Extrinsic (Impure) Semiconductors & n-type Semiconductor

Chapter 9.5 Extrinsic (Impure) Semiconductors

Fundamental Electrical and Electronic Principles Third Edition Book
Pages 304
Views 4,524
Downloads : 23 times
PDF Size : 3.0 MiB

Summary of Contents

Fundamental Electrical and Electronic Principles Third Edition Book

  • 268Fundamental Electrical and Electronic Principlesthe battery will attract free electrons towards the positive plate and the corresponding holes towards the negative plate. Since the external circuit is completed by conductors, and holes exist only in semiconductors, then how does current actually fl ow around the circuit without producing an excess of positive charge (the holes) at the left-hand end of the silicon? The answer is quite simple. For every electron that leaves the right-hand end and travels to the positive plate of the battery, another is released from the negative plate and enters the silicon at the left-hand end, where a recombination can occur. This recombination will be balanced by fresh electron-hole pair generation. Thus, within the silicon there will be a continuous drift of electrons in one direction with a drift of a corresponding number of holes in the opposite direction. In the external circuit the current fl ow is of course due only to the drift of electrons. 9.5 Extrinsic (Impure) Semiconductors Although pure silicon and germanium will conduct, as explained in the previous section, their characteristics are still closer to insulators than to conductors. In order to improve their conduction very small quantities (in the order of 1 part in 10 8 ) of certain other elements are added. This process is known as doping. The impurity elements that are added are either pentavalent (have fi ve valence electrons) or are trivalent (have three valence electrons) atoms. Depending upon which type is used in the doping process determines which one of the two types of extrinsic semiconductor is produced. 9.6 n-type Semiconductor To produce this type of semiconductor, pentavalent impurities are employed. The most commonly used are arsenic (As), phosphorus (P), and antimony (Sb). When atoms of such an element are added to the silicon a bonding process takes place such that each impurity atom joins the covalent bonding system of the silicon. However, since each impurity atom has fi ve valence electrons, one of these cannot fi nd a place in a covalent bond. These ‘ extra ’ electrons then tend to drift IelectronsholesI(electrons) Fig. 9.3