20 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



concerned only with differences between the occupied and unoccupied 

 states. 



The fraction of the traps which are occupied depends on the trap 

 energy and on the position of the Fermi level at the surface. The latter 

 in turn depends on the condition that the surface as a whole be neutral. 

 A very obvious mechanism for changing the total surface dipole is the 

 adsorption or desorption of ions on the surface. If this happens the 

 position of the Fermi level at the surface shifts until the total charge 

 on the surface, adsorbed ions, charge in the traps and charge in space- 

 charge layer, adds up to zero. The consequences of this model have been 

 carried through. 



The basis for the theory is illustrated in the energy level diagram of 

 Fig. 13. At the semiconductor surface there is a space-charge layer of 

 thickness (b which gives a change in electrostatic potential of Vb , 

 corresponding to a potential energy of an electron of —cVb . Outside 

 of the surface of the germanium proper, there is a surface film of thick- 

 ness Id . A double layer giving a potential change Vd , is formed from a 

 charge of ions, cti , on the outer surface of the film and charges in the 

 surface traps of types a and h. Changes in c.p. with ambient result 

 from changes in o-/ and consequent changes in Vb and Vd . It is as- 

 sumed that the remainder of the work function is independent of ambi- 



DOUBLE 

 LAYER 



SPACE CHARGE LAYER 

 Iq^IO-^CM 



X=0 INTERIOR 



Fif?. 13 — Schematic of energy level diagram at germanium surface. 



