are held together by covalent bonds, which are created by the interaction of the valence electrons of the atoms. Silicon is a crystalline material. After completing this section, you should be able to ❑ (^) Discuss insulators, conductors, and semiconductors and how they differ ◆ (^) Define the core of an atom ◆ (^) Describe the carbon atom ◆ (^) Name two types each of semiconductors, conductors, and insulators ❑ (^) Explain the band gap ◆ (^) Define valence band and conduction band ◆ (^) Compare a semiconductor atom to a conductor atom ❑ (^) Discuss silicon and gemanium atoms ❑ (^) Explain covalent bonds ◆ (^) Define crystal
8 ◆^ Introduction to Semiconductors
Insulators, Conductors, and Semiconductors
All materials are made up of atoms. These atoms contribute to the electrical properties of a material, including its ability to conduct electrical current. For purposes of discussing electrical properties, an atom can be represented by the valence shell and a core that consists of all the inner shells and the nucleus. This concept is illustrated in Figure 1–6 for a carbon atom. Carbon is used in some types of electrical resistors. Notice that the carbon atom has four electrons in the valence shell and two electrons in the inner shell. The nucleus consists of six protons and six neutrons, so the +6 indicates the positive charge of the six protons. The core has a net charge of +4 ( +6 for the nucleus and - 2 for the two inner-shell electrons). Insulators An insulator is a material that does not conduct electrical current under nor- mal conditions. Most good insulators are compounds rather than single-element materials and have very high resistivities. Valence electrons are tightly bound to the atoms; there- fore, there are very few free electrons in an insulator. Examples of insulators are rubber, plastics, glass, mica, and quartz. Conductors A conductor is a material that easily conducts electrical current. Most met- als are good conductors. The best conductors are single-element materials, such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al), which are characterized by atoms with only one valence electron very loosely bound to the atom. These loosely bound valence electrons can become free electrons with the addition of a small amount of energy to free them from the atom. Therefore, in a conductive material the free electrons are available to carry current. Semiconductors A semiconductor is a material that is between conductors and insula- tors in its ability to conduct electrical current. A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. Single-element semiconductors are an- timony (Sb), arsenic (As), astatine (At), boron (B), polonium (Po), tellurium (Te), silicon (Si), and germanium (Ge). Compound semiconductors such as gallium arsenide, indium phosphide, gallium nitride, silicon carbide, and silicon germanium are also commonly used. The single-element semiconductors are characterized by atoms with four valence electrons. Silicon is the most commonly used semiconductor.
Band Gap
In solid materials, interactions between atoms “smear” the valence shell into a band of energy levels called the valence band. Valence electrons are confined to that band. When an electron acquires enough additional energy, it can leave the valence shell, become a free electron, and exist in what is known as the conduction band. The difference in energy between the valence band and the conduction band is called an energy gap or band gap. This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom. Figure 1–7 shows energy diagrams for insulators, semiconductors, and conductors. The energy gap or band gap is the difference between two energy levels and electrons are “not allowed” in this energy gap based on quantum theory. Although an electron may not exist in this region, it can “jump” across it under certain conditions. For insulators, the gap can be crossed only when breakdown conditions occur—as when a very high voltage is ap- plied across the material. The band gap is illustrated in Figure 1–7(a) for insulators. In semiconductors the band gap is smaller, allowing an electron in the valence band to jump into the conduction band if it absorbs a photon. The band gap depends on the semicon- ductor material. This is illustrated in Figure 1–7(b). In conductors, the conduction band and valence band overlap, so there is no gap, as shown in Figure 1–7(c). This means that electrons in the valence band move freely into the conduction band, so there are always electrons available as free electrons.
Core (+4)
Valence electrons
▲ (^) FIGURE 1–
Diagram of a carbon atom.
Next to silicon, the second most common semiconductive material is gallium arsenide, GaAs. This is a crystalline compound, not an element. Its properties can be controlled by varying the relative amount of gallium and arsenic. GaAs has the advantage of making semiconductor devices that respond very quickly to electrical signals. It is widely used in high-frequency applications and in light-emitting diodes and solar cells.
F Y I
10 ◆^ Introduction to Semiconductors
The valence electrons in germanium are in the fourth shell while those in silicon are in the third shell, closer to the nucleus. This means that the germanium valence electrons are at higher energy levels than those in silicon and, therefore, require a smaller amount of ad- ditional energy to escape from the atom. This property makes germanium more unstable at high temperatures and results in excessive reverse current. This is why silicon is a more widely used semiconductive material. Covalent Bonds Figure 1–10 shows how each silicon atom positions itself with four adjacent silicon atoms to form a silicon crystal , which is a three-dimensional symmetrical arrangement of atoms. A silicon (Si) atom with its four valence electrons shares an electron with each of its four neighbors. This effectively creates eight shared valence electrons for each atom and produces a state of chemical stability. Also, this sharing of valence electrons produces a strong covalent bond that hold the atoms together; each valence electron is at- tracted equally by the two adjacent atoms which share it. Covalent bonding in an intrinsic silicon crystal is shown in Figure 1–11. An intrinsic crystal is one that has no impurities. Covalent bonding for germanium is similar because it also has four valence electrons.
Germanium atom
Silicon atom
Four valence electrons in the outer (valence) shell
▶ (^) FIGURE 1–
Diagrams of the silicon and germa- nium atoms.
(a) (b) Bonding diagram. The red negative signs represent the shared valence electrons.
- – – – - - - -
The center silicon atom shares an electron with each of the four surrounding silicon atoms, creating a covalent bond with each. The surrounding atoms are in turn bonded to other atoms, and so on.
Si
Si Si Si
Si
▶ (^) FIGURE 1–
Illustration of covalent bonds in silicon.
Current in Semiconductors ◆^11
**- – – – – – – – – – – –
- – – – – – – – – – – –
- – – – – – – – – – – –
- – – – – – – – – – – –** - - - - - - - - - **-
-**
Si Si Si Si Si
Si Si Si Si Si
Si Si Si Si Si
Si Si Si Si Si
◀ (^) FIGURE 1– Covalent bonds in a silicon crystal.
- What is the basic difference between conductors and insulators?
- How do semiconductors differ from conductors and insulators?
- How many valence electrons does a conductor such as copper have?
- How many valence electrons does a semiconductor have?
- Name three of the best conductive materials.
- What is the most widely used semiconductive material?
- Why does a semiconductor have fewer free electrons than a conductor?
- How are covalent bonds formed?
- What is meant by the term intrinsic?
- What is a crystal?
SECTION 1– CHECKUP
1–3 current In S emIconductorS The way a material conducts electrical current is important in understanding how electronic devices operate. You can’t really understand the operation of a device such as a diode or transistor without knowing something about current in semiconductors. After completing this section, you should be able to ❑ (^) Describe how current is produced in a semiconductor ❑ (^) Discuss conduction electrons and holes ◆ (^) Explain an electron-hole pair ◆ (^) Discuss recombination ❑ (^) Explain electron and hole current
As you have learned, the electrons in a solid can exist only within prescribed energy bands. Each shell corresponds to a certain energy band and is separated from adjacent shells by band gaps, in which no electrons can exist. Figure 1–12 shows the energy band diagram for the atoms in a pure silicon crystal at its lowest energy level. There are no electrons shown in the conduction band, a condition that occurs only at a temperature of absolute 0 Kelvin.
Current in Semiconductors ◆^13
Electron and Hole Current
When a voltage is applied across a piece of intrinsic silicon, as shown in Figure 1–15, 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.
**- – – – – – – – – –
- – – – – – – – – – – –
- – – – – – – – – – – –
- – – – – – – – – – – –**
Si Si Si Si Si
Si Si Si Si Si
Si Si Si Si Si
Si Si Si Si Si
Generation of an electron-hole pair
Recombination of an electron with a hole
Heat energy
- –
Si Si Si Si
Si Si Si Si Si
Si Si Si Si Si
V
Si
◀ (^) FIGURE 1– Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.
◀ (^) FIGURE 1– Electron current in intrinsic silicon is produced by the movement of ther- mally 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 1–16. Although current in the va- lence band is produced by valence electrons, it is called hole current to distinguish it from electron current in the conduction band. As you have seen, conduction in semiconductors is considered to be either the move- ment 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 differ- ent 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.
14 ◆^ Introduction to Semiconductors
A free electron leaves hole in valence shell.
A valence electron moves into 2nd hole and leaves a 3rd hole.
A valence electron moves into 4th hole and leaves a 5th hole. A valence electron moves into 1st hole and leaves a 2nd hole.
A valence electron moves into 3rd hole and leaves a 4th hole.
A valence electron moves into 5th hole and leaves a 6th hole.
When a valence electron moves left to right to fill a hole while leaving another hole behind, the hole has effectively moved from right to left. Gray arrows indicate effective movement of a hole.
5 3 1
6 4 2
Si Si Si
▶ (^) FIGURE 1–
Hole current in intrinsic silicon.
- Are free electrons in the valence band or in the conduction band?
- Which electrons are responsible for electron current in silicon?
- What is a hole?
- At what energy level does hole current occur?
SECTION 1– CHECKUP
Since semiconductors are generally poor conductors, their conductivity can be drasti- cally increased by the controlled addition of impurities to the intrinsic (pure) semiconduc- tive material. This process, called doping , increases the number of current carriers (elec- trons 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 im- purity atoms are added. These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb).
1–4 N - type And P - type S emIconductorS
Semiconductive 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 conduc- tion 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 conductiv- ity 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
16 ◆^ Introduction to Semiconductors
Hole from B atom
Si B
Si
Si
Si
▶ (^) FIGURE 1– Trivalent impurity atom in a silicon crystal structure. A boron (B) impu- rity atom is shown in the center.
- Define doping.
- What is the difference between a pentavalent atom and a trivalent atom?
- What are other names for the pentavalent and trivalent atoms?
- How is an n -type semiconductor formed?
- How is a p -type semiconductor formed?
- What is the majority carrier in an n -type semiconductor?
- What is the majority carrier in a p -type semiconductor?
- By what process are the majority carriers produced?
- By what process are the minority carriers produced?
- What is the difference between intrinsic and extrinsic semiconductors?
SECTION 1– CHECKUP
1–5 the PN J unctIon
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 be- tween 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 ◆ (^) State the values of barrier potential in silicon and germanium ❑ (^) 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.
The PN Junction ◆^17
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 1–19(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).
p region n region
pn junction
(a) The basic silicon structure at the instant of junction formation showing only the majority and minority carriers. Free electrons in the n region near the pn junction begin to diffuse across the junction and fall into holes near the junction in the p region.
p region n region
Depletion region
Barrier potential For every electron that diffuses across the junction and combines with a hole, a positive charge is left in the n region and a negative charge is created in the p region, forming a barrier potential. This action continues until the voltage of the barrier repels further diffusion. The blue arrows between the positive and negative charges in the depletion region represent the electric field.
(b)
▲ (^) FIGURE 1–
Formation of the depletion region. The width of the depletion region is exaggerated for illustration purposes.
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 dif- fuse across the junction into the p region where they combine with holes near the junction, as shown in Figure 1–19(b). 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 1–19(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
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.
H I S T O R Y N O T E
Summary ◆^19
Majority carriers Minority carriers
Majority carriers Minority carriers
Conduction band
Valence band
(^0) p region pn junction n region
Conduction band
Valence band
(^0) p region pn junction and depletion region
n region
Energy Energy
(a) At the instant of junction formation (b) At equilibrium
▲ (^) FIGURE 1–
Energy diagrams illustrating the formation of the pn junction and depletion region.
- What is a pn junction?
- Explain diffusion.
- Describe the depletion region.
- Explain what the barrier potential is and how it is created.
- What is the typical value of the barrier potential for a silicon diode?
- What is the typical value of the barrier potential for a germanium diode?
SECTION 1– CHECKUP
SUMMARY
Section 1–1 ◆^ 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 1–2 ◆^ 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 semiconduc- tive 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.
20 ◆^ Introduction to Semiconductors
Section 1–3 ◆^ The valence electrons that manage to escape from their parent atom are called conduction elec- trons 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 recombina- tion. 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 to- ward 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. Section 1–4 ◆^ 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 pro- cess, 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. Section 1–5 ◆^ 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 deple- tion region is formed by ionization. ◆ (^) The barrier potential is typically 0.7 V for a silicon diode and 0.3 V for germanium.
KEY TERMS 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 pn junc- tion 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 proper- ties. 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.
