PRINCIPLES OF TRANSISTOR ACTION 259 



in transit times for holes following difTerent paths there is a drop in amplitude 

 rather than simply a phase shift. 



Ill — Electrical Conductivity of Germanium 



Germanium, like carbon and silicon, is an element of the fourth group of 

 the periodic table, with the same crystal structure as diamond. Each 

 germanium atom has four near neighbors in a tetrahedral configuration 

 with which it forms covalent bonds. The specific gravity is about 5..S5 

 and the melting point 958°C. 



The conductivity at room temperature may be either n or p tyjje, de- 

 pending on the nature and concentration of impurities. Scaff, Theuerer, 

 and Schumacher^^ have shown that group III elements, with one less valence 

 electron, give p-type conductivity; group V elements, wdth one more va- 

 lence electron, give n-type conductivity. This applies to both germanium 



, -CONDUCTION BAND 



' *"~--FEBMI LEVEL 



Eg 

 i Ea 



ACCEPTORS-, 



i_ 1 i _ 



y/////////////////////// //}i'////y////////y^^^^ 



' ^-FILLED BAND 



Fig. 13 — Schematic energy level diagram for germanium showing filled and conduction 

 bands and donor and acceptor levels. 



and silicon. There is evidence that both acceptor (p-type) and donor 

 (n-type) impurities are substitutional'''. 



A schematic energy level diagram'^ which shows the allowed energy 

 levels for the valence electrons in a semi-conductor like germanium is given 

 in Fig. 13. There is a continuous band of levels, the filled band, normally 

 occupied by the electrons in the valence bonds; an energy gap. Eg, in which 

 there are no levels of the ideal crystal; and then another continuous band of 

 levels, the conduction band, normally unoccupied. There are just sufficient 

 levels in the filled band to accomodate the four valence electrons per atom. 

 The acceptor impurity levels, which lie just above the filled band, and the 

 donor levels, just below the conduction band, correspond to electrons local- 

 ized about the impurity atoms. Donors are normally neutral, but become 

 positively charged by excitation of an electron to the conduction band, an 

 energy Ed being required. Acceptors, normally neutral, are negatively 

 ionized by excitation of an electron from the filled band, an energy Ea 



