1. Semiconductor Materials

Semiconductors are a group of materials having conductivities between those of met als and insulators.

Gallium arsenide is one of the more common of the compound semiconductors. Its good optical properties make it useful in optical devices. GaAs is also used in specialized applications in which, for example, high speed is required. 

2. Types of Solids

Amorphous, polycrystalline, and single crystals are the three general types of solids.

An ordered region is a spatial volume in which atoms or molecules have a regular geometric arrangement or periodicity. Amorphous materials have order only within a few atomic or molecular dimensions, while polycrystalline materials have a high degree of order over many atomic or molecular dimensions.

The single-crystal regions are called grains.

Single-crystal materials,have a high degree of order, throughout the entire volume of the material. The advantage its electrical properties.

3. Space Lattices

A representative unit, or a group of atoms, is repeated at regular intervals in each of the three dimensions to form the single crystal. The periodic arrangement of atoms in the crystal is called the lattice.

3.1 Primitive and Unit Cell

 We can represent a particular atomic array by a dot that is called a lattice point.

The simplest means of repeating an atomic array is by translation. Each lattice point in Figure 1.2 can be translated a distance a1 in one direction and a distance b1 in a second nonco linear direction to generate the two-dimensional lattice. The translation directions need not be perpendicular.

A unit cell is a small volume of the crystal that can be used to reproduce the entire crystal.

 Figure 1.3 shows several possible unit cells in a two-dimensional lattice. 

 A primitive cell is the smallest unit cell that can be repeated to form the lattice.

the lattice is characterized by three vectors a , b , and c , which need not be perpendicular.

equivalent lattice point in the three-dimensional crystal can be found using the vector

where p , q , and s are integers.

The magnitudes of the vectors a , b , and c are the lattice constants of the unit cell. 

3.2 Basic Crystal Structures

The lattice constant of each unit cell in Figure 1.5 is designated as “ a .” The simple cubic (sc) structure has an atom located at each corner; the body-centered cubic (bcc) structure has an additional atom at the center of the cube; and the face-centered cubic (fcc) structure has addi tional atoms on each face plane.  

3.3 Crystal Planes and Miller Indices

Semiconductor devices are fabricated at or near a surface, so the surface proper ties may influence the device characteristics.

Each face plane of the cubic structure shown in Figure 1.8a is entirely equivalent. These planes are grouped together and are referred to as the {100} set of planes. We may also consider the planes shown in Figures 1.8b and 1.8c. The intercepts of the plane shown in Figure 1.8b are p = 1, q = 1, and s = infinity  . 

The Miller indices are found by taking the reciprocal of these intercepts and, as a result, this plane is referred to as the (110) plane. In a similar way, the plane shown in Figure 1.8c is referred to as the (111) plane. 

One characteristic of a crystal that can be determined is the distance between nearest equivalent parallel planes. Another characteristic is the surface concentration of atoms, number per square centimeter (#/cm 2 ), that are cut by a particular plane. 

Again, a single-crystal semiconductor is not infi nitely large and must terminate at some surface. The surface density of atoms may be important, for example, in determining how another material, such as an insulator, will “fi t” on the surface of a semiconductor material. 

4. The Diamond Structure

Silicon has a diamond crystal structure. Germanium has the same diamond structure.

An important characteristic of the diamond lattice is that any atom within the diamond structure will have four nearest neighboring atoms. 

The zincblende (sphalerite) structure differs from the diamond structure only in that there are two different types of atoms in the lattice.

5. Atomic Bonding

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6. Imperfection and Impurities in Solids

Imperfections tend to alter the electrical properties of a material and, in some cases, electrical parameters can be dominated by these defects or impurities.

6.1 Imperfections in Solids*

One type of imperfection that all crystals have in common is atomic thermal vibration

A perfect single crystal contains atoms at particular lattice sites, the atoms separated from each other by a distance we have assumed to be constant.

The thermal energy causes the atoms to vibrate in a random manner about an equilibrium lattice point.

The thermal energy causes the atoms to vibrate in a random manner about an equilib rium lattice point. This random thermal motion causes the distance between atoms to randomly fluctuate, slightly disrupting the perfect geometric arrangement of atoms. This imperfection, called lattice vibrations, affects some electrical parameters, as we will see later in our discussion of semiconductor material characteristics. 

Another type of defect is called a point defect. There are several of this type that we need to consider. Again, in an ideal single-crystal lattice, the atoms are arranged in a perfect periodic arrangement. However, in a real crystal, an atom may be missing from a particular lattice site. This defect is referred to as a vacancy; it is schemati cally shown in Figure 1.18a.

In another situation, an atom may be located between lattice sites. This defect is referred to as an interstitial and is schematically shown in Figure 1.18b. In the case of vacancy and interstitial defects, not only is the perfect geometric arrangement of atoms broken but also the ideal chemical bonding between atoms is disrupted, which tends to change the electrical properties of the material.

A vacancy and interstitial may be in close enough proximity to exhibit an interac tion between the two point defects. This vacancy–interstitial defect, also known as a Frenkel defect, produces different effects than the simple vacancy or interstitial.

The point defects involve single atoms or single-atom locations. In forming single-crystal materials, more complex defects may occur. A line defect, for example, occurs when an entire row of atoms is missing from its normal lattice site. This de fect is referred to as a line dislocation and is shown in Figure 1.19.

As with a point defect, a line dislocation disrupts both the normal geometric periodicity of the lattice and the ideal atomic bonds in the crystal. This dislocation can also alter the electrical properties of the material, usually in a more unpredictable manner than the simple point defects.

Other complex dislocations can also occur in a crystal lattice. However, this introductory discussion is intended only to present a few of the basic types of defect, and to show that a real crystal is not necessarily a perfect lattice structure. The effect of these imperfections on the electrical properties of a semiconductor will be consid ered in later chapters. 

6.2 Impurities in Solids 

Impurity atoms may be located at normal lattice sites, in which case they are called substitutional impurities. Impurity atoms may also be located between normal sites, in which case they are called interstitial impurities.

Both these impurities are lattice defects and are schematically shown in Figure 1.20. Some impurities, such as oxygen in silicon, tend to be essentially inert; however, other impurities, such as gold or phosphorus in silicon, can drastically alter the electrical properties of the material.

In Chapter 4 we will see that, by adding controlled amounts of particular impu rity atoms, the electrical characteristics of a semiconductor material can be favorably altered.

The technique of adding impurity atoms to a semiconductor material in order to change its conductivity is called doping. There are two general methods of doping: impurity diffusion and ion implantation.

The actual diffusion process depends to some extent on the material but, in gen eral, impurity diffusion occurs when a semiconductor crystal is placed in a high temperature (1000ºC) gaseous atmosphere containing the desired impurity atom. At this high temperature, many of the crystal atoms can randomly move in and out of their single-crystal lattice sites.

Vacancies may be created by this random motion so that impurity atoms can move through the lattice by hopping from one vacancy to another. Impurity diffusion is the process by which impurity particles move from a region of high concentration near the surface to a region of lower concentration within the crystal. 

When the temperature decreases, the impurity atoms become per manently frozen into the substitutional lattice sites. Diffusion of various impurities into selected regions of a semiconductor allows us to fabricate complex electronic circuits in a single semiconductor crystal. 

Ion implantation generally takes place at a lower temperature than diffusion. A beam of impurity ions is accelerated to kinetic energies in the range of 50 keV or greater and then directed to the surface of the semiconductor. 

The high-energy impurity ions enter the crystal and come to rest at some average depth from the surface. One advantage of ion implantation is that controlled numbers of impurity atoms can be introduced into specifi c regions of the crystal. 

A disadvantage of this technique is that the incident impurity atoms collide with the crystal atoms, causing lattice-displacement damage. However, most of the lattice damage can be removed by thermal annealing, in which the temperature of the crystal is raised for a short time. Thermal annealing is a required step after implantation.

7. Growth of Semiconductor Materials*

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7.1 Growth from a Melt

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7.2 Eptiaxial Growth

Chemical vapor-phase deposition (CVD).

1.8 Summary

• A few of the most common semiconductor materials were listed. Silicon is the most common semiconductor material and appears in column IV of the periodic table.  

• The properties of semiconductors and other materials are determined to a large extent by the single-crystal lattice structure. The unit cell is a small volume of the crystal that  is used to reproduce the entire crystal. Three basic unit cells are the simple cubic, body centered cubic, and face-centered cubic.  

• Silicon has the diamond crystal structure. Atoms are formed in a tetrahedral configuration with four nearest neighbor atoms. The binary semiconductors have a zincblende lattice that is basically the same as the diamond lattice.  

• Miller indices are used to describe planes in a crystal lattice. These planes may be used to describe the surface of a semiconductor material. The Miller indices are also used to describe directions in a crystal.  

• Imperfections do exist in semiconductor materials. A few of these imperfections are vacancies, substitutional impurities, and interstitial impurities. Small amounts of controlled substitutional impurities can favorably alter semiconductor properties as we will see in later chapters.  

• A brief description of semiconductor growth methods was given. Bulk growth, such as the Czochralski method, produces the starting semiconductor material or substrate. 

• Epitaxial growth can be used to control the surface properties of a semiconductor. Most semiconductor devices are fabricated in the epitaxial layer. 

Glossary of Important Terms  

binary semiconductor: A two-element compound semiconductor, such as gallium arsenide (GaAs).  

covalent bonding: The bonding between atoms in which valence electrons are shared.  

diamond lattice: The atomic crystal structure of silicon, for example, in which each atom has four nearest neighbors in a tetrahedral configuration.  

doping: The process of adding specifi c types of atoms to a semiconductor to favorably alter the electrical characteristics.  

elemental semiconductor: A semiconductor composed of a single species of atom, such as silicon or germanium.  

epitaxial layer: A thin, single-crystal layer of material formed on the surface of a substrate.  

ion implantation: One particular process of doping a semiconductor.  

lattice: The periodic arrangement of atoms in a crystal. 

Miller indices: The set of integers used to describe a crystal plane.  

primitive cell: The smallest unit cell that can be repeated to form a lattice.  

substrate: A semiconductor wafer or other material used as the starting material for further semiconductor processing, such as epitaxial growth or diffusion.  

ternary semiconductor: A three-element compound semiconductor, such as aluminum gallium arsenide (AlGaAs).  

unit cell: A small volume of a crystal that can be used to reproduce the entire crystal.  

zincblende lattice: A lattice structure identical to the diamond lattice except that there are two types of atoms instead of one.

Now, there are some review questions and some practice questions left. I will update as I proceed along the solving of questions.... 

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