Diodes are small electrical devices that are used to convey an electric current in one direction, and to inhibit the opposing current from moving in the opposite. They have two terminals, each with an electrode—one electrode is positively charged whereas the other is negatively charged. A diode’s ability to transfer a current only in one direction is also called a rectifying property. When a diode transmits a current in one direction it’s known as the forward biased condition; the reversed biased condition occurs when a diode blocks a current from moving in the opposite direction. However, a diode’s ability to be unidirectional depends on the type of diode and the technology employed. Different types of diodes, such as thermionic and various kinds of semiconductor diodes, use different technology to accomplish current transmission.
Thermionic diodes, also called vacuum tubes, are diodes that encase the electrodes in a glass vacuum—early models looked somewhat like miniature light bulbs. A heater filament is used to transfer heat which both causes a thermally induced emission of electrons within the vacuum, and heats the cathode. In this case, the anode becomes positive and attracts the electrons, transmitting the current in one direction. Because the anode will not release the electrons even when the temperature drops, the electrons can only move in one direction and the process cannot reverse direction.
Although thermionic diodes were a common early form of diode, most modern diodes are some type of semiconductor diode. Materials like silicon and germanium are often used because they have no free electrons, meaning they cannot easily transmit electricity and tend to serve as insulators. However, by doping these materials their chemical properties can be altered. When doping silicon, there are two types of impurities that can be added to turn silicon into a semi-conductive material: N-type and P-type.
An N-type impurity is either phosphorous or arsenic. Each of these has five outer electrons whereas silicon has four, so the extra phosphorous or arsenic electron has nothing to bond to. Instead, the extra electron serves as a means to transmit energy. Only a small amount of phosphorous or arsenic is needed to generate enough free electrons to transfer a current through silicon. Because the electrons carry a negative charge, this type of impurity is known as N-type.
In P-type doping, one of two different impurities are used: boron or gallium. Each of these impurities only has three outer electrons, so when added to silicon they form holes where they lack an electron as well as a positive charge. The positive charge enables boron or gallium to accept neighboring electrons, which in essence bumps the hole over within the lattice of electrons. The presence of holes is what enables the transfer of currents and the movement of electrons, making P-type doped silicon a conductive material. The name P-type derives from the material’s positive charge. Both N-type and P-type doping turn silicon into a conductor, but not a very strong one—this is why doped silicon is called a semiconductor.
P-type and N-Type silicon are used together in semiconductor diodes. To create a P-N diode, a P-type silicon material constitutes the anode, and transfers the current to the N-type cathode. Because of the charges and the materials’ properties, the current cannot be transferred in the opposite direction. In other kinds of semiconductor diodes, a metal is used to create one contact, while a P-type or N-type semiconductor serves as the other contact. When used in a reversed biased condition, the block most of the current. When used in a forward biased condition, enough voltage is transferred to start the diode and the transfer of electrons can begin.