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Diodes

Index

    Symbol
    Diode Function
    Forward Voltage Drop
    Reverse Voltage
    Diode Construction
    Standard Rectifiers
    Fast Rectifiers
    Ultra Fast Rectifiers
    Schottky Diodes
    Varicap Diodes
    Zener Diodes
    Transient Supressors
    Photo Diodes
    PIN Diodes
    Avalanche Photodiodes
    Germanium diodes
    Light Emitting Diodes (LED)
    SMD Diodes


Symbol

Diode Symbol

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Diode Function
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

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Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

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Reverse Voltage
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

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Diode Construction
The physical construction of a diode with a diffusion junction is shown in the figure below. When a diode is reverse biased ie. a positive voltage is applied to the cathode with respect to the anode, an electric field is formed between the cathode and anode specifically across the depletion region. The diode is 'reverse biased' and cannot conduct except for small leakage currents. However, if the electric field becomes too strong 'avalanche breakdown' occurs and the diode will become a short circuit and often be damaged. To counteract this the physical distance between the anode and cathode is increased by increasing the size of the bulk region and changing impurity atom doping levels.

Diode Construction

In the construction process, N type silicon substrate heated to ~1000oC in presence of vapour containing positive charged impurity atoms. P region diffused into N region. The resultant effect is to cause more charge carriers to be present within the diode when it is conducting. For the diode to switch OFF, the charge carriers must either recombine (minority) or be removed, the latter mechanism appearing as a reverse current (reverse recovery) flowing in the diode as it turns OFF. Put simply, diodes with higher voltage ratings have larger bulk regions, require more time to remove internal charges at turn OFF and are thus slower switching.

Standard Rectifiers
Rectifiers are electronic high voltage diodes, which allow current to flow in only one direction. Essentially, they act as one-way valves, and are used to convert AC current to DC current.


The performance of high voltage diodes is determined by a number of voltage, current and time coefficients:

VRRM: Maximum Reverse Voltage, which is the maximum reverse voltage of the diode.

VF: Forward Voltage, which is the voltage across the diode terminals resulting from the flow of current in the forward direction.

IR: Reverse Current flows when reverse bias is applied to a semiconductor junction.

trr: Reverse Recovery Time is the time required for the current to reach a specified reverse current (IR) after instantaneous switching from a specified forward condition (IF).

IF: Forward Current is the current flowing through the diode in the direction of lower resistance.

Tj: Junction Operating Temperature is the range of temperatures in which the high voltage diodes are designed to operate.

You can find listings for standard rectifier diodes at the links below:
Asahi Engineering
Mouser Electronics

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Fast Rectifiers
Figure 3a and b show typical styles of reverse recovery. The area within the negative portion of each curve, , is the total reverse recovery charge Qrr and represents the charge removal from the junction and the bulk regions of the diode and is effectively independent of the forward current in the diode. The recovery time t2 - t1 is dependant on the size of the bulk region thus high di/dt currents can be obtained when using fast diodes. If the di/dt of the snap recovery is too high and stray inductance exists in the circuit then extremely high and possibly damaging voltage spikes can be induced.
(Note: ). Qrr can be found from manufacturers specifications thus the maximum reverse recovery current Irr is given by:

If ta is very small compared to ta then ta trr and knowing the rate of decrease of current di/dt = Irr/ta Irr/trr leads to:

Figure 3:
(a) Reverse recovery of a general purpose diode, (b) fast diode. Reverse recovery time trr = t2 - t0.

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The effect of reverse recovery on the output voltage of a rectifier feeding a resistive load is shown in figure 4.

Figure 4: Bridge rectifier output voltage showing diode reverse recovery effects.

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Ultra Fast Rectifiers
ABSTRACT:   International Rectifier's new series of Ultra-fast recovery diodes are aimed specifically at the 12/24/48V SMPS output stage, and extend the company's current product range of Ultra-fast recovery diodes with industry standard part number products. The new product series has been developed to meet today's requirement of high frequency operation and power ratings, using a technology platform flexible enough to match the performance improvement curve of the market requirements in the years to come. The new IR Ultra-fast recovery diode series (200-400V) adopts platinum diffusion in order to overcome the limitation of gold diffusion and the electron irradiation technology. With this approach, the best trade off for leakage current, forward voltage drop and reverse recovery, has been achieved with a maximum operating junction temperature of 175 degrees Celsius and a reverse recovery time as low as 15-20ns. With this type of performance, the maximum allowable switching frequency for this Ultra- fast diode family would be up to 500-750kHz. This assumption is verified by the diode loss calculation used for the IR MUR1620 operating in a typical output rectification in a forward converter.

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Schottky Diodes
To achieve very fast switching, schottky diodes can be used, although their current and voltage ratings are restricted. Rectifying action is dependant solely on majority carriers therefore no minority carrier recombination. Recovery is dependant on the capacitance of the metal-silicon junction. Polished pre-doped N+ epitaxial substrate with a thin N layer barrier metal deposit. Interface between the metal and N layer creates a barrier potential.

Schottky Diode Construction

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Varicap Diodes
Symbol

Varicap Diode Symbol

It provides an electrically controllable capacitance, which can be used in tuned circuits. It is small and inexpensive, which makes its use advantageous in many applications. Its disadvantages compared to a manually controlled variable capacitor are a lower Q, nonlinearity, lower voltage rating and a more limited range.
The capacitance decreases as the reverse bias increases, according to the relation C = Co/ (1 + V/Vo)n, where Co and Vo are constants. Vo is approximately the forward voltage of the diode. The exponent n depends on the doping density of the semiconductors PN junction and the distance away from the junction. For a graded junction (linear variation), n = 0.33. For an abrupt junction (constant doping density), n = 0.5. If the density jumps abruptly at the junction, then decreases (called hyperabrupt), n can be made as high as n = 2.

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Zener Diodes
Symbol

Zener Diode Symbol

The diagram below shows the forward conduction and reverse breakdown voltage of a zener diode.

Zener Diode Diagram

Notice that as the reverse voltage is increased the leakage current remains essentially constant until the breakdown voltage is reached where the current increases dramatically. This breakdown voltage is the zener voltage for zener diodes. While for the conventional rectifier or diode it is imperative to operate below this voltage; the zener diode is intended to operate at that voltage, and so finds its greatest application as a voltage regulator.

The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available.
When specifying zener diodes, a tolerance of the specified voltage must be stated. The most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available . A test current (Iz) must be specified with the voltage and tolerance.
The power handling capability also needs to be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1 , 5, 10, and 50 Watts.
To help in your selection process I have prepared a table of the more common zener diodes aranged in order of watage.

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Transient Supressors
Although there are other types of transient suppressor we are looking here at an enhanced zener diode that has improved clamping properties.
There are two main forms of zener diode transient suppressor, Uni-directional for use in DC circuits and Bi-directional for use in AC circuits. They should be specified according to their working peak reverse voltage Vrwm often abbreviated to Vr which should be equal or greater than the peak operating voltage. When these devices are not active they dissipate less than a milli-watt making them very efficient. The other important factor being the power handling of the device. When these devices are active they dissipate a lot of power and substantial heat is generated in a small area, which is the main limitation of this device.
A uni-directional suppressor acts just like a zener diode clamping voltage spikes. The bi-directional suppressor is made from two zener diodes connected by their cathodes, this means that they can act on either negative or positive spikes.

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Photo Diodes
Photodiodes are fabricated from semiconductor materials. The most popular choices are silicon (Si) or gallium arsenide (GaAs), and others include indium antimonide (InSb), indium arsenide (InAs), lead selenide (PbSe), and lead sulfide (PbS). These materials absorb light over a characteristic wavelength range, for example: 250 nm to 1100 nm for silicon, and 800 nm to 2.0 µm for GaAs. When a photon of light is absorbed, it excites an electron and produces a single pair of charge carriers—an electron and a hole, where a hole is simply the absence of an electron in the semiconductor lattice. Current passes through a semiconductor when the charge carriers separate and move in opposite directions.

Photo Diode Representation

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PIN Diodes
The trick in a photodiode is to collect the photon-induced charge carriers as current or voltage at the electrodes, before they have a chance to recombine. This is achieved using a pn or P-(Intrinsic layer)-N diode junction structure—hence the term p-i-n photodiode. In a generic p-i-n photodiode, light enters the device through the thin p-type layer. Absorption causes light intensity to drop exponentially with penetration depth. Any photons absorbed in the depletion region produce charge carriers that are immediately separated and swept across the junction by the natural internal bias. Charge carriers created outside the depletion region will move randomly, many of them eventually entering the depletion region to be swept rapidly across the junction. Some of them will recombine and disappear without ever reaching the depletion region. This movement of charge carriers across the junction upsets the electrical balance and produces a small photocurrent, which can be detected at the electrodes.

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Avalanche Photodiodes
One limitation of the p-i-n photodiode is the lack of internal gain—an incoming photon produces only one electron-hole pair. Low-light applications require detectors with internal gain to boost the signal above the noise floor of subsequent electronics and signal processors. For many years, however, the only device that provided such gain was the photomultiplier tube (PMT). Although it offers high gain, the PMT has a number of practical limitations: It is a bulky vacuum tube; it generates heat; and compared to a photodiode, it offers limited linearity, a narrow spectral response range, and a low QE (< 25%). Fortunately, the avalanche photodiode (APD) now offers a solid-state alternative for most PMT applications (see figure below).

Avalanche Photodiode Representation

In an APD, as with any other photodiode, incoming photons produce electron-hole pairs; however, the APD is operated with a large reverse bias (up to 2 kV for beveled-edge designs), which accelerates the photon-generated electrons. The electrons collide with the atomic lattice, releasing additional electrons via secondary ionization. These secondary electrons also are accelerated, which results in an avalanche of carriers, hence the name.

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Germanium diodes
Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V (as opposed to the 0.6V drop of a Silicone based diode) and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.

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Light Emitting Diodes (LED)
Symbol

Light Emitting Diode Symbol

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LED Picture

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MELF/SOD80 Diode Identification

 

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