Electron and Hole Current

• When a voltage is applied across a piece of intrinsic silicon, as shown in Figure 1-15, the thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attracted 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 another hole where it came from;
• Effectively the hole has moved from one place to another in the crystal structure, as illustrated in Figure 1–16;

• Although current in the valence band is produced by valence electrons, it is called hole current to distinguish it from electron current in the conduction band;
• As you have seen, conduction in semiconductors is considered to be either the movement of free electrons in the conduction band or the movement of holes in the valence band, which is actually the movement of valence electrons to nearby atoms, creating hole current in the opposite direction;
• It is interesting to contrast the two types of charge movement in a semiconductor with the charge movement in a metallic conductor, such as copper;
• Copper atoms form a different type of crystal in which the atoms are not covalently bonded to each other but consist of a “sea” of positive ion cores, which are atoms stripped of their valence electrons;
• The valence electrons are attracted to the positive ions, keeping the positive ions together and forming the metallic bond;
• The valence electrons do not belong to a given atom, but to the crystal as a whole;
• Since the valence electrons in copper are free to move, the application of a voltage results in current;
• There is only one type of current—the movement of free electrons—because there are no “holes” in the metallic crystal structure;