SEMICONDUCTOR - HOW IT WORKS? BASIC INFORMATION

In a semiconductor, electrons flow, but not as well as they do in a conductor. You might imagine the people in the line being lazy and not too eager to pass the balls along.

Some semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost as bad as insulating materials. The people might be just  little sluggish, or they might be almost asleep.

Semiconductors are not exactly the same as resistors. In a semiconductor, the material is treated so that it has very special properties. The semiconductors include certain substances, such as silicon, selenium, or gallium, that have been “doped” by the addition of impurities like indium or antimony.

Perhaps you have heard of such things as gallium arsenide, metal oxides, or silicon rectifiers. Electrical conduction in these materials is always a result of the motion of electrons.

However, this can be a quite peculiar movement, and sometimes engineers speak of the movement of holes rather than electrons. A hole is a shortage of an electron—you might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons.


When most of the charge carriers are electrons, the semiconductor is called N-type, because electrons are negatively charged. When most of the charge carriers are holes, the semiconducting material is known as P type because holes have a positive electric charge.

But P-type material does pass some electrons, and N-type material carries some holes. In a semiconductor, the more abundant type of charge carrier is called the majority carrier. The less abundant kind is known as the minority carrier.

Semiconductors are used in diodes, transistors, and integrated circuits in almost limitless variety. These substances are what make it possible for you to have a computer in a briefcase.

That notebook computer, if it used vacuum tubes, would occupy a skyscraper, because it has billions of electronic components. It would also need its own power plant, and would cost thousands of dollars in electric bills every day.

But the circuits are etched microscopically onto semiconducting wafers, greatly reducing the size and power requirements.

Semiconductor is so little that we barely recognize it in our daily life. Share us your experience on a time wherein we recognized the existence  of semiconductors.

SEMICONDUCTOR THEORY BASIC INFORMATION AND TUTORIALS

Semiconductor materials have physical characteristics that are totally different from those of metals. Whereas metals conduct electricity at all temperatures, semiconductors conduct well at some temperatures and poorly at others.

In the preceding section, it was shown that semiconductors are covalent solids. That is, the atoms form covalent bonds with themselves, the most important being silicon and germanium.

Others may form semiconductor compounds where two or more elements form covalent bonds, such as gallium (Group III) and arsenic (Group V), which combine to form gallium arsenide.

Typical semiconductor materials used in the fabrication of IC chips are

■ Elemental semiconductors
– Silicon
– Germanium
– Selenium

■ Semiconducting compounds
– Gallium arsenide (GaAs)
– Gallium arsenide–phosphide (FaAsP)
– Indium phosphide (InP)

Germanium is an elemental semiconductor that was used to fabricate the first transistors and solid state devices. But, because it is difficult to process and inhibits device performance, it is rarely used now.

The other elemental semiconductor, silicon, is used in approximately 90 percent of the chips fabricated. Silicon’s popularity can be attributed to its abundance in nature and retention of good electrical properties, even at high temperatures. In addition, its silicon dioxide (SiO2) has many properties ideally suited to IC manufacturing.

Gallium arsenide is classified as a semiconducting compound. Some of its properties, such as faster operating frequencies (two to three times faster than silicon), low heat dissipation, resistance to radiation, and minimal leakage between adjacent components, makes GaAs an important semiconductor for use in high-performance applications. Its drawbacks are the difficulty of growing the ingots and fabricating the ICs.

An elemental or compound semiconductor that was not contaminated by the introduction of impurities is called an intrinsic semiconductor. At an absolute zero temperature, intrinsic semiconductors form stable covalent bonds that have valence shells completely filled with electrons.

These covalent bonds are very strong, so that each electron is held very strongly to the atoms sharing it. Thus, there are no free electrons available, and no electrical conduction is possible. As the temperature is raised to relatively high temperatures, the valence bonds sometimes break, and electrons are released.

The free electrons behave in the same way as free electrons in a metal; therefore, electrical conduction is now possible when an electric field is applied.

If an impurity, such as phosphorus or boron, is introduced into the crystal structure of an intrinsic semiconductor, its chemical state is altered to where the semiconductor will have an excess or deficiency of electrons, depending on the impurity type used. The process of adding a small quantity of impurities to an intrinsic semiconductor is called doping.

SEMICONDUCTORS DEFINITION AND BASIC KNOWLEDGE

SEMICONDUCTORS BASIC INFORMATION
What Are Semiconductors?


Semiconductors are a category of materials with an electrical conductivity that is between that of conductors and insulators. Good conductors, which are all metals, have electrical resistivities down in the range of 10−6 -cm.

Insulators have electrical resistivities that are up in the range from 10^6 to as much as about 1012 -cm. Semiconductors have resistivities that are generally in the range of 10^−4 up to 10^ 4 ohm-cm. The resistivity of a semiconductor is strongly influenced by impurities, called dopants, that are purposely added to the material to change the electronic characteristics.


We will first consider the case of the pure, or intrinsic semiconductor. As a result of the thermal energy present in the material, electrons can break loose from covalent bonds and become free electrons able to move through the solid and contribute to the electrical conductivity. The covalent bonds left behind have an electron vacancy called a hole.

Electrons from neighboring covalent bonds can easily move into an adjacent bond with an electron vacancy, or hole, and thus the hold can move from one covalent bond to an adjacent bond. As this process continues, we can say that the hole is moving through the material. These holes act as if they have a positive charge equal in magnitude to the electron charge, and they can also contribute to the electrical conductivity.

Thus, in a semiconductor there are two types of mobile electrical charge carriers that can contribute to the electrical conductivity, the free electrons and the holes. Since the electrons and holes are generated in equal numbers, and recombine in equal numbers, the free electron and hole populations are equal.

In the extrinsic or doped semiconductor, impurities are purposely added to modify the electronic characteristics. In the case of silicon, every silicon atom shares its four valence electrons with each of its four nearest neighbors in covalent bonds.

If an impurity or dopant atom with a valency of five, such as phosphorus, is substituted for silicon, four of the five valence electrons of the dopant atom will be held in covalent bonds. The extra, or fifth electron will not be in a covalent bond, and is loosely held. At room temperature, almost all of these extra electrons will have broken loose from their parent atoms, and become free electrons.

These pentavalent dopants thus donate free electrons to the semiconductor and are called donors. These donated electrons upset the balance between the electron and hole populations, so there are now more electrons than holes. This is now called an N-type semiconductor, in which the electrons are the majority carriers, and holes are the minority carriers.

 In an N-type semiconductor the free electron concentration is generally many orders of magnitude larger than the hole concentration. If an impurity or dopant atom with a valency of three, such as boron, is substituted for silicon, three of the four valence electrons of the dopant atom will be held in covalent bonds. One of the covalent bonds will be missing an electron.

An electron from a neighboring silicon-to-silicon covalent bond, however, can easily jump into this electron vacancy, thereby creating a vacancy, or hole, in the silicon-to-silicon covalent bond. Thus, these trivalent dopants accept free electrons, thereby generating holes, and are called acceptors.

These additional holes upset the balance between the electron and hole populations, and so there are now more holes than electrons. This is called a P-type semiconductor, in which the holes are the majority carriers, and the electrons are the minority carriers. In a P-type semiconductor the hole concentration is generally many orders of magnitude larger than the electron concentration.

As a result of the concentration difference of the free electrons and holes there will be an initial flow of these charge carriers across the junction, which will result in the N-type side attaining a net positive charge with respect to the P-type side. This results in the formation of an electric potential hill or barrier at the junction.

Under equilibrium conditions the height of this potential hill, called the contact potential is such that the flow of the majority carrier holes from the P-type side up the hill to the N-type side is reduced to the extent that it becomes equal to the flow of the minority carrier holes from the N-type side down the hill to the P-type side.

Similarly, the flow of the majority carrier free electrons from the N-type side is reduced to the extent that it becomes equal to the flow of the minority carrier electrons from the P-type side. Thus, the net current flow across the junction under equilibrium conditions is zero.