126 CHAPTER 3. CHARGE TRANSPORT IN MATERIALS
– – – –
+ +
Ec
Equilibrium Fermi
level for an n-type
material
Ev
Excess electron
injection
Excess electron
and hole injection
EF
(a)– – – –
+ +
Ec
Ev
EFp
(b)EFn
– – –
– – – –
+ +
Ec
Ev
EFp
(c)EFn
– – –
+++ ++
EFn moves towards Ec
EFn moves towards Ec
EFp moves towards Ev
Figure 3.18: (a) Schematic of an equilibrium Fermi level position in ann−type semiconductor.
(b) The positions of the quasi-Fermi levels for the case where excess electrons are injected in the
conduction band. (c) The position of the quasi-Fermi levels when excess electrons and holes are
injected.
jump to the conduction band, then fall into a trap, etc. On a microscopic level there are generation
recombination processes occurring in a material which cause electrons to jump between valence
band, conduction band and trap states, as shown in figure 3.19.