The electrons of an atom can exist olywithin prescribed energy bands. Each shell around the nucleus corresponds to a certain energy band and is separated from adjacent shell by energy gaps.
Conduction electrons and Holes:
An instrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal)energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons. Free electrons are also called conduction electrons. This is illustrated in Energy diagram and Bonding diagram below:
When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal. This vacancy is called a hole. For every electron raised to the conduction band by external energy , there is one hole left in the valence band, creating what is called an electron-hole pair. Recombination orrurs when a conduction-band electron loses energy and falls back into a hole in the valence band.
A piece of intrinsic silicon at room temperature has , at any instant, a number of conduction-band (free) electrons that are unattached to any atom and are essentially drifting randomly , throughout the material. There is also an equal number of holes in the valence band created when these electrons jump into the conduction band.
Electron Current , Hole Current:
When a voltage is applied across a piece of instrinsic silicon , the thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attrated toward the positive end. This movement of free electrons is one type of current in a semiconductive material and is called electron current.
Another type of current occurs in the valence band, where the holes created by the free electrons exist. Electrons remaining in the valence band are still attached to their atoms and are not free to move randomly in the crystal structure as are the free electrons. However, a valence electron can move into a nearby hole with little change in its energy level, thus leaving anogher hole where it came frome. Effectively the hole has moved from one place to another in crystal structure.