Rectifier and Schottky Diodes

Rectifiers and Schottky diodes are very common components in electronics. They are used in devices that range from very low power examples to very large industrial equipment.

Both of these components fall under the category of diodes, electronic components that regulate the flow of current, permitting its passage in one direction and obstructing it in the other. Ideally, these devices would be able to allow a limitless amount of current to flow in one direction and to block a limitless amount in the other but this, of course, is not the case. Diodes come rated to operate and different levels of current and different voltages and using them in capacities for which they are not rated can cause them to fail.

These are among the simplest components used in electronics. They consist of two materials brought together and the materials used greatly dictate the applications for any given type. Those applications include simple ones, such as power supplies, and very advanced examples, such as radiation detectors.

 

What Is a Rectifier?

A rectifier is utilized to change AC current to DC current. Because a diode only allows current to pass in one direction, the alternating current is obstructed from flowing backward. This process of eliminating current in one direction is called rectification.

While diodes are commonly used for this application, there are other electronic components that can be used to rectify voltage. Switches have been used to perform this process, as have vacuum tubes and other components.

Because one direction of current flow is eliminated with the application of a rectifier to a circuit, the DC current that results from the placement of the diode is delivered in pulses. For devices that would not be able to perform with a pulsed current, filters are utilized to deliver a stable current that can provide power to the device.

 

Which Rectifier Types Exist?

There are several variations on rectifiers, each suited to their own applications.

A full-wave rectifier takes an input power and converts it to a single polarity at the output. This means, for instance, that AC current could be converted to purely negative or positive current, depending upon the needs of the builder. This requires the combination of several different components and, due to the common usage of these devices, such components are often items sold as readymade configurations.

Half-wave rectifiers take an AC sine wave and permit only one phase to pass through the component to the output. The negative or positive portion of the phase can be passed through, depending upon need. This is enormously simple to construct, requiring only a single diode. However, these types of rectifiers do produce the aforementioned pulsing current, and have a great deal of ripple, so filtering needs are generally more pronounced with this type of a rectifier.

Three-phase rectifiers are commonly used in industrial applications. These rectifiers can be of either aforementioned type or others. These typically employ a thyristor rather than a diode to construct the device.

 

What Is a Schottky Diode?

A Schottky diode is differentiated from other types by two principal qualities. First, the diode has a very low voltage drop compared to most other types. The result of that lowered voltage drop allows for the other particularly useful quality of these diodes, their fast switching action. That can help the systems into which these diodes are integrated to perform much more efficiently.

That fast switching action is the result of the construction of these diodes, detailed below. The diode, when it switches from passing current though to blocking current from passing in the other direction, takes an incredibly small amount of time, even compared to the 100 ns time typical of conventional diodes.

 

How Did the Schottky Diode Get its Name?

The Schottky diode is named after Walter H. Schottky (1886-1976), a physicist and a prolific inventor. He was associated with the Siemens company and has several electronic devices to his credit. He is also the developer of a mathematical formula that is fundamental to modern electronics. His contributions to science are most appreciated by the most technically-adept individuals and, thus, aren’t particularly well known among the public. They are significant, despite not being popularly understood.

The diode gets its name from the metal and semiconductor junction at the heart of his namesake diodes. This is called a Schottky barrier. The diode depends upon the asymmetric function at the interface of the metal and the semiconductor material.

 

What Are the Various Parts Within a Schottky Diode?

A Schottky diode has the same basic parts and any diode, but the materials at the core of it are different than p-n type diodes, the most common type.

The basic parts of the diode consist of the leads that attach it to the circuit and semiconductors within the diode that provide the required function. In standard semiconductor diodes, this consists of a p-type semiconductor and an n-type semiconductor. The anode is the positive lead of the diode, the cathode the negative.

A Schottky has, rather than a p and an n type semiconductor at its core, a core that is made out of an n type semiconductor and a metal. The metals used include chromium and tungsten, as well as precious metals such as platinum and palladium.

The Schottky has a very low voltage drop, which makes it useful in myriad applications. Combined with the fast switching, the simple variation in construction makes these very useful devices.

 

How Does a Schottky Diode Function?

One of the most useful applications of a diode takes advantage of the fact that it can go from a conducting state to a non-conducting state when the direction of the current applied is reversed. While the intervals are very small—in the nanosecond range—a standard p-n junction diode requires more time to complete this transition than does a Schottky diode.

When semiconductor diodes make the switch from conducting to non-conducting, they have to recover. This is eliminated in a Schottky diode by replacing one of the semiconductor materials with the metal chosen. Thus, the Schottky, because it doesn’t have the two semiconductor junction that necessitates recovery time after switches, can change states almost instantly. That translates to very much increased efficiency in the circuit.

 

How Is a Schottky Diode Constructed?

The construction of these diodes is very simple. The metal leads meet in the middle of the component. One lead connects to the semiconducting material and the other to the metal. This is wrapped in an insulating material, preventing it from being damaged. These devices are generally smaller electronic components.

 

Which Applications Require a Schottky Diode?

The great efficiency of these diodes largely dictates the applications to which they are put. They are used widely in radio mixers and in radio frequency detectors. This flows from their ability to operate at very high frequencies, into the MHz range.

Schottky diodes are also used to protect transistor components through a process called voltage clamping. Their usefulness in this regard is primarily based on their very low voltage drop.

Diodes often function as a sort of value. To use a plumbing analogy, they can be thought of as backflow prevention valves. They allow current to flow in one direction, but completely block it in another. This valve-like operation is what makes them useful for protecting devices sensitive to current flowing in the opposite direction. One obvious application for this is in charging systems.

For instance, if a battery or group of batteries are being charged by an outside power source, the current needs to be able to flow to the batteries to replenish them but not allowed to flow backward, creating a drain on the batteries. Schottky diodes are used commonly in this particular application.

Switched mode power supplies also employ these devices to control the flow of current. Such power supplies are used on numerous devices, including computers.

 

Which Companies Make Schottky Diodes?

Schottky diodes are very common components and they are manufactured by a variety of different companies. Some of the names include Fuji, Dynex, Cree, Bourns, Beckman, DAWIN Electronics, DiodesZetex, Fairchild Semiconductor, Lite-On and many others.

 

What Is Considered the Maximum Continuous Forward Current?

Electronic components sometimes vary between how they function ideally and how they function practically. In a diode, there should be no limit to how much forward current can flow through the device, as it should offer no resistance. However, in practical terms, there are limits. This describes those limits and the safe range that the diode can operate within.

This is measured in Amps and can range from tiny to very substantial levels of current in diodes intended to be used in industrial applications.

 

What Is the Peak Reverse Repetitive Voltage?

This is the highest voltage that the diode can handle in the reverse direction. If this voltage is exceeded, the diode may undergo what is called an avalanche breakdown. This means that the diode will start conducting current through materials that would normally function as insulators against current. In this case, the “repetitive” aspect of the name refers to receiving pulses of voltage in reverse-bias orientation.

 

What Is the Peak Forward Voltage?

This is the highest level of voltage that can be put through the diode in forward bias mode. The peaking phenomena has to do with the capacity of the diode to retain its properties as it heats up.

 

What Is the Peak Reverse Recovery Time?

For a very short amount of time, diodes may conduct in the reverse direction when the current direction changes, particularly when that direction changes rapidly. This describes the peak time that it takes for the diode to stop doing this. In Schottky diodes, there is a very low recovery time compared to p-n semiconductor diodes.

 

Which Diode Types exist?

The various types of diodes can be differentiated by both their construction and their function. There are, for instance, many different types of diodes that fall under the category of semiconductor diodes, but they have much different functions between them, and are used toward different ends because of this.

Schottky diodes, rectifier diodes, switching diodes and vacuum tube diodes are all different types of diodes, but how they differ is significant.

Vacuum tube diodes and solid-state diodes are two broad categories. Thermionic diodes are vacuum tube diodes, and are constructed by placing a filament and a plate in a glass tube which has all the air removed from it. These predated solid-state, semiconductor diodes that are seen in modern day power supplies.

Among those semiconductor diodes, there are many different types. There are the aforementioned Schottky diode, desirable for their very fast switching performance. There are also light emitting diodes which, when current flows through them, emit photons. The high efficiency and durability of these devices has led to them being the most likely replacements for incandescent and fluorescent lighting in many applications.

A photo diode is similar to an LED, but it produces current in the presence of light rather than producing light in the presence of current. A tunnel diode is a very specialized type of semiconductor diode that can operate at very high speeds and that are utilized in tuners, counters and pulse generators.

As mentioned, rectifiers are diodes used in the capacity of changing the polarity of a voltage from an alternating state to a constant one.

Diodes can also be differentiated by how their junctions are made. In the common p-n junction diode, there are two semiconductors in contact with one another. Two of the common materials used are gallium arsenide and germanium. These are doped to create the desired type.

The aforementioned Schottky diodes can be described in terms of their junctions, which are metal-semiconductor functions, in this particular case.

Point contact diodes have a metal contact that interfaces with the semiconductor and that produces a p-type semiconductor as a result.

Thermionic diodes are far less frequently employed today than they were in the past. Semiconductor diodes are the norm. They offer very good performance and reliability, are very efficient and very compact.

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