Semiconductors

While I'm talking about silicon and covalent network materials, let me deal with the special case of semiconductors. Silicon is a network marginal nonmetal. It can be represented using electron dot diagrams in this way. This is a two dimensional drawing rather than three, but it will serve for our purposes. Around the edge I've shown the pairs of electrons that would result from this atom bonding to the next atom over. Pure silicon is an intrinsic semiconductor, but impurities enhance its properties.

Silicon

What if an atom from a neighboring family like phosphorus or arsenic took the place of one of the silicon atoms?

Phosphorus has five valence electrons, which is normal for it, but the bonding network is set up using 4 valence electrons from each atom. In that sense the fifth valence electron of the P is an "extra" electron, it is not "tied down" like the rest, it is somewhat free to move around, it increases the electrical conductivity of the network. Since the electron is negative, we have what is called a negative or n-type semiconductor.

n-type semicondictor

Diagram of silicon network with phosphorus impurity.

Conversely, what would happen if an element like Al or Ga from the family to the left of Si on the periodic table were used to replace one of the silicon atoms?

Ga has only 3 valence electrons. Again that is normal for Ga, but in a network that is set up using 4 valence electrons from each atom that leaves a gap. Because it represents no charge where there "should be" a negative charge, it is often referred to as a "positive hole."

p-type semiconductor

Diagram of silicon network with gallium impurity.

The "hole" also offers a place to which neighboring electrons can move. When they do move, it is almost like the hole moved. Because the electrons can move a little, electrical conductivity is increased. Because it seems like the "positive hole" is moving, this is called a positive or p-type semiconductor.

Moving "positive hole"

Silicon network (p-type) with hole identified.

Silicon network (p-type) with hole in new position.

By carefully manipulating the size and location of the n-type and p-type regions in a chip of silicon or the similar metalloid germanium, a wide variety of diodes, transistors and other electronic components have been created.

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E-mail instructor: Eden Francis

Clackamas Community College
Š1998, 2002 Clackamas Community College, Hal Bender