3.2 Semiconductors and pn Junctions

A semiconductor is a crystal lattice structure that can have free electrons (which are negative carriers) and/or free holes (which are an absence of electrons and are equivalent to positive carriers). The type of semiconductor typically used is silicon (commonly found in sand). This material has a valence of four, implying that each atom has four free electrons to share with neighboring atoms when forming the covalent bonds of the crystal lattice. Intrinsic silicon (i.e., undoped silicon) is a very pure crystal structure having equal numbers of free electrons and holes. These free carriers are those electrons or holes that have gained enough energy due to thermal agitation to escape their bonds. At room temperature, there are approximately 1.5 10 ¹⁰ carriers of each type/cm ³, or equivalently 1.5 10 ¹⁶ carriers/m ³. The number of carriers approximately doubles for every 11 C increase in temperature.

If silicon is doped with a pentavalent impurity (i.e., atoms of an element having a valence of five, or equivalently five electrons in the outer shell available when bonding with neighboring atoms), there will be almost one extra free electron for every impurity atom. ^[3] These free electrons can be used to conduct current. A pentavalent impurity is said to donate free electrons to the silicon crystal, and thus the impurity is known as a donor. Examples of donor elements are phosphorus, P, and arsenic, _As. These...