In thi s chapter we develop a basic understanding of the properties of intrinsic and
extrinsic semiconductors. Although most of our discussions and examples will be
based on Si , the ideas are applicable to Ge and to the compound semiconductors such
as GaAs, InP, and others. By intrinsic Si we mean an ideal perfect crystal of Si that has
no impurities or crystal defects such as dislocations and grain boundaries. The crystal
thus consists of Si atoms perfectly bonded to each other in the diamond structure. At
temperatures above absolute zero, we know that the Si atoms in the crystal lattice will
be vibrating with a distribution of energies. Even though the average energy of the vibrations i s at most ?>kT and incapable of breaking the Si-Si bond, a few of the lattice
vibrations in certain crystal regions may nonetheless be sufficiently energetic to "rupture" a Si-Si bond . When a Si-Si bond i s broken, a "free" electron i s created that can
wander around the crystal and also contribute to electrical conduction in the presence
of an applied field . The broken bond has a missing electron that causes thi s region to
be positively charged . The vacancy left behind by the missing electron in the bonding
orbital i s called a hole. An electron in a neighboi ng bond can readily tunnel into this
broken bond and fill it, thereby effectively causing the hole to be displaced to the original position ofthe tunneling electron . By electron tunneling from a neighboring bond,
holes are therefore also free to wander around the crystal and also contribute to electrical conduction in the presence of an applied field . In an intrinsic semiconductor, the
number of thermally generated electrons i s equal to the number of holes (broken
bonds). In an extrinsic semiconductor, impurities are added to the semiconductor that
can contribute either excess electrons or excess holes. For example, when an impurity
such as arsenic i s added to Si , each As atom acts as a donor and contributes a free electron to the crystal . Since these electrons do not come from broken bonds, the numbers
of electrons and holes are not equal in an extrinsic semiconductor, and the As-doped Si
in this example will have excess electrons. It will be an n-type Si since electrical conduction will be mainly due to the motion of electrons. It i s also possible to obtain a
p-type Si crystal in which hole concentration i s in excess of the electron concentration
due to, for example, boron doping .