Tag Archives: Diodes

What are Zener, Schottky and Avalanche Diodes?

Diodes are very commonly used semiconductor devices. They are mostly used as rectifiers for converting Alternating to Direct current. Their special characteristic of allowing current flow in only one direction makes them indispensable as rectifiers. Apart from rectification, various types of diodes are available for different purposes such as for generating light, microwaves, infrared rays and for various types of switching at high speeds.

For example, the power supply industry has been moving towards high speed switching because higher speed reduces the volume of magnetics used, which ultimately reduces the bulk and price of the units. For switching at high frequencies, diodes are also required to react at high speeds. Schottky diodes are ideal for this purpose, as their switching speeds approach nearly zero time. Additionally, they have very low forward voltage drop, which increases their operating efficiency.

As their switching speed is very high, Schottky diodes recover very fast when the current reverses, resulting in only a very small reverse current overshoot. Although the maximum average rectified currents for Schottky diodes are popularly in the range of 1, 2, 3 and 10 Amperes, Schottky diodes that can handle up to 400A are also available. The corresponding maximum reverse voltage for Schottky diodes can range from 8 to 1200V, with most popular values being 30, 40, 60 and 100 Volts.

Another very versatile type of diode used in the power supply industry is the Zener diode. All diodes conduct current only when they are forward biased. When they are reverse biased, there is only a very small leakage current flowing. As the reverse voltage increases to beyond the rated peak inverse voltage of the diode, the diode can breakdown irreversibly and with permanent damage.

A special type of diode, called the Zener diode, blocks the current through it up to a certain voltage when reverse biased. Beyond this reverse breakdown voltage, it allows the current to flow even when biased in the reverse. That makes this type of diode very useful for generating reference voltages, clamping signals to specific voltage levels or ranges and more generally acting as a voltage regulator.

Zener diodes are manufactured to have their reverse breakdown voltage occur at specific, well-defined voltage levels. They are also able to operate continuously in the breakdown mode, without damage. Commonly, Zener diodes are available with breakdown voltage between 1.8 to 200 Volts.

Another special type of diode called the Avalanche diode is used for circuit protection. When the reverse bias voltage starts to increase, the diode intentionally starts an avalanche effect at a predetermined voltage. This causes the diode to start conducting current without damaging itself, and diverts the excessive power away from the circuit to its ground.

Designers use the Avalanche diode more as a protection to circuits against unwanted or unexpected voltages that might otherwise have caused extensive damage. Usually, the cathode of the diode connects to the circuit while its anode is connected to the ground. Therefore, the Avalanche diode bypasses any threatening voltage directly to the ground, thus saving the circuit. In this configuration, Avalanche diodes act as clamping diodes fixing the maximum voltage that the circuit will experience.

How did the diode get it’s name?

Although most diodes are made of silicon nowadays, it was not always so. Initially, there were two types – thermionic or vacuum tube and solid state or semiconductor. Both the types were developed simultaneously, but separately, in the early 1900s. Early semiconductor diodes were not as capable as their vacuum tube counterparts, which were extensively used as radio receiver detectors. Various types of these thermionic valves were in use and had different functionalities such as double-diode triodes, amplifiers, vacuum tube rectifiers and gas-filled rectifiers.

The diode gets its name from the two electrodes it has. Both the thermionic as well as the semiconductor type possess the peculiar asymmetric property of conductance, whereby a diode offers low resistance to flow of current in one direction and high resistance in the other. Similar to its vacuum counterpart, several types of semiconductor diodes exist.

The first semiconductor diode was the cat’s whisker type, made of mineral crystals such as galena and developed around 1906. However, these were not very stable and did not find much use at the time. Different materials such as selenium and germanium are also used for making these devices.

In 1873, Frederick Guthrie discovered that current flow was possible only in one direction and that was the basic principle of the thermionic diodes. Guthrie found that it was possible to discharge a positively charged electroscope when a grounded piece of white-hot metal was brought close to it. This did not happen if the electroscope was negatively charged. This gave him proof that current can flow only in one direction.

Although Thomas Edison rediscovered the same principle in 1880 and took out a patent for his discovery, it did not find much use until 20 years later. In 1900s, John Ambrose Fleming used the Edison effect to make and patent the first thermionic diode, also called the Fleming valve. He used the device as a precision radio detector.

To put it simply, a diode functions as a one-way valve. It allows electricity to flow in one direction while blocking all current flow in the reverse direction. The semiconductor diode has an anode (A, p-type or positive) and a cathode (K, n-type or negative). Since the cathode is more negatively charged compared to the anode, electric current will not flow if the cathode and anode are charged to the same or very similar voltage.

This property of the diode allowing current to flow in only one direction is utilized during rectification, when alternating current is changed to direct current. Such rectifier diodes are mostly used in low current power supplies. For turning a circuit on or off, you need a switching diode. If you are working with high-frequency signals, band-switching diodes are useful. Where a constant voltage is necessary, there are zener diodes.

Diodes are also used for various purposes such as the production of different types of analog signals, microwave frequencies and even light of various colors. When current passes through Light Emitting Diodes or LEDs, it emits light of a specific wavelength. Such diodes are used for displays, room lighting and for decoration.

Schottky Diodes – What makes them so special?

Some of the most common questions we get are about Schottky diodes.

Schottky Diode

The simple definition of a Schottky diode is a diode with a very fast switching action as well as a lower forward voltage drop.

As the current flows through a diode, it experiences a slight voltage drop across the diode terminals. Normally, a diode has approximately 0.7-1.7V drops. A Schottky diode, however, will see a drop in voltage between 0.15-0.45V. The benefit of this lower drop? A much higher system efficiency.

The construction of a Schottky diode also effects the voltage drop and switching time. A Schottky diode has a metal semiconductor junction as the Schottky barrier rather than the traditional semiconductor to semiconductor junction seen in conventional diodes. It is this barrier that affects the voltage drop and the speed of the switching times.

Sometimes Schottky diodes are misspelled by adding an ‘e’ to the end: Schottkey. The correct spelling is Schottky which is the surname of the man that is credited with putting these electronic components in the history books.

Opening Up and Tearing Down an IPOD Shuffle

Opening up and tearing down an IPOD Shuffle to see what’s inside…

The 3rd Generation of the IPOD Shuffle is a wonder of technology….1000 songs stored in an aluminum case smaller than a disposable lighter.

Did you ever wonder what electronic components make up the guts of an IPOD Shuffle?

You might be surprised at what goes into the circuitry of the IPOD Shuffle. In descending order by percentage of cost, the main components are:

logic, memory, metals, rechargeable materials, connectors, PCB, crystal, misc, capacitors, transistors, analog, diodes, magnetic, and plastics.

Here’s a partial breakdown by number of electronic components:

Capacitors – 65+
Resistors – 50+
Diodes – 4+

Pretty amazing what goes into equipment that measures only 45.2mm x 17.5mm x 7.8mm when fully assembled! This is possible because the components are extremely small surface mount components.

If you look at the cost breakdown by component family, it’s just as revealing. Naturally, the largest share is for memory in the form of IC’s. Over 70% (about $12.00 worth) is for logic and memory.

Types of Diodes

Diodes are an important part of today’s electronic components and are widely used for a number of applications. Accordingly, a large number of different types of diodes have been created to cater to their wide array of uses. The more popular types of diodes are described below.

Schottky Diodes: These diodes are made from a semiconductor to metal contact instead of semiconductor-semiconductor junction. This gives them a lower forward drop voltage as compared to pn junction diodes. They have a faster reverse recovery time and high switching speeds due to the low junction capacitance. They are used in voltage clamping applications and as low loss rectifiers.

LEDs (Light Emitting Diodes): These electronic components are formed from direct band gap semiconductors like gallium arsenide, and as the carriers cross the junction and recombine with the majority carriers, they emit photons. Infrared to ultraviolet wavelengths can be obtained depending upon the material used for making the LEDs. They are often used in signaling operations.

Varactor Diodes: These diodes are used as voltage controlled capacitors and have important applications in frequency locked loop and phase locked loops used in tuning circuits.

Zener Diodes: Zener diodes permit current to flow in the forward direction as in a normal diode, but where it differs is that it also allows current to flow in the reverse direction when the voltage exceeds the breakdown voltage also referred to as Zener voltage or the Zener knee voltage. It can be used as a precision voltage reference.

Avalanche diodes: These diodes are also used for conducting in the reverse direction once the reverse bias voltage increases the breakdown voltage. The reverse bias causes a wave of ionization, like an avalanche, and leads to a large current.

Tunnel diodes: These diodes have a negative resistance region of operation that is caused due to quantum tunneling. This allows for amplification of signals. These diodes offer most resistance to nuclear radiation.

Gunn Diodes: These diodes are similar to tunnel diodes except that they are made of different materials, like InP, GaAs, and exhibit negative differential resistance.

If you are just beginning to work with diodes, you might want to purchase a small amount of each type listed above. A good source for all diodes and other electronic components is West Florida Components.

Early Crystal Radios

Requiring no battery, the crystal radio was one of the earliest forms of radio having been developed in the late 1800s and early 1900s. At this time the crystal radio sets were used to receive Morse code messages, but as time progressed the voice messages could also be received by such sets. This progression had much to do with an improvement in materials, which included the diodes and tuning coil. Even with an improvement in materials though, the construction of a radio set was fairly simple to achieve.

By the 1920s and 1930s radio was taking off, but the sets were expensive objects to buy and so the crystal radio was the cheap alternative that could be built at home. Most major newspapers would run guides on how to build such radios, and it was information that was put to good use during the Second World War. During the war, allied Prisoners of War made use of the materials that they had on hand, to build their own radios, to find out news of the fighting. The soldiers would use recovered wire for the tuning coil and antenna, and make diodes from everyday material, like the pencil lead.

The crystal radio is still used by many people around the world today, although now it is usually a hobby rather than a necessity. In most cases, radio sets are now fairly cheap and mass produced making the building of a crystal radio a pleasure rather than something that needs to be done.