FORMATION OF SOLAR CELL USING SEMI-CONDUCTOR MATERIAL
The vacuum tubes were widely used for various purposes in electronics, mostly voltage and power amplification, before the invention of solid state semiconductor devices in the 1940’s. Since then semiconductor devices have gradually replaced vacuum tubes in most of such applications, due to many of their favorable properties such as small size, light weight, low energy consumption, high frequency capability, and high reliability. However, vacuum tubes can still find some applications even today, such as high power radio frequency transmitters and microwave ovens. Although the physics of vacuum tubes and semiconductor devices is very different, the basic functions are similar, such as controlling current through small voltage.
- Conductors and Insulators:Good conductors, such as copper (Cu ), silver (Ag ), and gold (Au ) can conduct electricity with little resistance because the atoms only have one electrons on the out-most layer or shell, called valence electron (VE), which is only loosely bound to the atom and can easily become free electrons freely movable to conduct electricity.On the other hand, insulators do not conduct electricity as no free electrons exist in the material.
- Semiconductors:The conductivity of those elements with four valence electrons is not as good as the conductors but still better than the insulators, and they are given the name semiconductors. The two semiconductors of great importance are silicon (Si ) and germanium (Ge ), both of which have four valence electrons. Their crystal structure crystal lattice has a tetrahedral pattern with each atom sharing one valence electron with each of its four neighbors to form the covalent bonds.If an electron gains enough thermal energy (1.1 eV for Si or 0.7 eV for Ge), it may break the covalent bond and becomes a free electron of negative charge, while leaving a vacancy or a hole of positive charge. In an electric field, a free electron may move to a new location to fill a hole there, i.e., both such electrons and holes contribute to electrical conduction. Such crystal is called intrinsic semiconductor .
At room temperature, relatively few electrons gain enough energy to become free electrons, the over all conductivity of such materials is low, thereby their name semiconductors.
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