Doping

Doping is the addition of controlled amounts of specific impurity atoms for the purpose of changing the conductivity of a semiconductor sample by increasing either the electron or hole concentration. Some common dopants for silicon are listed in the table below.

Dopant atoms can either be "donors" or "acceptors." Donors increase the elctron concentration in the silicon, whereas acceptors increase the hole concentration. This is due to the fact that silicon, being an element from Column IV of the Periodic Table of the Elements, has four valence electrons. In an undoped silicon crystal, each atom shares these electrons with its four nearest neighbors via covalent bonding (see Figure 1).

When Column V elements such as arsenic or phosphorus are introduced into the silicon crystal lattice, four of the five valence electrons fit into the silicon bonding structure. The fifth electron, however, is weakly bound, and at room temperature is free to move through the crystal (Figure 2a). It is thus said to be "donated" to the semiconductor.

Likewise, Column III elements such as boron have only three valence electrons, and therefore cannot complete all four of the available bonds when substituted for a silicon atom. These elements, however, readily "accept" an electron from a nearby silicon-silicon bond. This creates a hole which is also free to wander through the crystal lattice at room temperature (Figure 2b).