ABSTRACT
FromChapter 4, it was stated that an atom contains both negative charge carriers (electrons) and positive charge carriers (protons). Electrons each carry a single unit of negative electric charge while protons each exhibit a single unit of positive charge. Since atoms normally contain an equal number of electrons and protons, the net charge present will be zero. For example, if an atom has 11 electrons, it will also contain 11 protons. The end result is that the negative charge of the electrons will be exactly balanced by the positive charge of the protons. Electrons are in constant motion as they orbit around
the nucleus of the atom. Electron orbits are organized into shells. The maximum number of electrons present in the first shell is two, in the second shell eight, and in the third, fourth andfifth shells it is 18, 32and50, respectively. In electronics, only the electron shell furthermost from the nucleus of an atom is important. It is important to note that the movement of electrons between atoms only involves those present in the outer valence shell. If the valence shell contains the maximum number
of electrons possible, the electrons are rigidly bonded together and the material has the properties of an insulator (see Figure 13.2). If, however, the valence shell does not have its full complement of electrons, the electrons can be easily detached from their orbital bonds, and the material has the properties associated with an electrical conductor. In its pure state, silicon is an insulator because the
covalent bonding rigidly holds all of the electrons, leaving no free (easily loosened) electrons to conduct
current. If, however, an atom of a different element (i.e. an impurity) is introduced that has five electrons in its valence shell, a surplus Figure 13.3). These free use as charge carriers and through the lattice by difference to the material. Similarly, if the
pure silicon lattice has shell, the absence of the proper covalent bonding spaces into which These spaces are referred rent will flow when an is applied to the material. Regardless of whether
surplus electrons or holes,
behave as an insulator, neither will it have the properties that we normally associate with a metallic conductor. Instead, we call the material a semiconductor – the term simply serves to indicate that the material is no longer a good insulator nor is it a good conductor but is somewhere in between. Examples of semiconductor materials include silicon (Si), germanium (Ge), galliumarsenide (GaAs) and indiumarsenide (InAs). Antimony, arsenic and phosphorus are n-type
impurities and form an n-type material when any of these impurities are added to pure semiconductor material such as silicon or germanium. The amount of impurity addedusuallyvaries from1part impurity in10 5
parts semiconductor material to 1 part impurity to 108
parts semiconductor material, depending on the resistivity required. Indium, aluminium and boron are all p-type impurities and form a p-typematerial when any of these impurities are added to a pure semiconductor. The process of introducing an atom of another (impu-
rity) element into the lattice of an otherwise pure material is called doping. When the pure material is doped with an impurity with five electrons in its valence shell (i.e. a pentavalent impurity) it will become an n-type (i.e. negative type) semiconductormaterial. If, however, the purematerial is dopedwith an impurity having three electrons in its valence shell (i.e. a trivalent impurity) it will become a p-type (i.e. positive type) semiconductor material. Note that n-type semiconductor material contains an excess of negative charge carriers, and p-type material contains an excess of positive charge carriers. In semiconductormaterials, there are very few charge
carriers per unit volume free to conduct. This is because the ‘four electron structure’ in the outer shell of the atoms (called valency electrons), form strong covalent bonds with neighbouring atoms, resulting in
held fairly rigidly in place.
