What is a Semiconductor?

Semiconductors are elements whose conductivity is between that of an insulator - which has almost no conductivity - and a conductor - which has almost full conductivity. Most semiconductors are crystals, with silicon being the most common.


Some background: The electrons in an atom are organized in layers called shells, with the outermost layer being the valence shell. When atoms form bonds, the electrons in the valence shells are the ones that are shared or exchanged. Most conductors have just one electron in the valence shell. Semiconductors, on the other hand, typically have four electrons in their valence shell.

If all of an atom’s the neighbors are the same element, then all of its valence electrons will bond with valence electrons from other atoms. As a result, the atoms will arrange themselves into crystals. Semiconductors are made out of these distinct crystals.

Pure silicon crystals are useful for their conductivity. However, if trace amounts of other elements are introduced to a crystal, the atoms take on new and interesting qualities with respect to their conductivity. This process, called ‘doping’, introduces impurities to an element in order to modulate its conductivity. Semiconductors that are doped lightly or moderately are called ‘extrinsic semiconductors’. A semiconductor that is doped so much that it acts almost like a conductor is referred to as a degenerate semiconductor.

 The element introduced by doping is called a dopant, and the type of dopant affects the type of semiconductor and its properties. In fact, crystals can transform into one of two types of conductors dependent on the dopant used.

 When the dopant has five electrons in its valence layer, the result is an n-type semiconductor. For example, phosphorus atoms join the crystal structure of the silicon, each bonding with four adjacent silicon atoms. Because the phosphorus atom has five electrons in its valence shell, but only four of them are bonded to adjacent atoms, the fifth valence electron is left hanging without a bond. N-type semiconductors, then, are characterized by having extra electrons.

When the dopant has only three electrons in the valence shell, the result is a p-type semiconductor. The atom is able to bonds with four other atoms, but since it has only three electrons to offer, a hole is created. This hole behaves like a positive charge, which is what ‘p’ stands for.

Much like a positive charge, holes attract electrons. But when an electron moves into a hole, the electron leaves a new hole at its previous location. Thus, in a p-type semiconductor, holes are constantly moving around within the crystal as electrons constantly try to fill them up.
 

References

Moore, J. S., Jones, K. S., Kennel, H., & Corcoran, S. (2008). 3-D analysis of semiconductor dopant distributions in a patterned structure using LEAP. Ultramicroscopy, 108(6), 536-539.

Twitchett-Harrison, A. C., Yates, T. J., Newcomb, S. B., Dunin-Borkowski, R. E., & Midgley, P. A. (2007). High-resolution three-dimensional mapping of semiconductor dopant potentials. Nano Letters, 7(7), 2020-2023.