Fundamental Electrical and Electronic Principles, Third Edition

268 Fundamental Electrical and Electronic PrincipleselectronsIholesI(electrons)the battery will attract free electrons towards the positive plateand the corresponding holes towards the negative plate. Since theexternal circuit is completed by conductors, and holes exist onlyin semiconductors, then how does current actually flow around thecircuit without producing an excess of positive charge (the holes) atthe left-hand end of the silicon? The answer is quite simple. For everyelectron that leaves the right-hand end and travels to the positive plateof the battery, another is released from the negative plate and entersthe silicon at the left-hand end, where a recombination can occur. Thisrecombination will be balanced by fresh electron-hole pair generation.Thus, within the silicon there will be a continuous drift of electronsin one direction with a drift of a corresponding number of holes in theopposite direction. In the external circuit the current flow is of coursedue only to the drift of electrons.9.5 Extrinsic (Impure) Semiconductors9.6 n-type SemiconductorFig. 9.3Although pure silicon and germanium will conduct, as explained inthe previous section, their characteristics are still closer to insulatorsthan to conductors. In order to improve their conduction very smallquantities (in the order of 1 part in 10 8 ) of certain other elements areadded. This process is known as doping. The impurity elements thatare added are either pentavalent (have five valence electrons) or aretrivalent (have three valence electrons) atoms. Depending upon whichtype is used in the doping process determines which one of the twotypes of extrinsic semiconductor is produced.To produce this type of semiconductor, pentavalent impurities areemployed. The most commonly used are arsenic (As), phosphorus (P),and antimony (Sb). When atoms of such an element are added to thesilicon a bonding process takes place such that each impurity atomjoins the covalent bonding system of the silicon. However, since eachimpurity atom has five valence electrons, one of these cannot find aplace in a covalent bond. These ‘ extra ’ electrons then tend to drift