2.2. ENERGY BANDS
27
A
X '
Wavevector A
Si
c
z
Figure 2.18. Band structure plots of the energy bands of the indirect-gap semiconductors Si and
Ge. The bandgap (absence of bands) lies slightly above the Fermi energy E = 0 on both figures,
with the conduction band above and the valence band below the gap. The figure shows that the
lowest point or bottom of the conduction band of Ge is at the energy E = 0.6 at the symmetry
point L (labeled by b+), and for Si it is 85% of the way along the direction A, from r,5 to X,. It is
clear from Fig. 2.15 that the bottom of the conduction band of the direct-gap semiconductor
GaAs is at the symmetry point r,. The top of the valence band is at the center point r of the
Brillouin zone for both materials.