Modern Spectroscopy

9.2 EXAMPLES OF LASERS 351
Figure 9.9 Impurity levels I in (a) an n-type and (b) a p-type semiconductor; C is the conduction
band and V the valence band
converts silicon from a high-temperature semiconductor into a room-temperature
semiconductor.
Alternatively, as in Figure 9.9(b), a dopant with one valence electron fewer than the host
contributes an impurity band I which is empty but more accessible to electrons from the
valence band. An example of such a p-type semiconductor is silicon doped with aluminium
ðKL3s23p1Þ in which the band gap is about 0.08 eV.
A semiconductor laser takes advantage of the properties of a junction between a p-type
and an n-type semiconductor made from the same host material. Such an n–p combination is
called a semiconductor diode. Doping concentrations are quite high and, as a result, the
conduction and valence band energies of the host are shifted in the two semiconductors, as
shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy EF. If a voltage is applied to the junction with the negative and positive terminals attached to
the n and p regions, respectively, electrons flow from the n to the p region, and positive holes
in the opposite direction. The levels are also displaced, as shown in Figure 9.10(b), and the
Fermi energies E0 FðnÞ and E00 FðpÞ are now unequal, resulting in a population inversion in the
region of the junction and leading to laser action. The semiconductor laser is, unusually, an
example of a two-level system, but the population inversion is not obtained by pumping: we
saw in Section 9.1.2 that this could not be done.
Figure 9.10 (a) The Fermi level E F in the region of a p–n junction. (b) The result of applying a
voltage across the junction; C is the conduction band and V the valence band