Doping to Enhance Conductivity of Semiconductors

LEDs are made of semiconductors, but the conductivity of the semiconductors in LEDs has been specially enhanced to allow for the unique properties of LEDs. The conductivity of semiconductors like Si can be increased by adding small, controlled amounts of "impurities" that have roughly the same atomic size, but more or fewer valence electrons than the semimetal. This process is known as doping. An impurity with fewer valence electrons (such as Al; see the periodic table) takes up space in the solid structure, but contributes fewer electrons to the valence band, thus generating an electron deficit (Figure 8). This type of dopant creates a space (or "hole") in the lower-energy (valence) band, making room for electrons to travel. Hence, the electrons in the valence band can move from one orbital to another within this band with a small input of energy (smaller than required for the semiconductor without the doping), and thus move throughout the solid. Alternately, an impurity with more valence electrons (such as P; see the periodic table) contributes extra electrons to the band (Figure 9). Since the valence band is already filled by the semimetal, the extra electrons must go into the higher-energy (conduction) band. These electrons now occupy a partially-filled band (the conduction band), can move easily between the orbitals of this band, and thus move easily throughout the solid. Semiconductors whose conductivity has been enhanced with valence-electron-deficient dopants are known as p-type semiconductors (p for "positive", because it is deficient in negatively-charged electrons), and semiconductors whose conductivity has been enhanced with valence-electron-enriched dopants are known as n-type semiconductors (n for "negative", because it is enriched with negatively-charged electrons).

Now we know that LEDs are doped semiconductors, and hence conduct electricity with a small input of energy (i.e., LEDs are very efficient). However, we have not yet answered the questions, how does an LED give off light, and why does an LED give off only one specific color of light?

p-type

n-type

Figure 8

A schematic diagram showing the solid crystal-lattice structure and bands for silicon doped with aluminum, a p-type semiconductor.

Figure 9

A schematic diagram showing the solid crystal-lattice structure and bands for silicon doped with phosphorus, an n-type semiconductor.