Direct Energy, 2018a

110 6.3 Semiconductors and Energy Level Diagrams
6.3 contains the same information, but is zoomed in vertically to show the
valence electron levels more clearly.
The P atom has two more electrons than the Al atom. Phosphorous
atoms have more protons, so the electrons are a bit more tightly bound to
the nucleus. For this reason, it takes a bit more energy to rip the electrons
o, and the allowed energy levels are a bit dierent than for Al.
The amount of energy required to rip a 3p electron o the atom is
the vertical distance from the 3p level to the ground line at the top of the
gure. The amount of energy required to rip a 2p electron o is the vertical
distance from the 2p level to the ground line. As expected, these gures
show that it requires more energy to rip o the inner shell 2p electron than
the valence shell 3p electron. If enough energy is supplied, an electron will
be ripped o, and the electron will ow freely through the material. If
some energy is supplied but not enough to rip o the electron, the electron
can get excited to a higher energy level. The energy required to excite an
electron is given by the vertical distance in the gure from an occupied to
an unoccupied energy level. In either case, we say that an electron-hole pair
forms. If the amount of energy supplied is too small to excite an electron
from a lled to unlled state, the external energy will not be absorbed.
Energy Levels of Electrons of Isolated Al and Isolated P Atoms
at T>0 K
How do the energy levels change when the Al and P atoms are at temperatures
above absolute zero, where electrons are continuously vibrating
and moving? First, the energy levels broaden. The electrons can still only
take certain energy levels, but there is a wider range to the allowed energy
levels. Second, occasionally, electrons spontaneously get excited into
higher states. For example, a 3p electron may get excited into the 4s state
temporarily. If it does, it will quickly return to the ground state.
Energy Levels of AlP at T =0K
How much energy does it take to rip an electron o an AlP crystal at
T =0K? The three valence electrons of each Al atom and the ve valence
electrons of each P atom form chemical bonds. The energy required to rip
o these electrons is slightly dierent than the energy required to rip o
the equivalent electrons of isolated Al and isolated P atoms. Figure 6.4
illustrates the energy levels of the valence electrons of AlP. Unlike in the
previous gures, these energy levels do not come from actual data. Instead,
they are meant as a rough illustration of the eect. The amount of energy