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What Are the Physiochemical Properties of Semiconductors?

To understand how semiconductors work requires understanding both the chemistry and the physics of semiconductors. Semiconductors are constructed in an elaborate chemical procedure and they operate according to physical principles. The usual distinction between chemistry and physics is that chemistry involves how atoms combine to make substances and physics does not. Semiconductors demonstrate how some phenomena can only be described by a combination of chemistry and physics. Some devices must be described by their physiochemical properties.

Silicon Substrate
- Typical semiconductors start off as pure crystals of silicon. Atoms of silicon have a complete shell of electrons and four extra electrons left over. These atoms are easily arranged in a crystalline structure in which each atom shares an electron with the four atoms surrounding it. This is an extremely stable configuration; when atoms are arranged like this, each atom has its normal number of atoms plus the four atoms that are shared with the surrounding atoms, so each atom seems to have a complete shell. Atoms are most stable when they have complete shells. Pure silicon with its atoms arranged this way makes a crystal that is very stable.
Making Semiconductors
- Pure silicon crystals are baked in an oven with a trace of phosphorus gas -- phosphorus has one more electron than silicon -- until a carefully controlled number of silicon atoms are replaced by phosphorus atoms to make an N type (Negative type) semiconductor that has a surplus of free electrons. If baked in an oven with a trace of aluminium gas -- aluminimum has one less electron than silicon -- it will make a P type (Positive type) semiconductor that has a defeciency of electrons. P type semiconductors are said to have free "holes" -- places that are missing an electron.
Diodes
- Diodes are electronic components that allow current to flow in one direction but not in the other direction. Diodes are composed of two slices of semiconductor material -- one of them P type and the other N type. If a negative potential is applied to the P type semiconductor, and a positive potential is applied to the N type semiconductor, the free electrons and free holes are pulled apart and no current flows. If the potentials are applied the reverse way, the electrons and holes are pushed together and current flows. Current flows in one direction, but not in the other.
Transistors
- Transistors are composed of three slices of semiconductor material -- the middle slice different from the identical outer slices. There are PNP transistors and NPN transistors. Current flowing across the entire transistor will encounter a lot of resistance. No matter which way the current is flowing, it will encounter one PN or NP junction that is biased in the wrong direction. Changing the potential on the middle slice changes the across-the-entire-transistor resistance dramatically -- a small change in the signal to the middle element controls the larger current across the entire transistor, Therefore, transistors can be used as amplifiers.

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