Conduction In a Semiconductor

Semiconductors are substances that are used to make electronic components such as diodes, transistors, solar cells, integrated circuits, microprocessors etc. These semiconductor electronic components are used in the construction of all electronic gadgets and devices ranging from radios, music systems, television, solar panels, telephones, mobile phones, computers, laptops, e notebooks etc. Therefore semiconductors are fundamental in the field of electronics and construction of electronic devices.

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The semiconductors have replaced the valves in most electronic circuitry. Semiconductor devices are light weight and have small physical dimensions, high efficiency and low power losses, ability to overcome vibrations, long life and durability of the components, and requires low voltage and power supplies.

 

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A semiconductor is a substance with an electrical conductivity that lies between a conductor and an insulator.  The semiconducting materials are mostly crystalline in nature and include carbon, silicon, germanium, mixtures of arsenic, selenium, tellurium, etc. The atoms of germanium and silicon  have 4 electrons in their outer orbits.  As there are many atoms in their crystalline structure, electron sharing occurs. This sharing behavior causes their outer electrons become tightly bonded to their respective places in the crystal.  This type of bonding is called  covalent bond.  Although these are covalently bonded in their crystalline structure, these covalent bonds are weak in semiconductors.  The bonds are broken at higher temperatures above absolute zero and more  bonds are broken when the temperature goes higher. When the covalent bonds are broken,  an electron becomes free that can be moved by an electric field.  The place that is left by the free electron becomes a hole. A free place is available in the atom where the electron has left which is the hole. An electron is electrically negative and a hole is electrically positive in charge.

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In the absence of any electrical charge, free electrons are distributed evenly throughout the semiconductor. The electrical charge in semiconductor exists as electron-hole pairs. The electrons are being captured by the holes resulting in the destruction of the  electron-hole pairs. When a hole captures an electron, the hole no longer exists and the electron is no longer a free electron.  Electron and hole pairs are being constantly created and destroyed in about equal numbers.

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When an electrical charge is applied across a block of semiconductor, electrons are attracted to the positive end of the semiconductor block. Electron hole pairs are formed rapidly throughout the block due to the electric charge. There is heavy concentration of electrons at the positive end and only fewer electrons are left at the negative end of the block. More electrons move towards the positive end by the steady application of electrical charge. The holes at the negative end of the block cannot capture electrons because there are almost none for them to capture. At the positive end of the semiconductor block there is an excess of electrons and at the negative end there is an excess of holes. More electron hole pairs are formed at the positive end as long as the temperature breaks the electrons loose from their bonds. Thus the conduction on a pure semiconductor is dependent on the temperature and the charge given.