SURFACE PROPERTIES OF GERMANIUM 21 



ent. When light shines on the surface, Vb is changed to Vb + ^Vb and 

 there is an additional potential drop, 8Vi , in the body of the germanium 

 resulting from the recombination current which flows to the interior. 

 The change in contact potential with light is equal to 8Vb + dVi . 



The film thickness ^d is shown on an exaggerated scale. We expect 

 //> ^ 10"^ cm and ^b '^ 10~^ cm, so that Ib^ ^d . 



TABLE OF SYMBOLS 



A. Energies: 



Ea = Ea (true) + kT In {oiunoc/fj^o^, is the effective energy of the 

 a-traps for F^ = 0. Here oiunoc and Woc are the statistical 

 weights of the unoccupied and occupied states, respectively. 



Eh = effective energy of the 6-traps for Vb = 0. 



Ec = energy of lowest state of conduction band in interior of semi- 

 conductor just beyond the space-charge layer. 



Ev = energy of highest state of valence band at the same position. 



Ef = Fermi energy. 



Ei = Ef when material is intrinsic. 



Vd = potential drop across surface film. 



Vb = potential drop across space-charge layer. 



Vbo = value of Vb for which Ua = Pb , see below. 



Vq = value of Vbo for an intrinsic sample. 



B. Concentrations: 



n = Nc exp [{Ef — Ec)/kT] = equilibrium concentration 



(no./cm^) of conduction electrons in interior of semicon- 

 ductor just beyond the space-charge layer. 



p = Nv exp [{E^ - EF)/kT] = corresponding hole concentra- 



tion. 



ni = intrinsic concentration. 



ns , Ps = equilibrium concentrations of electrons and holes, re- 

 spectively, at the surface. 



Na,Nb = concentration (no./cm^) of a- and 5-traps, respectively. 



Ua = equilibrium concentration (no./cm^) of occupied a-traps. 



Pj, = iVft — rib = equilibrium concentration (no./cm ) of unoc- 



cupied 6-traps. 



UaOyPbo = values of na and pb for an intrinsic sample with Vb = 0. 



ni = n + 8n,pi = p + 8p, and n,i , psi , riai , Pbi = concentra- 



tions in presence of light. Electrical neutrality requires that 

 8n = 8p. 



