e high resistivity of semiconductors decreases as the temperature israised or the material illuminated. The excitation of valence electrons to the conductionband is best accomplished when the semiconductor is in the crystalline state, i.e. when theatoms are arranged in a precise geometrical formation or "lattice". At room temperatureand low illumination, pure or so-called "intrinsic" semiconductors have a high resistivity.But the resistivity can be greatly reduced by "doping", i.e. introducing a very small amountof impurity, of the order of one in a million atoms. There are 2 kinds of dopant. Thosewhich have more valence electrons that the semiconductor itself are called "donors" andthose which have fewer are termed "acceptors In a silicon crystal, each atom has 4valence electrons, which are shared with a neighbouring atom to form a stable tetrahedralstructure. Phosphorus, which has 5 valence electrons, is a donor and causes extraelectrons to appear in the conduction band. Silicon so doped is called "n-type" [Book 5].On the other hand, boron, with a valence of 3, is an acceptor, leaving so-called "holes" inthe lattice, which act like positive charges and render the silicon "p-type".The p-typesilicon crystals, in which the atomic nucleii in the lattice are indicated by circles andthe bonding valence electrons are shown as lines between the atoms. Holes, like electrons,will remove under the influence of an applied voltage but, as the mechanism of theirmovement is valence electron substitution from atom to atom, they are less mobile thanthe free conduction electrons. In a n-on-p crystalline silicon solar cell, a shadow junction is formed by diffusingphosphorus into a boron-based base. At the junction, conduction electrons from donoratoms inthe n-region diffuse into the p-region and combine with holes in acceptor atoms,producing a layer of negatively-charged impurity atoms. The opposite action also takesplace, holes from acceptor atoms...
