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What is a diode?

Diode fundamentals and History

Tale of a Diode

Below we have compiled a simple explanation of the history and principles behind diodes so that you can have a background of what we think is common knowledge. If you feel it's necessary, give it a glance to refresh your memory. "Well I already knew that!" is what some of you are thinking, and if that is case, feel free to skip this section.

1.Before the vacuum tube...

Rectifying properties and the Edison effect were discovered in the two-pole vacuum tube 1884. But, actually eight years before that, the rectifying effect of selenium was discovered in 1876. As you can see, the history of using the properties of semiconductors to create diodes that possess rectifying effects is very old. It may be hard to believe, but the history of semiconductors dates back before that of the vacuum tube!

2.From germanium to silicon

The first primitive diodes, like the selenium rectifier or crystal detectors, used iron pyrites and galena and other natural copper oxides (polycrystalline semiconductors). After that, as refining techniques advanced, we entered the age where highly sensitive single crystalline semiconductors could be produced with reliability-the age of germanium and silicon. Later we learned that germanium has a low resistance to heat, and so nowadays, most all semiconductors are made with silicon.

3.Rectification from the pn junction

The diode element consists of a structure called the pn junction. The terminal attached to the p-type semiconductor is called the anode, and the terminal attached to the n-type semiconductor is called the cathode. Current is allowed to flow from the anode to the cathode, but almost completely prevented from flowing in the reverse direction. This phenomenon is called rectification, and, put simply, it converts alternating current to a unidirectional current.

Diode model
Diode model

Diode schematic electrical symbol
Diode schematic electrical symbol

4.In other words, a diode is a valve!

If you intuitively conjure up an image for the effects of a diode, you might call it a "valve"-a valve for electrical current. If you think of electrical current as flowing water, the anode could be considered the upstream side and the cathode the downstream side. Water flows from upstream to downstream (or shall I say, the electrical current does), but the "valve" prevents it from flowing from downstream to upstream. This is the principle operation of a diode.

The valve is open and electricity flows (forward direction)
●The valve is open and electricity flows
(forward direction)

The valve is closed and electricity does not flow (reverse direction)
●The valve is closed and electricity does not flow
(reverse direction)

5.The many types of junctions

The current types of junctions in today's diodes can be divided into two main classifications: the pn junction and the Schottky barrier junction. The first one is a semiconductor-to-semiconductor junction, and this type of junction can be further classified into diffusion type junctions and mesatype junctions. The latter one uses the effects caused between a semiconductor and a metal and therefore is not actually called a junction in terms of diodes. However, to make things easier to understand, it will be considered a junction here. Currently, the Schottky barrier diode is known for its low power requirements and high speeds, and ROHM is making great advances with its Schottky barrier diode series.

6.Forward bias characteristics and reverse bias characteristics
Forward bias characteristics and reverse bias characteristics

A diode has two electrodes: the anode and the cathode. The anode is the (+) terminal and the cathode is the (-) terminal. The characteristics of the diode when current is flowing from the anode to the cathode are called the forward bias characteristics and VF and IF are examples of these characteristics. Conversely, if a (-) voltage is applied to the anode and a (+) voltage is applied to the cathode, current is prevented from flowing through the diode. The characteristics at this time are called the reverse bias characteristics and VR and IR are examples of these characteristics.

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