Figure 1-23.Different types of crystal and metallic rectifiers.
There are many known metal-semiconductor combinations that can be used for contact rectification.
Copper oxide and selenium devices are by far the most popular. Copper oxide and selenium are
frequently used over other types of metallic rectifiers because they have a large forward current per unit
contact area, low forward voltage drop, good stability, and a lower aging rate. In practical application, the
selenium rectifier is used where a relatively large amount of power is required. On the other hand,
copper-oxide rectifiers are generally used in small-current applications such as ac meter movements or for
delivering direct current to circuits requiring not more than 10 amperes.
Since metallic rectifiers are affected by temperature, atmospheric conditions, and aging (in the case
of copper oxide and selenium), they are being replaced by the improved silicon crystal rectifier. The
silicon rectifier replaces the bulky selenium rectifier as to current and voltage rating, and can operate at
higher ambient (surrounding) temperatures.
In addition to their use as simple rectifiers, diodes are also used in circuits that mix signals together
(mixers), detect the presence of a signal (detector), and act as a switch "to open or close a circuit." Diodes
used in these applications are commonly referred to as "signal diodes." The simplest application of a
signal diode is the basic diode switch shown in figure 1-24.