Fundamental Electrical and Electronic Principles, Third Edition

Semiconductor Theory and Devices 267SiSiSiSiSiholeSifree electronSiSiFig. 9.2some stage drift into the vicinity of one of these positive ions, and becaptured, i.e. the hole will once more be filled by an electron. Thisprocess is known as recombination, and when it occurs the normalcharge balance of that atom is restored.The hole-pair generation and recombination processes occurcontinuously, and since heat is a form of energy, will increase as thetemperature increases. This results in more mobile charge carriers beingavailable, and accounts for the fact that semiconductors have a negativetemperature coefficient of resistance, i.e. as they get hotter they conductmore easily. It must be borne in mind that although these thermallygenerated mobile charge carriers are being produced, the sample ofmaterial as a whole still remains electrically neutral. In other words, if a‘ head count ’ of all the positive and negative charged particles could bemade, there would still be a balance between positive and negative, i.e.for every free electron there will be a corresponding hole.The concept of the drift of free electrons through the material may bereadily understood, but the concept of hole mobility is more difficult toappreciate. In fact the holes themselves cannot move — they are merelygenerated and filled. However, when a bond breaks down the electronthat drifts away will at some point fill a hole elsewhere in the lattice.Thus the hole that has been filled is replaced elsewhere by the newlygenerated hole, and will appear to have drifted to a new location. Inorder to simplify the description of conduction in a semiconductor, theholes are considered to be mobile positive charge carriers whilst thefree electrons are of course mobile negative charge carriers.9.4 Conduction in Intrinsic SemiconductorsFigure. 9.3 illustrates the effect when a source of emf is connectedacross a sample of pure silicon. The electric field produced by