Conduction in Semiconductors

Semiconductors behavior

Semiconductors behave differently based on temperature, acting as insulators at lower temperatures while acting as conductors at the higher temperatures. At higher temperature, conduction occurs because the electrons around the semiconductor atom can break the covalent bond and  move freely around the lattice. The semiconductors conductive property is the basis for understanding how these materials can be used in different electrical devices.The semiconductor material properties can be determined by the bond structure of the semiconductor. A major effect is the energy levels that the electrons can occupy and the way electrons move about the crystal lattice. The electrons within the formed covalent bond between each of the atoms in the lattice structure are kept in place by this bond and so they become localized to the region around the atom. The bonded electrons can’t move or change energy, and so they are not considered free and can’t contribute to the current flow, absorption, or any other physical processes.  But only at zero all electrons reach this stuck bonded arrangement. At higher temperatures, enough energy can be gained by electrons to escape from their bonds. If this happens, the electrons become free to move about the crystal lattice and participate in conduction. At normal room temperature, the semiconductor has adequate free electrons to make it conduct current. At absolute zero the semiconductor acts like an insulator.

Energy states of electrons

The electron is at a high energy state when it gains enough energy to participate in conduction, while it is at a low energy state when it is bound, and therefore can’t participate in conduction. So, two different energy states for the electrons are introduced by the presence of the bond between the two atoms. The electron can’t attain values of energy in-between these two levels; as it is either at the low energy position in the bond, or it has gained enough energy to break the bond and therefore has a definite minimum energy. The band gap of a semiconductor is this minimum energy. The number and energy of the free electrons participating in conduction is basic to the electronic devices operation. The space the electrons leave behind, allows the movement of the covalent bond from one electron to the other, therefore appearing as a positive charge that moves through the crystal lattice. This empty space is commonly referred to as a hole, and is like an electron but it has a positive charge.  

Conduction of Semiconductors and Solar cells

The most critical parameters of the semiconductor material for solar cell operation are:

• the band gap
• the number of electrons or holes (free carriers) which are available for conduction
• Free carriers (electrons or holes) generation and recombination as a response to light rays striking the material.