 the covalent band that can move around the structure. Because the material now has an excess of nega- tively charged electrons, it is also referred to as an n-type material and rep- resents one-half of a p-n semiconductor. As you might expect, the other half of the p-n semiconductor will have an excess of positively charged material. p-TYPE MATERIAL To illustrate the creation of material with an excess of positively charged electrons, let's assume that other material with a com- position of only three valence electrons is added to silicon. Because four valence electrons are required for a covalent bond, one would now be missing. This electron vacancy is referred to as a "hole" in the form of a positively charged particle. Since the resulting material has an excess of positively charged holes, it is referred to as p-type material. DOPING MATERIAL The p- and in-type material described above is cre- ated by doping the material with atoms. Since the 1970s, gallium arsenide (GaAs), gallium arsenide phosphide (GaAs, P), aluminum gallium arsenide (Al Ga .,As), aluminum gallium indium phosphide Chapter Four Silicon atom Figure 4.4 Covalent bonds in a series of silicon atoms. Number of valence electrons covalent bond Ga 1-x y ) In-P], and gallium indium arsenide phosphide (Ga X In 1-x As P1-y) (where X represents a percent of the elements that are aluminum and 1-x, the percent that are gallium) have been used as doping material. Such material represents elements from groups III and V of the periodic table that have three and five electrons in their valence shells. Through the use of elements from groups III and V of the periodic table, it becomes possible to create a p-n semiconductor that generates energy in the form of light. In comparison, silicon generates energy in the form of heat resulting from the vibrations in the crystal structure of silica. ELECTRON FLOW By joining p and n material, holes and electrons flow across the p-n junction and recombine. Here the process of recombining means that a free electron falls 