Electronics 104: Conduction in Semiconductors

How does electrical current conduct in Semiconductors.

Guide

Conduction

The way all material conduct electrical current is important in understanding the operation of any semiconductor component or device.

Conduction Electrons

Electrons of an atom can only exist within a prescibed energy band (a Shell), which directly correlates to the amount of energy an electron is carrying. Each Energy Band is seperated by an Energy Gap; an area that requires more energy for an electron to cross than electron possess. Within these energy gaps, no electron can exist. 
An external force may be applied, such as heat, to provide atoms in a shell to posses the energy needed to cross the energy gap and move to the next level of energy and become a free electrons.

Free Electrons arealso called Conduction Electrons  because these are the electrons that will doing all the work to conduct electrical current.

Conduction Bands

Just as every atom has a Valence Band (an outermost shell for electrons to orbit in) so too do all atoms which have combined/connected with each other a band of their own, the "Coduction Band". The Conduction Bandl of the combined material/group of atoms can interact with the Valence Band of the individual atoms that make up the group.

Conduction Holes

When enough external energy is applied for some valence electrons to jump to the conduction band then a vacancy is left in the valance band of the of the source atom. This vacancy is called an Electron Hole. For every electron raised to the level of the conduction band there is a vacant hole created; this combination is called the Electron-Hole pair. Electrons in the conduction band will recombine with any vacant holes in the valence band of the closest atom.

Electron and Hole Current

When a voltage is applied accross a piece of semiconductive material, some electrons within each atom will gain enough energy to jump the conduction band, which are then free to move randomly in the crystal structure, are now easily attracted toward the positive end. This movement of free electrons within the material of a semiconductor is called Electron Current.

Another type of current exists at the valence shell level of each atom within the material by the holes created when an electron in the valence shell becomes a free electron in the conduction band. Valence electrons cannot move freely throughout the material as they are still attached to their atom but they can easily move to an empty hole, thereby created another empty hole from the position it came from. Effectively, the hole has move from one location in the structure to another location in the structure. This movement of holes throughout the material is called Hole Current. 

 Next: ELECTRONICS 105: P-TYPE AND N-TYPE SEMICONDUCTORS