light [Book 4]. This explains why the high resistivity of semiconductors decreases as the temperature is raised or the material illuminated. The excitation of valence electrons to the conduction band is best accomplished when the semiconductor is in the crystalline state, i.e. when the atoms are arranged in a precise geometrical formation or "lattice". At room temperature and 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 amount of impurity, of the order of one in a million atoms. There are 2 kinds of dopant. Those which have more valence electrons that the semiconductor itself are called "donors" and those which have fewer are termed "acceptors" [Book 2]. In a silicon crystal, each atom has 4 valence electrons, which are shared with a neighbouring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a donor and causes extra electrons 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" in the lattice, which act like positive charges and render the silicon "p-type"[Book 5]. The drawings in Figure 1.2 are 2-dimensional representations of n- and p-type silicon crystals, in which the atomic nucleii in the lattice are indicated by circles and the 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 their movement is valence electron substitution from atom to atom, they are less mobile than the free conduction electrons [Book 2]. In a n-on-p crystalline silicon solar cell, a shadow junction is formed by diffusing phosphorus into a boron-based base. At the junction, conduction electrons from donor atoms in the n-region diffuse into the p-region and combine...
