How is a semiconductor different from a conductor?

Can you explain how a semiconductor different from a conductor?

Answer:
A semiconductor's conductivity can be controlled over a wide range. They are similar to insulators in their structure, and like insulators have very few electrons that are capable of gaining enough energy to jump the band gap, which is necessary for conduction.

Semiconductors and insulators, in the absence of electric fields, have very similar electrical properties.

Semiconductors, however, have a much smaller band gap, allowing for many other methods (in addition to temperature) to control their electrical conductivity.

Wikipedia has a good overview...
http://en.wikipedia.org/wiki/semiconduct...
A conductor will conduct current in either direction or in the case of alternating current both the positive and negative half cycles. A semiconductor will only conduct in one direction or in the case of alternating current on one half cycle only.
To discuss this properly we must visualize the behavior of atoms. Copper, the second best conductor after silver, has an atomic number of 29. This means there are 29 protons in its nucleus and an equal number of "orbiting" electrons, so that the net charge of a neutral copper atom is zero. The electrons are arranged in shells (roughly, layers) and those closest to the nucleus are tightly held. As we move up through the shells, we find the electrons are progressively more energetic and less tightly bound to the nucleus. For copper there is a single loosely held outermost "conduction" electron. In a copper wire, all of the atomic nuclei and 28 of the 29 electrons are stationary. (The 28 electrons "orbit" their own nucleus, but the atoms themselves remain in place.) The single outermost conduction electron of each copper atom is free to move from atom to atom, so long as the total number of electrons in the wire remains exactly equal to the total number of protons. Thus a conducting wire carries no net charge. A good way to think of this is to imagine a pipe (which represents the wire) completely filled with glass marbles (which represent the individual conduction electrons). There are no atoms required in this analogy because the atoms, including 28/29th's of the electrons are stationary. Now, if we push one additional marble into one end of the pipe, another marble "instantly" pops out of the pipe at the other end. The faster we shove marbles into one end, the faster they pour out of the other. Note that the total number of marbles in the pipe never changes. This is analogous to conduction electrons in a wire. A power supply pushes one electron into one end of the wire and a different electron pops out of the far end, at the speed of "light" in copper. Note that while the propagation of the electric "signal" is nearly instantaneous, the "drift" of the individual conduction electrons is only a few centimeters per second. It is this electron "drift" which is responsible for the magnetic field which surrounds every conducting wire.

The situation is much different for semiconductor atoms. Silicon has an atomic number of 13 and so each silicon atom has that many protons and electrons. The outermost electron shell of silicon carries 4 electrons and they are much more tightly bound to their atoms. Unlike copper, which is amorphus, silicon is crystalline. Its atoms form molecular bonds with each other in a regular geometric pattern. Each atom shares its outer electrons with its neighbors and none are free to move from atom to atom. Absolutely pure silicon will not conduct electricity because there are no available conduction electrons. If an impurity is added to the silicon (called "doping") the regular crystalline pattern of molecular bonds is interrupted. If phosphorus or arsenic atoms are used to dope the silicon crystal, extra electrons are introduced into the crystal lattice and an N-type semiconductor results. If aluminum or gallium atoms are used, then insufficient electrons are present to counter all the nuclear protons and a P-type semiconductor is produced. It is convenient to visualize this excess positive charge as holes in the electron shell. Because semiconductors do not have conduction electrons it is important to understand that the excess electrons in N-type and the excess "holes" in P-type semiconductors are the charge carriers which move from place to place within the crystal and conduct electricity.

The effects for which semiconductors are famous arise from the interaction of N-type and P-type semiconductors. And that should be the subject of at least two more questions, one for diodes and one for bipolar transistors, and maybe a third for field effect transistors.

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