9.2.2 Electrons, Holes, and Doping

Electronic devices require higher conductivity. All it takes is a few impurities. To understand how that works, we will start by looking at silicon, the semiconductor most widely used in electronics, although silicon is a very poor material for making diode lasers.

Silicon atoms have four electrons in their outer shell, and in silicon crystals each of those four electrons forms a bond with an adjacent atom, as shown in Figure 9-2A. Very few of the valance electrons in such pure silicon crystals can escape to the conduction band at room temperature.

In reality, any silicon crystal inevitably includes a few impurity atoms, and some of them can fit into the crystal in positions that otherwise would be occupied by silicon atoms. Suppose the impurity atom had five electrons in its outer shell, such as phosphorus or arsenic. As shown in Figure 9-2B, four of the electrons would form bonds with adjacent silicon atoms, but the fifth would be left over and it easily could move about through the crystal.

An atom with only three outer electrons such as aluminum or gallium also could fit into a space normally occupied by silicon, with its three outer electrons each bonding to adjacent silicon atoms. However, this would leave a hole at the place where the fourth outer electron of silicon normally would form a bond to an adjacent silicon atom, as shown in Figure 9-2C. The hole can "move" if a nearby electron moves to fill the hole, leaving behind a hole elsewhere in the crystal lattice. In that way, holes can move through the crystal as carriers of positive charge, to complement the negative charge carried by electrons.

Intentionally adding impurities can increase the conductivity of the semiconductor, making it more useful for electronic devices. A semiconductor doped with atoms that donate electrons to create free carriers, like phosphorus in silicon, is called n-type because it has negative current carriers. Semiconductors doped with atoms that produce holes (or electron acceptors) are called p-type because they contain extra holes that serve as positive current carriers. The degree of conductivity in n- and p-type materials depends on the impurity doping. Undoped semiconductors are called intrinsic or i-type.