336 HEI.L SYSTEM TECIIXICAL JOLKXAL 



Silicon and germanium form what are called "covalent crystals," the 

 atoms being held together by "electron-pair bonds" formed by the valence 

 electrons. The covalent bond in the hydrogen molecule is the simplest elec- 

 tron-pair bond. Figure 1 represents two hydrogen atoms and a hydrogen 

 molecule.-' Each atom consists of a proton and one electron. The proton 

 weighs approximately 2,000 times as much as the electron and is a relatively 

 immobile particle about which the electron moves in its orbit or quantum 

 mechanical wave function. In an isolated atom, this wave function has 

 spherical symmetry and the electronic charge is distributed on the average 

 as a diffuse sphere centered about the proton. When the two atoms are 

 brought close together, interaction between the wave functions of the two 

 electrons takes place and the electronic cloud of each becomes modified, as 

 suggested in the figure. The result is to produce an extra accumulation of 

 charge between the two protons which acts to bind them together. Accord- 

 ing to quantum mechanical laws associated with the "Pauli exclusion prin- 

 ciple," the bond is especially stable when it contains precisely two electrons. 

 It is weakened considerably by removal of one electron and is not greatly 

 strengthened by the addition of a third electron. This special stability of the 

 electron-pair bond or covalent bond is a fundamental fact of chemistry which 

 is now quite well understood on the basis of wave mechanics. 



The elements carbon, silicon and germanium have the common feature of 

 being tetravalent. iVlthough they possess respectively 6, 14 and 32 electrons 

 all together, in each case only four of these are able to enter into chemical 

 reactions. The remaining electrons are closely bound to the nucleus produc- 

 ing a stable "ionic core" having a net charge of +4 units. This core can be 

 regarded as completely inactive so far as electronic processes in chemical 

 reactions and in semiconductors are concerned. 



Each of these atoms tends to form covalent or electron-pair bonds with 

 four other atoms. This tendency is completely satisfied in the diamond lat- 

 tice which is the crystalline form of all three elements. The lattice, shown in 

 Fig. 2, is a cubic arrangement and may be regarded as made up of four in- 

 terpenetrating simple cubic lattices like the one formed by the atoms on the 

 four corners of the cube shown. In this structure each typical atom is sur- 

 rounded by four neighbors regularly placed about it, with which it forms four 



Company, British Thompson-Houston Ltd., Telecommunications Research Establishment 

 and Oxford University; and in the United States: the Bell Telephone Laboratories, West- 

 inghouse Research L'aborator\-, General Electric Company, Sylvania Electric Products, 

 Inc., and the E.L duPont deXemours and Company. It is also pointed out that the crystal 

 groui)S at the University of Pcnns\lvania and Purdue Universit\-, who operated under 

 N.D.R.C. contracts, were responsible for much fundamental work. 



-The figures in this introduction and the text associated with them, like the following 

 pa])er on "Hole Injection in Germanium", follow closely the presentation in a book en- 

 titled "Holes and Electrons, an Introduction to the Physics of Transistors" now under 

 prejniration by \V. .Sliockley. 



