Conduction in Intrinsic Semiconductors

Chapter 9.4 Conduction in Intrinsic Semiconductors

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • Semiconductor Theory and Devices 267some stage drift into the vicinity of one of these positive ions, and be captured, i.e. the hole will once more be fi lled by an electron. This process is known as recombination, and when it occurs the normal charge balance of that atom is restored. The hole-pair generation and recombination processes occur continuously, and since heat is a form of energy, will increase as the temperature increases. This results in more mobile charge carriers being available, and accounts for the fact that semiconductors have a negative temperature coeffi cient of resistance, i.e. as they get hotter they conduct more easily. It must be borne in mind that although these thermally generated mobile charge carriers are being produced, the sample of material as a whole still remains electrically neutral. In other words, if a ‘ head count ’ of all the positive and negative charged particles could be made, 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 be readily understood, but the concept of hole mobility is more diffi cult to appreciate. In fact the holes themselves cannot move — they are merely generated and fi lled. However, when a bond breaks down the electron that drifts away will at some point fi ll a hole elsewhere in the lattice. Thus the hole that has been fi lled is replaced elsewhere by the newly generated hole, and will appear to have drifted to a new location. In order to simplify the description of conduction in a semiconductor, the holes are considered to be mobile positive charge carriers whilst the free electrons are of course mobile negative charge carriers. 9.4 Conduction in Intrinsic Semiconductors Figure. 9.3 illustrates the effect when a source of emf is connected across a sample of pure silicon. The electric fi eld produced by SiSiSiSiSiSiSiSifree electronhole Fig. 9.2