where carriers are not permitted. However, as the atoms of a material are brought closer
together to form the crystal lattice structure, there is an interaction between atoms, which
will result in the electrons of a particular shell of an atom having slightly different energy
levels from electrons in the same orbit of an adjoining atom. The result is an expansion
of the fixed, discrete energy levels of the valence electrons of Fig. 1.6a to bands as shown
in Fig. 1.6b . In other words, the valence electrons in a silicon material can have varying
energy levels as long as they fall within the band of Fig. 1.6b . Figure l.6b clearly reveals
that there is a minimum energy level associated with electrons in the conduction band and
a maximum energy level of electrons bound to the valence shell of the atom. Between the
two is an energy gap that the electron in the valence band must overcome to become a free
carrier. That energy gap is different for Ge, Si, and GaAs; Ge has the smallest gap and GaAs
the largest gap. In total, this simply means that:
An electron in the valence band of silicon must absorb more energy than one in the
valence band of germanium to become a free carrier. Similarly, an electron in the
valence band of gallium arsenide must gain more energy than one in silicon or
germanium to enter the conduction band.
This difference in energy gap requirements reveals the sensitivity of each type of
semiconductor to changes in temperature. For instance, as the temperature of a Ge sample
increases, the number of electrons that can pick up thermal energy and enter the conduction
band will increase quite rapidly because the energy gap is quite small. However, the number
of electrons entering the conduction band for Si or GaAs would be a great deal less. This
sensitivity to changes in energy level can have positive and negative effects. The design of
photodetectors sensitive to light and security systems sensitive to heat would appear to be
an excellent area of application for Ge devices. However, for transistor networks, where
stability is a high priority, this sensitivity to temperature or light can be a detrimental factor.