Semiconductors

Most materials are conductors, insulators, or something in-between, but a special class of materials known as semiconductors can be persuaded to exhibit both conducting and insulating properties. The first semiconductor to undergo evaluation was the element germanium (chemical symbol Ge). However, for a variety of reasons, silicon (chemical symbol Si) replaced germanium as the semiconductor of choice. As silicon is the main constituent of sand and one of the most common elements on earth (silicon accounts for approximately 28% of the earth’s crust), we aren’t in any danger of running out of it in the foreseeable future.

Pure crystalline silicon acts as an insulator; however, scientists at Bell Laboratories in the United States found that, by inserting certain impurities into the crystal lattice, they could make silicon act as a conductor. The process of inserting the impurities is known as doping, and the most commonly used dopants are boron atoms with three electrons in their outermost electron shells and phosphorus atoms with five.

If a pure piece of silicon is surrounded by a gas containing boron or phosphorus and heated in a high-temperature oven, the boron or phosphorus atoms will permeate the crystal lattice and displace some silicon atoms without disturbing other atoms in the vicinity. This process is known as diffusion. Boron-doped silicon is called P-type silicon and phosphorus-doped silicon is called N-type (Figure 10.1).

Figure 10.1 Creating P-type and N-type silicon

Because boron atoms have only three electrons in their outermost electron shells, they can only make bonds with three of the silicon atoms surrounding them. Thus, the site (location) occupied by a boron atom in the silicon crystal will accept a free electron with relative ease and is therefore known as an acceptor. Similarly, because phosphorus atoms have five electrons in their outermost electron shells, the site of a phosphorus atom in the silicon crystal will donate an electron with relative ease and is therefore known as a donor.

10.2 Semiconductor Diodes

As was noted above, pure crystalline silicon acts as an insulator. By comparison, both P-type and N-type silicon are reasonably good conductors (Figure 10.2).

Figure 10.2 Pure P-type and N-type silicon

When things start to become really interesting, however, is when a piece of silicon is doped such that part is P-type and part is N-type (Figure 10.3).

Figure 10.3 Mixing P-type and N-type silicon

The silicon with both P-type and N-type conducts electricity in only one direction; in the other direction it behaves like an OPEN (OFF) switch. These structures, known as semiconductor diodes, come in many shapes and sizes; an example could be as shown in Figure 10.4. (Note that the “semiconductor” portion of semiconductor diode was initially used to distinguish these components from their vacuum tube-based cousins. As semiconductors took over, everyone started to just refer to them as diodes.)

Figure 10.4 Diode: Component and symbol (a) Diode component; (b) Symbol

If the triangular body of the symbol is pointing in the classical direction of current flow (more positive to more negative), the diode will conduct. An individually packaged diode consists of a piece of silicon with connections to external leads, all encapsulated in a protective package (the silicon is typically smaller than a grain of sand). The package protects the silicon from moisture and other impurities and, when the diode is operating, helps to conduct heat away from the silicon.

Due to the fact that diodes (and transistors as discussed below) are formed from solids—as opposed to vacuum tubes, which are largely formed from empty space—people started to refer to them as solid-state electronics.