A silicon diode is a semiconductor that has positive and negative polarity, and can allow electrical current to flow in one direction while restricting it in another. The element silicon, in its pure form, acts as an electrical insulator. To enable it to conduct electricity, minute amounts of other elements — in a process known as doping — are added to it. These make the charged semiconductor material used to build silicon diodes, which are then often utilized in radios, computers, alternating current-direct current (AC/DC) power supplies, and as temperature and radiation sensors, among other applications.
When a silicon diode is made, it has both a positive and negative side, and a connection between the two, known as the p–n junction. The two differently-charged sides are a result of differing elements being added to the silicon. The positive side, known as an anode and made from p-type silicon, is doped with boron or gallium. Their atomic structure, combined with the silicon, creates the positive charge. Adding phosphorous or arsenic creates the negative cathode, made from n-type silicon, in a similar manner.
Silicon diode voltage has a forward bias of 0.7 volts. This means that 0.7 volts is needed to power the diode. Once this amount of energy passes through it, it will conduct electrical current across its p–n junction. It will also stop most current from flowing in the reverse. Every silicon diode has a maximum voltage that can be applied to it in reverse before it will break down. This tends to be at least 50 volts or more.
In a way, the diode can be thought of as an electrical check valve because it passes excess energy forward, but generally does not let a measurable amount back through in the reverse. A very small amount of current can, in fact, flow in reverse through the diode, but it is so minute that a circuit breakdown from this amount would typically be rare.
Since a silicon diode only generally allows power to flow one way, it can be used to protect other devices in a circuit, such as transistors, from receiving too much power and burning out. In addition, specially configured silicon diodes, known as Zener diodes, can be used to maintain a fixed voltage. These are made to intentionally conduct some electricity backward when necessary, in order to keep a precise amount of it flowing.