The effect of this doping process on the relative conductivity can best be described
through the use of the energy-band diagram of Fig. 1.8 . Note that a discrete energy level
(called the donor level ) appears in the forbidden band with an E g significantly less than that
of the intrinsic material. Those free electrons due to the added impurity sit at this energy
level and have less difficulty absorbing a sufficient measure of thermal energy to move into
the conduction band at room temperature. The result is that at room temperature, there are a
large number of carriers (electrons) in the conduction level, and the conductivity of the material
increases significantly. At room temperature in an intrinsic Si material there is about
one free electron for every 10 12 atoms. If the dosage level is 1 in 10 million (10 7 ), the ratio
10 12 >10 7 10 5 indicates that the carrier concentration has increased by a ratio of 100,000:1.