Floyd Full Summary Chapter 1

SEMICONDUCTOR BASICS

CHAPTER 1

·  According to the classical Bohr model, the atom is viewed as having a planetary-type structure with electrons orbiting at various distances around the central nucleus.

·  The nucleus of an atom consists of protons and neutrons. The protons have a positive charge and the neutrons are uncharged. The number of protons is the atomic number of the atom.

·  Electrons have a negative charge and orbit around the nucleus at distances that depend on their energy level. An atom has discrete bands of energy called shells in which the electrons orbit. Atomic structure allows a certain maximum number of electrons in each shell. These shells are designated 1, 2, 3, and so on. In their natural state, all atoms are neutral because they have an equal number of protons and electrons.

·  The outermost shell or band of an atom is called the valence band, and electrons that orbit in this band are called valence electrons. These electrons have the highest energy of all those in the atom. If a valence electron acquires enough energy from an outside source such as heat, it can jump out of the valence band and break away from its atom.

·  Semiconductor atoms have four valence electrons. Silicon is the most widely used semiconductive material.

·  Materials that are conductors have a large number of free electrons and conduct current very well. Insulating materials have very few free electrons and do not conduct current at all under normal circumstances. Semiconductive materials fall in between conductors and insulators in their ability to conduct current.

·  Semiconductor atoms bond together in a symmetrical pattern to form a solid material called a crystal. The bonds that hold a crystal together are called covalent bonds. Within the crystal structure, the valence electrons that manage to escape from their parent atom are called conduction electrons or free electrons. They have more energy than the electrons in the valence band and are free to drift throughout the material. When an electron breaks away to become free, it leaves a hole in the valence band creating what is called an electron-hole pair. These electron-hole pairs are thermally produced because the electron has acquired enough energy from external heat to break away from its atom.

·  A free electron will eventually lose energy and fall back into a hole. This is called recombination. But, electron-hole pairs are continuously being thermally generated so there are always free electrons in the material.

·  When a voltage is applied across the semiconductor, the thermally produced free electrons move in a net direction and form the current. This is one type of current in an intrinsic (pure) semiconductor.

·  Another type of current is the hole current. This occurs as valence electrons move from hole to hole creating, in effect, a movement of holes in the opposite direction.

·  An n-type semiconductive material is created by adding impurity atoms that have five valence electrons. These impurities are pentavalent atoms. A p-type semiconductor is created by adding impurity atoms with only three valence electrons. These impurities are trivalent atoms.

·  The process of adding pentavalent or trivalent impurities to a semiconductor is called doping.

·  The majority carriers in an n-type semiconductor are free electrons acquired by the doping process, and the minority carriers are holes produced by thermally generated electron-hole pairs. The majority carriers in a p-type semiconductor are holes acquired by the doping process, and the minority carriers are free electrons produced by thermally generated electron-hole pairs.

·  A pn junction is formed when part of a material is doped n-type and part of it is doped p-type. A depletion region forms starting at the junction that is devoid of any majority carriers. The depletion region is formed by ionization.