Direct Energy, 2018a

114 6.3 Semiconductors and Energy Level Diagrams
levels broaden. Second, some electrons get excited to higher energy levels
and quickly, perhaps in a few microseconds, decay back down.
6.3.3 Denitions of Conductors, Dielectrics, and Semiconductors
Conductors, dielectrics, and semiconductors were dened in section 1.5.1.
Now that we have seen example energy level diagrams, we should revisit
these denitions as well as dene the term semimetal. In the presence of an
applied external voltage, electric eld, optical eld, or other energy source,
valence electrons ow easily in a conductor [10, p. 429] [11, ch. 4]. In a
conductor, the conduction band is partially lled with electrons, so there
are many available energy states for electrons remaining in the conduction
band. With just a little bit of external energy, possibly even from vibrations
that naturally occur at T>0 K, valence electrons ow easily. Inner shell
electrons can be ripped o their atoms and ow, but signicantly more
energy is needed to ripo inner shell than valence electrons.
In the presence of an applied external voltage, electric eld, optical eld,
or other energy source, electrons do not ow easily in an insulator [10, p.
429] [11, ch. 4]. The valence band is lled and the conduction band is
empty. The energy gap between valence band and conduction band in an
insulator is typically above 3 eV. A little heat or energy from vibrations is
not enough to excite an electron from one allowed energy state to another.
If a large enough external source of energy is applied, though, an electron
can be excited or ripped o of an insulator.
In Sec. 3.3, electro-optic materials were discussed. Some insulators
are electro-optic which means that in the presence of an external electric
or optical eld, the spatial distribution of electrons changes slightly which
cause a material polarization to build up. Photons of the external electric
or optical eld in this case do not have enough energy to excite electrons
in the insulator, so the internal momentum of electrons in the material
does not change. The electro-optic eect occurs in insulators and involves
external energies too small to excite electrons from one allowed energy state
to another while the aects discussed in Sec. 6.3 involve semiconductors
and external energies large enough to excite electrons from one energy level
to another.
At T = 0 Kina semiconductor, the valence band is full, and the
conduction band is empty. The energy gap of a semiconductor is small, in
the range 0.5 eV E g 3 eV. In the presence of a small applied voltage,
electric eld, or optical eld, a semiconductor acts as an insulator. In the
presence of a large applied voltage or other energy source, a semiconductor