Basics Of Semiconductors

INTRODUCTION :

To understand how diodes, transistors and integrated circuits work, you first have to study semiconductors : materials that are neither conductors nor insulators.

Semiconductors contain some free electrons , but what makes different is the presence of holes. In this article you wil get to know about semiconductors, holes & other related topics.

CONDUCTORS : We all know that copper is a good conductor the reason is clear when we look at its atomic structure (fig:1& fig:2).the nucleus of the atom contains 29 protons(positive charges).the electrons travel in distinct orbits.There are two electrons in the first orbit, 8 electrons in the second, 18 in the third and 1 in the outer most orbit.

STABLE ORBITS : The positive nucleus of fig1 attracts the planetary electrons. The reason why these electrons are not pulled into the nucleus is the centrifugal force created by their circular motion. The centrifugal force is exactly to the inward pull of the nucleus, so that the orbit is stable.

The larger the orbit of an electron the smaller the attraction of the nucleus. In a larger orbit an electron travels more slowly, producing less centrifugal force. the outer most electron travels very slowly and feels almost no attraction to the nucleus.

THE CORE : In electronics, all that matters is that outer orbit. It is called the valance orbit. The orbit controls the electrical properties of the atom. To emphasize the importance of the valance orbit, we define the core of an atom as the nucleus and all the inner orbits. For a copper atom, the core is the nucleus (+29), and the first three orbits.

The core of a cu atom has a net charge of +1 because it contains 29 protons and 28 inner electrons.the valance electron is in a large orbit around a core and has a net charge of only +1.Because of this, the inward pull felt by the valance electron is very small.

FREE ELECTRON : Since the attraction between the core and the valance electron is very weak, an outside force can easily dislodge this electron from the cu atom. This is why we often call the valance electron a free electron. This is also why cu is a good conductor. The slightest voltage causes the free electrons to flow from one atom to t

he next. The best conductors are silver, copper and gold.

SEMICONDUCTORS : The best conductors have one valance electron, whereas the best insulators have eight valance electrons. A semiconductor is an element with electrical properties b/w those of a conductor and those of a insulator.

GERMENIUM : Germenium is an example of a semiconductor. It has four valance electron. But these germanium devices had a fatal flaw that engineers could not overcome. Eventually, another semiconductor named silicon became practical and made germanium obsolete in most electronic applications.

SILICON : Next to oxyzen, silicon is the most abundant element on the earth. But there were refining problems that prevented the use of silicon in the early days of semiconductors. Once these problems were solved, the advantages of silicon immediately mase it the semiconductor of choice. Without it, modern electronics, communications,, and computers would be impossible.

An isolated silicon atom has the 14protons and 14 electrons. As shown in fig3 the first orbit contains 2 electrons and the 2nd orbit contains 8 electrons. The re

maining 4 electrons are in the valance orbit.The core has a net charge of +4 because it contains 14 protons in the nucleus and 10 electrons in the first two orbits.

SILICON CRYSTALS : When silicon atoms combine forms a solid, they arrange themselves into an orderly pattern called a crystal. Each silicon atom shares its electrons with four neighboring atoms in such a way as to have eight electrons in its valance orbit. For instance a central atom with four neighbors. The shaded circles represents the silicon cores. Although the central originally had four electrons in its valance orbit, it now has eight.

COVALENT BONDS : Each neighboring atom shares an electron with the central atom.In this way, the central atom has four additional electrons< giving it total of eight electrons in the valance orbit. The electrons no longer belong to any single atom. Each central atom and its neighbors share the electrons.

The same idea is true for all they other silicon atoms. In other words, every atom inside a silicon crystals has four neighbors.

Each core has a charge of +4.Look at the central core and the one to its right. These two cores attract the pair of electrons b/w them with equal and opposite force. This pulling in opposite directions is what holds the silicon atoms together.

Since each shared electron is being pulled in opposite direction the electron become bonds b/w the opposite cores. We call this type of chemical bond a covalent bond. In a silicon crystal there are a billion of silicon atoms, each with 8 valance electrons. These valance electrons are the covalent bonds that hold the crystal to gather- that gives it solidity.