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What is a diode?
Outline of a Diode
Diodes can be classified by the functions of the circuit in which it is being used, or more commonly, by the shape that is demanded by the size of the products in which it will be mounted. The complicated point is that there is no direct relation between the two and you must keep them both in your mind at all times. However, you can think of the function as the base, and since this function can then be supported by many different shapes, it can then be further classified by those shapes. |
| 1.Classification by the working frequency The most basic types of diodes |
How a Zener diode works
Zener diodes are usually placed in a series with a resistance. At this time, E = R × IZ (voltage at both edges of R) + VZ (Zener voltage, constant) holds true. In other words, even if the power supply voltage E fluctuates, the Zener voltage VZ stays constant (however, E must be greater than VZ), and the voltage difference between E and VZ is generated at both edges of R.
* Furthermore, a Schottky barrier diode extends from the high-frequency range to the normal-frequency range.
Why does rectification occur?
---This can be explained by the movement of the carrier density.---
When a (+) voltage is applied to the p-type region from the anode, and a (-) voltage is applied to the n-type region from the cathode, the electrons in the n-type region are pulled towards the p-type region and the holes in the p-type region are pulled towards the n-type region. The result is that the pn junction does not stop the flow of the electrons or holes and the electrical current flows smoothly along the circuit. On the other hand, if a (-) voltage is applied to the p-type region from the anode, and a (+) voltage is applied to the n-type region from the cathode, the holes in the p-type region gather at the anode and the electrons in n-type region gather at the cathode. In other words, no holes or electrons end up existing at the pn junction and the electrical current does not flow.
<Diode static characteristics>
■pn junction (forward bias)
■pn junction (reverse bias)
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■Zener diode (constant voltage diode)
Since a Zener diode generates a constant voltage from the Zener effect that occurs when a reverse bias voltage is applied, it is also called a constant voltage diode. By using the resulting constant voltage, it can be used in constant voltage circuits and other circuits that require a reference voltage.
■Schottky barrier diode
A Schottky barrier diode attaches a Schottky gate electrode directly to a n-type semiconductor and makes use of the fact that reverse bias voltages are prevented from causing current flow across the junction of the metal and semiconductor. Some are for high frequencies and some are for general rectification. Those for high frequencies are often used in high-speed switching for wave detectors and mixers in the UHF and microwave bands. Since the forward voltage is small and the reverse breakdown voltage cannot be made too high (currently, approx. 100 to 200 volts) compared to normal diodes for general rectification, Schottky barrier diodes for general rectification are used for the rectification of power supplies for low voltages and high currents, or power supply switching for the rectification of high frequencies with its small reverse recovery time.
■Switching diode
By using the rectifying properties of the pn junction (the extreme difference in the forward and reverse bias resistances), these diodes are mainly used for switching the circuit on and off. Generally, when you speak of switching diodes, it includes Schottky barrier diodes and band switching diodes, but we at Rohm think of them as separate types. Switching diodes are the normal silicon small-signal switching diodes. They are used for demodulation, modulation, switching, mixing, and the like and make up approximately half of the total diodes produced in Japan. Rohm holds the largest share of this market.
■Rectifier diodes
In general, these diodes are aimed at current rectification for currents of 1 ampere or more and are used in rectification circuits for power supplies. There are many types from low power to high power with a wide range of different packages. The most common type is the low power 1 ampere class that makes up approximately 70% of the rectifier diodes produced.
■Band switching diodes
The band switching diode was developed as a general small signal diode for high frequency use. Since it is used for switching the frequency bands for high frequency tuners, the resistance at high frequencies is small and the capacitance between the electrodes is kept as small as possible. |
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2.Classification by structure |
This allows classification by the different configurations of the diode elements. Currently the main constructions are the flat planar diode types and high breakdown voltage mesa diode types. |
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*Diffused-junction rectifier (pn junction type)
By introducing impurities (boron or phosphorous) to the silicon semiconductor by heat diffusion (by causing the impurities to permeate into the semiconductor with high temperatures), this type of diode refers to a configuration where both the p-type and n-type regions are diffused with impurities. At the junctions, a type of barrier called an electric potential barrier is formed, and this barrier produces the rectification for the electrical current.
*Schottky barrier diode
Schottky barrier diodes make use of the electric potential barrier formed at the junction between metals and semiconductors. The knowledge that rectifying properties were present at the junctions between metals and semiconductors has been around for a long time, but the man that theoretically explained the phenomenon was a physicist named Walter Schottky, and this structure now bears his name. Compared to the pn junction of a diffused-junction rectifier, there is no storage of minority carriers and the reverse recovery time is extremely short. This enables an excellent efficiency for rectification in high frequencies, a low forward voltage (Vf), and a minimal amount of power loss. This is why Schottky diodes are often used in applications for high frequency rectification. |
■Planar diode
 Today, the most popular method for creating a semiconductor junction, is forming an oxidized layer on a silicon substrate, opening holes in the required locations, and then introducing impurities into the holes for a diffused junction. Since the oxidized membrane on the silicon is resistant to the introduction of the impurities, the junctions can be made at only the required locations. Also, the portions of the surface of the silicon substrate where the junctions can occur are protected by this oxidized membrane for a structure that is very resistant to external contaminants. |
■Mesa diode
 With a structure that is shaped like a geological mesa, this configuration allows for very large breakdown voltages (Vr). This is the reason mesa diodes are often used for rectification purposes. However, even though the breakdown voltage can be made very high, since the structure causes the pn junction to be exposed, the reverse current (leak current) is often much larger (worse) than that for planar diodes. For Rohm, the MSR, GSR, and RLR are diodes of this configuration. |
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3.Classification by forward current |
When classifying diode by their forward current (IF), those with a forward current (IF) of less than 1 ampere are called small-signal diodes, and those with a forward current (IF) of 1 ampere or more are called rectifier diodes.
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4.Classification by intergratability |
One of the strong points of Rohm is that it offers diode arrays. Opposed to discrete types, diode arrays are an integration of many diodes.
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5.Classification by shape |
Diodes come in many different shapes to accommodate different packages, mounting types, and functions. The two major types are the lead type and the surface mounting type.
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Tale of a Diode