.SKM/CU.XDl CJURS 341 



If (he conductivity of the sample is due to excess electrons it is called 

 n-lype, since the current carriers act like negative charges; if due to holes, 

 it is called p-lype, since the carriers act like positive charges. 



Either tyi>e of conduction can be produced at will by admixture of a suit- 

 able "impurity," a donor such as arsenic yielding an excess of free electrons, 

 while an acceptor like boron causes an electron deficit or a surplus of positive 

 holes. The reason why arsenic and boron serve in these opposite capacities 

 comes readily to hand. 



The arsenic atom has five valence electrons surrounding a core having a 

 net charge of +5 units and, when introduced (e.g. in silicon) as a low-fraction 

 impurity, it is believed that each arsenic atom displaces one of the silicon 

 atoms from its regular site and forms four covalent bonds with the nighbor- 

 ing silicon atoms, thus using four of its five valence electrons (see Fig. 4). 

 The extra electron cannot fit into these four bonds and is free to move about 

 the crystal. This excess electron therefore constitutes a mobile localized 

 negative charge. The arsenic atom, on the other hand, is an immobile local- 

 ized positive charge, since its core, with a charge of +5 units, is not neutral- 

 ized by its share (—4) of the charge in the valence bonds. Its net charge, 

 therefore, just balances that of the excess electron it sets free in the crystal. 

 Thus arsenic impurity atoms add excess electrons but do not disturb the 

 over-all electrical neutrality of the crystal. The negative electrons, however, 

 are somewhat attracted by the positive arsenic atoms and at low tempera- 

 tures become weakly bound to them. At room temperature, thermal agita- 

 tion shakes them ofif so that they become free. 



To produce a p-type semiconductor we choose an added impurity, such 

 as boron, having three valence electrons and therefore not enough to com- 

 plete the valence bond structure surrounding it. The hole in one of the bonds 

 to the boron atom can be filled by an electron from an adjacent bond, and 

 when this occurs the hole migrates away to the bond which just gave up one 

 of its electrons. The boron atom thus becomes an immobile localized nega- 

 tive charge. Because of the symmetry between the behavior of holes and 

 electrons, we can describe the situation by saying that the negative boron 

 atom attracts the positively charged hole but that thermal agitation shakes 

 the latter ofif at room temperature so that it is free to wander about and con- 

 tribute to the conductivity. 



Because of their valencies, phosphorous and antimony, as well as arsenic 

 are in the donor class while aluminum, gallium and indium are additional 

 examples of the acceptor class. 



It is beyond the scope of this prefatory note to describe how, b\' measure- 

 ments of conductivity and the Hall efifect as inlluenced by the amount of 

 added donor or acceptor, it has been possible to determine the concentration 

 of electrons and holes, as well as to fix the energies needed to remove an elec- 

 tron from a donor, a hole from an acceptor, and to break a covalent bond 



