Section 1–2

• Insulating materials have very few free electrons and do not conduct current at all 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 a crystal together are called covalent bonds;

 

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 such as heat, it can jump out of the valence band and break away from its atom;

 

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
  1–20(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;
• 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 1-20(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 1–20(b);
• At this point, the junction is at equilibrium; and the depletion region is complete because diffusion has ceased;
• There is an energy gradiant 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;

 

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 1–19(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 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 1–19(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 0.7 V for silicon and 0.3 V for germanium at 25°C;
• Because germanium devices are not widely used, silicon will be used throughout the rest of the book;

 

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;
• 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 1–19(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-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 1–18, 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;
• 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;

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;

 

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 1–17, 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;

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;

 

N-Type and P-Type Semiconductors

• 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 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;

 

• 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;

 

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;
• 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 valence 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 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;

 

Conduction Electrons and Holes

• An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons;
• Free electrons are also called conduction electrons;
• This is illustrated in the energy diagram of Figure 1–13(a) and in the bonding diagram of Figure 1–13(b);

• When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal;
• This vacancy is called a hole;
• For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating what is called an electron-hole pair;
• Recombination occurs when a conduction-band electron loses energy and falls back into a hole in the valence band;
• To summarize, a piece of intrinsic silicon at room temperature has, at any instant, a number of conduction-band (free) electrons that are unattached to any atom and are essentially drifting randomly throughout the material;
• There is also an equal number of holes in the valence band created when these electrons jump into the conduction band. This is illustrated in Figure 1–14;