Band Gap

Overview

  1. The band gap is the minimum amount of energy required for an electron to break free of its bound state.
  2. When the band gap energy is met, the electron is excited into a free state, and can therefore participate in conduction.
  3. The band gap determines how much energy is needed from the sun for conduction, as well as how much energy is generated.
  4. A hole is created where the electron was formerly bound. This hole also participates in conduction.

The band gap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can participate in conduction. The band structure of a semiconductor gives the energy of the electrons on the y-axis and is called a "band diagram". The lower energy level of a semiconductor is called the "valence band" (EV) and the energy level at which an electron can be considered free is called the "conduction band" (EC). The band gap (EG) is the gap in energy between the bound state and the free state, between the valence band and conduction band. Therefore, the band gap is the minimum change in energy required to excite the electron so that it can participate in conduction.

Schematic of the energy bands for electrons in a solid.

Once the electron becomes excited into the conduction band, it is free to move about the semiconductor and participate in conduction. However, the excitation of an electron to the conduction band will also allow an additional conduction process to take place. The excitation of an electron to the conduction band leaves behind an empty space for an electron. An electron from a neighboring atom can move into this empty space. When this electron moves, it leaves behind another space. The continual movement of the space for an electron, called a "hole", can be illustrated as the movement of a positively charged particle through the crystal structure. Consequently, the excitation of an electron into the conduction band results in not only an electron in the conduction band but also a hole in the valence band. Thus, both the electron and hole can participate in conduction and are called "carriers".

The concept of a moving "hole" is analogous to that of a bubble in a liquid. Although it is actually the liquid that moves, it is easier to describe the motion of the bubble going in the opposite direction.