What is a diode?
A diode is a semiconductor device with one PN junction or alternatively one metal-semiconductor junction. It has two terminals called anode and cathode.
It has the property of a switch that allows current to flow or not to flow depending on the direction of the voltage applied between the anode and cathode. This action is called rectification.
A semiconductor has an electrical conductivity value falling between that of a conductor that conducts electricity well such as a metal and that of an insulator that does not readily conduct electricity such as glass or rubber. For example, silicon and germanium are semiconductor materials.
(For more information on semiconductors, see the e-learning "Basic Knowledge of Discrete Semiconductor Device")
Basic Knowledge of Discrete Semiconductor Device
As diode materials, p-type and n-type semiconductors are used. Semiconductors that have been doped with boron (B) or another electron acceptor atom are called p-type semiconductors because a majority of charge carriers in them are positive holes. Semiconductors that have been doped with arsenic or another electron donor atom are called n-type semiconductors because a majority of charge carriers in them are negative electrons. A pn junction diode is formed by combining p-type and n-type semiconductors. For example, a pn junction is created by injecting p-type (n-type) dopant ions accelerated at high energy into an n-type (p-type) semiconductor.
A pn junction formed in this way with an electrode terminal attached is called a diode. A pn junction formed in this way with an electrode terminal attached is called a diode. (Fig. 1)
A diode conducts electric current only when it is forward-biased (i.e., when the anode is more positively biased than the cathode) (Fig. 2). This function of allowing current to flow in only one direction is called rectification.
Typically, a diode does not conduct when it is reverse-biased. However, as reverse-bias voltage is increased, large current suddenly passes through a diode at a given voltage. This voltage is called breakdown voltage (VBR). In the breakdown region above VBR, a slight change in voltage causes a large change in current. In other words, voltage changes only slightly with respect to a change in current. There are diodes designed to exploit this constant-voltage behavior such as Zener diodes and transient voltage suppression (TVS) diodes (also called ESD protection diodes).
Fig. 3 shows the IF-VF curve of a pn junction diode.
In addition, there is a type of diode in which a metal is used instead of either the p-type or n-type semiconductor. Because of a difference in work function between a metal and a p-type (n-type) semiconductor, the metal-semiconductor junction can be a Schottky junction that exhibits rectification characteristics like a pn junction, depending on the metal-semiconductor combination. Called a Schottky barrier diode (SBD), this type of diode is characterized by low forward voltage.
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