energy levels
The farther an electron is from the nucleus, the higher is the energy state, and any electron that has left its parent atom has a higher energy state than any electron in the atomic structure.
Only specific energy levels can exist for the electrons in the atomic structure of an isolated atom. The result is a series of gaps between allowed energy levels where carriers are not permitted.
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.
Ge devices:
- photodetectors sensitive to light
- security system sensitive to heat
Si and GaAs:
- transistor networks, stability is a high priority
The wider the energy gap, the greater is the possibility of energy being released in the form of visible (infrared) light waves. For GaAs the gap is sufficiently large to result in significant light radiation.
The units of measurement are electron volts (eV). The unit
of measure is appropriate because W (energy) = QV (as
derived from the defining equation for voltage: V =
W/Q). Substituting the charge of one electron and a
potential difference of 1 V results in an energy level referred to as
one electron volt.
每个小电子都要一步一步往上爬，最终达到一个free的状态, 人生不是这样吗？