Understanding Semiconductor Materials: Intrinsic, Extrinsic, N-Type, and P-Type

To understand how electronic devices work, you need to have a basic understanding of the structure of atoms and the interaction of atomic particles. This section expands the basic information of the previous section and introduces the P-N junction. P-N junctions are the basis of operation for many electronic devices, such as diodes and transistors.

Intrinsic Material

Materials such as silicon, germanium, and carbon are natural elements found in crystalline form. Instead of being a random mass, these atoms are arranged orderly. A crystal of silicon or germanium forms a definite geometric pattern, namely, a cube. Figure 1 illustrates a two-dimensional simplification of the silicon crystal showing only the nucleus and valence electrons. Note that the electrons of individual atoms are covalently bonded together. Germanium has a similar type of crystal structure.

Crystals of silicon and germanium can be manufactured by melting the natural elements. The process is somewhat complex and rather expensive. Manufactured crystals must be made extremely pure to be useable in semiconductor devices. A very pure semiconductor crystal is called an intrinsic material. Germanium is considered to be intrinsic when only 1 part of impurity exists in 1010 parts of germanium. Silicon is intrinsic when the impurity ratio is 1:1013. In more sophisticated solid-state devices, the ratio may be even higher.

An intrinsic crystal of silicon would appear as the structure in Figure 3. Covalent bonding changes the electrical conductivity of the material, causing each group of atoms to have a simulated condition of stability. Thus, only a limited number of free electrons are available for conduction. Therefore, silicon and germanium crystals respond to some extent as insulators.

Intrinsic silicon at −273◦ C is considered to be a perfect insulator. The valence electrons of each atom are firmly bonded together in perfect covalent bonds. No free electrons are available for conduction. In actual circuit operation, the absolute zero condition is not very meaningful because it cannot readily be attained. Any temperature above −273◦C causes silicon to become somewhat conductive. The insulating quality of a semiconductor material is, therefore, dependent on its operating temperature.

At room temperature, which is approximately 25◦C, silicon atoms receive enough energy from heat to break their bonding. A number of free electrons become available for conduction. At room temperature, intrinsic silicon becomes somewhat conductive.

Figure 1 Intrinsic crystal of silicon. The electrons of the individual atoms are covalently bonded together.

Holes

An important solid-state event takes place when intrinsic silicon goes into conduction. For every electron that is freed from its covalent bonding, a void is created. This spot, which is normally called a hole, represents an electron deficiency. A hole in a covalent bonding group occurs when an electron is released. An increase in the temperature of a piece of silicon causes the number of free electrons and holes to increase.

It should be remembered that when a neutral atom loses an electron, it acquires a positive charge. The atom then becomes a positive ion. Since an atom acquires a hole at the same time as the positive charge, the hole bears a positive charge. It should be noted, however, that the crystal remains electrically neutral. For every free electron, there is an equivalent hole. These two balance the overall charge of the crystal.

Hole Flow

When a valence electron leaves its covalent bond to become a free electron, a hole appears in its place. This hole can then attract a different electron from a nearby bonded group. On leaving its bonded group, the electron creates a new hole. The original hole is then filled and becomes electrically neutral. Each electron that leaves its bonding to fill a hole creates a new hole in its original group. In a sense, this means that electrons move in one direction and holes move in the opposite direction. Since electron movement is considered to be an electric current, holes are also representative of current flow.

Electrons are called negative current carriers and holes are positive current carriers that move in opposite directions in a semiconductor material. When voltage is applied, holes move toward the negative side of the source. Electron current flows toward the positive side of the source. This condition takes place only in a semiconductor material. In a conductor, such as copper, there is no covalent bonding. The current carriers in metallically bonded materials are electrons.

Figure 2 shows how an intrinsic piece of silicon responds at room temperature when voltage is applied. Note that the free electrons move toward the positive terminal of the battery. Electrons leaving the semiconductor flow into the copper connecting wire. Each electron that leaves the material creates a hole in its place. The holes appear to move by jumping between covalent bonded groups. Holes are attracted by the negative terminal connection. For each electron that flows out of the material, a new electron enters at the negative connection point.

Figure 2 Current carrier movement in silicon. Electrons and holes move in opposite directions.

 The process of hole flow and electron flow through the material is continuous as long as energy is supplied. Current flow is the resulting carrier movement. An intrinsic semiconductor has an equal number of current carriers moving in each direction. The resulting current flow of a semiconductor is limited primarily to the applied voltage and the operating temperature of the material.

Extrinsic Material

Pure silicon or germanium in its intrinsic state is rarely used as a semiconductor. Useable semiconductors must have controlled amounts of impurities added to them. The added impurities change the conduction capabilities of a semiconductor. The process of adding an impurity to an intrinsic material is called doping. The impurity is called a dopant. Doping a semiconductor causes it to be an extrinsic material. Extrinsic semiconductors are the operational basis of nearly all solid-state devices.

N-Type Material

An N-type material is formed when intrinsic silicon is mixed with a Group VB element, such as arsenic (As) and antimony (Sb). When these impurities are added to silicon or germanium, the crystal structure is unaltered.

Figure 3. N-type crystal material. The extra electron of each impurity atom does not take part in a covalent bonding group and, therefore, does not alter the crystal structure or bonding process.

 Atoms of arsenic and antimony have five electrons in their valence band. Adding this type of impurity to silicon does not alter the crystal structure or bonding process. Each impurity atom has an extra electron that does not take part in a covalent bonding group. These electrons are loosely held together by their parent atoms. Figure 3 shows how a silicon crystal is altered with the addition of an impurity atom.

When arsenic is added to pure silicon, the crystal becomes an N-type material. It has extra electrons or negative (N) charges that do not take part in the covalent bonding process. These electrons are free to move about through the crystal structure. Impurities that add electrons to a crystal are generally called donor atoms. An N-type material, therefore, has more extra free electrons than an intrinsic piece of material. A piece of N-material is not negatively charged; rather, its atoms are electrically neutral.

An extrinsic silicon crystal of the N-type will go into conduction with a very small amount of voltage applied. In contrast, an intrinsic crystal (pure silicon) requires a rather substantial amount of voltage or energy for its electrons to go into conduction. Essentially, this means that an N-type material is a fairly good electrical conductor. In this type of crystal, electrons are considered to be the majority current carriers; holes are the minority current carriers. The amount of donor material added to silicon determines the number of majority current carriers in its structure.

Figure 4 Current carriers in an N-type material. Electrons are the majority current carriers and holes are the minority current carriers.

The number of electrons in a piece of N-type silicon is a million or more times greater than the number of electron−hole pairs of a piece of intrinsic silicon. At room temperature, there is a decided difference in the electrical conductivity of this material. Extrinsic silicon becomes a rather good electrical conductor because there are a larger number of current carriers to take part in conduction. Current flow is achieved primarily by electrons in this material. Figure 4 shows how the current carriers respond in a piece of N-type material. There are more electrons indicated than holes; so electrons are the majority current carriers and holes are the minority carriers.

If the voltage source of Figure 4 were reversed, the current flow would reverse its direction. This means that N-type silicon conducts equally well in either direction. The flow of current carriers is simply reversed. This is an important consideration in the operation of a device that employs N-type material in its construction. The polarity of the external voltage determines the direction of current flow through the N-material.

P-Type Material

A P-type material is formed when intrinsic silicon is mixed with Group IIIA elements, such as indium (In) or gallium (Ga). Group IIIA elements are often called acceptors because they readily seek a fourth electron. This type of dopant material has three valence electrons. Each covalent bond that is formed with an indium atom has an electron deficiency or hole, which represents a positively charged area in the covalent bonding structure. Each hole in the P-type material can be filled with an electron. Electrons from neighboring covalent bond groups require very little energy to move in and fill a hole. Holes in the P-type material tend to wander from one covalent bond group to another.

The ratio of doping material to silicon is typically in the range of 1−106 or 1−1 million. This means that the P-type material has a million times more holes than the heat-generated electron−hole pairs of pure silicon. At room temperature, there is a very decided difference in the electrical conductivity of this material. Figure 5 shows how the crystal structure of silicon is altered when doped with an acceptor element. In this case, the dopant is indium.

Figure 5. P-type crystal material. The atoms of the crystal material form a covalent bond with indium atoms, creating a deficiency or hole in the covalent bonding structure. This hole represents a positively charged area.

Figure 6. Current carriers in a P-type material. Holes are the majority current carriers and electrons are the minority current carriers.

A P-type material will go into conduction with only a small amount of applied voltage. Intrinsic silicon, by comparison, requires substantially more voltage to produce conduction. Extrinsic silicon, therefore, is considered to be a rather good electrical conductor. In this material, holes are the majority carriers and electrons are the minority carriers. The amount of acceptor material added determines the number of majority current carriers in its structure.

Figure 6 shows how a P-type crystal responds when connected to a voltage source. Note that there are more holes than electrons. With voltage applied, electrons are attracted to the positive battery terminal. This is illustrated in Figure 6. Holes move, in a sense, toward the negative battery terminal. An electron is picked up at this point. The electron immediately fills a hole. At the same time, an electron is pulled from the material by the positive battery terminal, forming a hole. The hole is attracted to the negative battery terminal. Holes, therefore, move toward the negative battery terminal because electrons shift between different bonded groups. With energy applied, hole flow is continuous.

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