3.8. CARRIER GENERATION AND RECOMBINATION 127
– ––––––
Ec
Band to band
electron-hole
generation
Ev +––
–
– –––
– –––––
– donor ionization:
free electron +
ionized donor
– ––
–
– ––
+
+
– ––––– –
Band to band
electron-hole
recombination
+ ––– –––
–
– –––––
electron
recombination
with a donor
–
– ––
–
– ––
+ +
Figure 3.19: A schematic of carrier generation and recombination. Processes involving band to
band transitions are shown along with processes involving dopant or other impurity levels.
At equilibrium, thermal energy is responsible for exciting electrons from the valence band to
the conduction band. Such a generation process is calledthermalgeneration. We can also see
that if electrons are continuously excited up from the valence band into the conduction band,
there will be a build-up of free carriers. In order to reach an equilibrium concentration there has
to be carrierrecombination as well. Under steady state conditions we have
G=R (3.8.1)whereGis the generation rate andRis the carrier recombination rate.
In figure 3.19 we show a schematic description of carrier generation and recombination. Free
carriers can be generated if an electron leaves the valence-band and goes to the conduction-
band. They can also be generated if electrons leave a donor and go into the conduction-band.