PHOTODIODES BASIC AND TUTORIALS

A photodiode can be regarded as a high-impedance non-ohmic photosensitive device whose current is almost independent of applied voltage. The incident light falls on a reverse-biased semiconductor junction, and the separation of electrons from holes will allow the junction to conduct despite the reverse-bias.

Photodiodes are constructed like any other diodes, using silicon, but without the opaque coating that is normally used on signal and rectifier diodes. The junction area may be quite large, so the photodiode may have more capacitance between electrodes than a conventional signal diode.

This can be compensated by using a feedback capacitor in the circuit, illustrated in Figure 5.6, which shows a typical circuit for using a photodiode along with an operational amplifier for a voltage output.

The feedback resistor R will determine the output voltage, which will be RI, where I is the diode current.
• Some LEDs can be used as photodiodes with peak sensitivity values in the infra-red or in the visible spectrum, and in some circuits it can be convenient to use the same device as both a receiver and an indicator.


Characteristics for photodiodes specify the output current into a short circuit, and the current will be much lower into a resistance of appreciable value. The sensitivity can be quoted in terms of incident light measurements, but Table 5.2, shows, more usefully, the output of some types when the incident light is provided by various typical sources.

Figure 5.8 shows typical circuits using a photodiode using an operational amplifier as a load. The circuit in (a) is used for high sensitivity and operation down to DC levels. The circuit in (b) is preferred when speed of response is preferred to operation at very low frequencies.

SCHOTTKY DIODES BASIC AND TUTORIALS

Schottky diodes are named for their discoverer, the physicist Walter Schottky. A Schottky diode consists of a metal-semiconductor junction, in which the semiconductor is usually silicon, and the metal can be, typically, silver, aluminium, gold, chromium, nickel, platinum or tungsten, or alloys of exotic metals.

The diode conducts using majority carriers, so that the forward drop is small, only about 0.2 V compared to the 0.6 V of a silicon diode. In addition, the diodes have very fast switching times, meaning that when the voltage is switched off the current also turns off with only a very small delay.

This feature makes the Schottky diode useful in RF applications such as RF demodulation and in high frequency switch-mode power supplies.

Because of the low voltage drop, the diodes also make excellent power rectifiers, particularly for high-frequency supplies, though the reverse current is too high for some applications.

Figure 5.4a shows the relevant symbol.

Schottky diodes are also used embedded into ICs (see later) in logic circuits, and as part of complex devices ranging from photodiodes to MOSFETs.

Silicon carbide Schottky diodes are now being used for high-current diodes with very high voltage ratings (up to 1200 V).

BREAKDOWN DIODES BASIC AND TUTORIALS

Although reverse breakdown of a diode is a departure from its rectifying action, practical use can be made of this effect. If a diode is supplied with reverse current from a current source with a sufficiently high voltage capability (>|BV|), the diode voltage is substantially constant over a wide range of current.

The diode, now used as a breakdown (or Zener) diode, has wide application in providing stabilized voltages ranging from 2.7 V to 200 V or more.


A breakdown diode is characterized by its nominal breakdown voltage and the reciprocal of the reverse characteristic in the reverse region, the dynamic slope resistance (rz).

An ideal breakdown diode has a well specified breakdown voltage and zero slope resistance giving a constant reverse voltage (in breakdown) indepen dent of temperature and reverse current.

In practice, however, the breakdown characteristic is curved in the low reverse current region and the reverse current supplied must be of sufficient magnitude to ensure that the breakdown diode operates beyond the knee of the characteristic in a region of low slope resistance.

Further, even beyond the knee, slope resistance varies with reverse current and depends on the nominal breakdown voltage and temperature. Manufacturers’ data should be consulted for accurate figures. In general, rz is a minimum for devices with a |BV| of approximately 6 V and operated at high reverse currents.

At lower currents and for both higher and lower values of |BV|, rz increases. The temperature coefficient of breakdown voltage depends on both the nominal breakdown voltage and on the reverse current.

Below approximately 5 V the temperature coefficient is negative and above is positive. This is because different breakdown mechanisms occur for low and high breakdown voltages.

At approximately 5 V both mechanisms are present and produce a zero temperature coefficient. The device rating which is important for a breakdown diode is the power dissipation, the product of reverse current and breakdown voltage.

LASER DIODES BASIC AND TUTORIALS

LASER DIODES BASIC INFORMATION
What Are Laser Diodes?

The laser diode is a further development upon the regular light-emitting diode, or LED. The term "laser" itself is actually an acronym, despite the fact it's often written in lower-case letters.

"Laser" stands for Light Amplification by Stimulated Emission of Radiation, and refers to another strange quantum process whereby characteristic light emitted by electrons transitioning from high-level to low-level energy states in a material stimulate other electrons in a substance to make similar "jumps," the result being a synchronized output of light from the material.


This synchronization extends to the actual phase of the emitted light, so that all light waves emitted from a "lasing" material are not just the same frequency (color), but also the same phase as each other, so that they reinforce one another and are able to travel in a very tightly-confined, nondispersing beam.

This is why laser light stays so remarkably focused over long distances: each and every light wave coming from the laser is in step with each other:

Incandescent lamps produce "white" (mixed-frequency, or mixed-color) light. Regular LEDs produce monochromatic light: same frequency (color), but different phases, resulting in similar beam dispersion.

 Laser LEDs produce coherent light : light that is both monochromatic (single-color) and monophasic (single phase), resulting in precise beam confinement.

Laser light finds wide application in the modern world: everything from surveying, where a straight and nondispersing light beam is very useful for precise sighting of measurement markers, to the reading and writing of optical disks, where only the narrowness of a focused laser beam is able to resolve the microscopic "pits" in the disk's surface comprising the binary 1's and 0's of digital information.

Some laser diodes require special high-power "pulsing" circuits to deliver large quantities of voltage and current in short bursts. Other laser diodes may be operated continuously at lower power.

In the latter case, laser action occurs only within a certain range of diode current, necessitating some form of current-regulator circuit. As laser diodes age, their power requirements may change (more current required for less output power), but it should be remembered that low-power laser diodes, like LEDs, are fairly long-lived devices, with typical service lives in the tens of thousands of hours.

MULTIMETER CHECK OF DIODES BASIC AND TUTORIALS

CHECKING DIODES USING METER BASIC INFORMATION
How To Check Diodes Using Multimeter Tutorials?

Being able to determine the polarity (cathode versus anode) and basic functionality of a diode is a very important skill for the electronics hobbyist or technician to have.

Since we know that a diode is essentially nothing more than a one-way valve for electricity, it makes sense we should be able to verify its one-way nature using a DC (battery-powered) ohmmeter.


Connected one way across the diode, the meter should show a very low resistance. Connected the other way across the diode, it should show a very high resistance ("OL" on some digital meter models):

Of course, in order to determine which end of the diode is the cathode and which is the anode, you must know with certainty which test lead of the meter is positive (+) and which is negative (-) when set to the "resistance" or "­" function.

With most digital multimeters I've seen, the red lead becomes positive and the black lead negative when set to measure resistance, in accordance with standard electronics color-code convention. However, this is not guaranteed for all meters.

Many analog multimeters, for example, actually make their black leads positive (+) and their red leads negative (-) when switched to the "resistance" function, because it is easier to manufacture it that way!

One problem with using an ohmmeter to check a diode is that the readings obtained only have qualitative value, not quantitative. In other words, an ohmmeter only tells you which way the diode conducts; the low-value resistance indication obtained while conducting is useless.

If an ohmmeter shows a value of "1.73 ohms" while forward-biasing a diode, that figure of 1.73 ­ doesn't represent any real-world quantity useful to us as technicians or circuit designers. It neither represents the forward voltage drop nor any "bulk" resistance in the semiconductor material of the diode itself, but rather is a figure dependent upon both quantities and will vary substantially with the particular ohmmeter used to take the reading.

For this reason, some digital multimeter manufacturers equip their meters with a special "diode check" function which displays the actual forward voltage drop of the diode in volts, rather than a "resistance" ¯gure in ohms. These meters work by forcing a small current through the diode and measuring the voltage dropped between the two test leads:

The forward voltage reading obtained with such a meter will typically be less than the "normal" drop of 0.7 volts for silicon and 0.3 volts for germanium, because the current provided by the meter is of trivial proportions. 

If a multimeter with diode-check function isn't available, or you would like to measure a diode's forward voltage drop at some non-trivial current, the following circuit may be constructed using nothing but a battery, resistor, and a normal voltmeter:

Connecting the diode backwards to this testing circuit will simply result in the voltmeter indi-

cating the full voltage of the battery.

If this circuit were designed so as to provide a constant or nearly constant current through the diode despite changes in forward voltage drop, it could be used as the basis of a temperature measurement instrument, the voltage measured across the diode being inversely proportional to diode junction temperature. 

Of course, diode current should be kept to a minimum to avoid self-heating (the diode dissipating substantial amounts of heat energy), which would interfere with temperature measurement.

Beware that some digital multimeters equipped with a "diode check" function may output a very low test voltage (less than 0.3 volts) when set to the regular "resistance" (­) function: too low to fully collapse the depletion region of a PN junction. 

The philosophy here is that the "diode check" function is to be used for testing semiconductor devices, and the "resistance" function for anything else. By using a very low test voltage to measure resistance, it is easier for a technician to measure the resistance of non-semiconductor components connected to semiconductor components, since the semiconductor component junctions will not become forward-biased with such low voltages.

Consider the example of a resistor and diode connected in parallel, soldered in place on a printed circuit board (PCB). Normally, one would have to unsolder the resistor from the circuit (disconnect it from all other components) before being able to measure its resistance, otherwise any parallel- connected components would affect the reading obtained. 

However, using a multimeter that outputs a very low test voltage to the probes in the "resistance" function mode, the diode's PN junction will not have enough voltage impressed across it to become forward-biased, and as such will pass negligible current. 

Consequently, the meter "sees" the diode as an open (no continuity), and only registers the resistor's resistance:

If such an ohmmeter were used to test a diode, it would indicate a very high resistance (many mega-ohms) even if connected to the diode in the "correct" (forward-biased) direction:

Reverse voltage strength of a diode is not as easily tested, because exceeding a normal diode's PIV usually results in destruction of the diode. There are special types of diodes, though, which are designed to "break down" in reverse-bias mode without damage (called Zener diodes), and they are best tested with the same type of voltage source / resistor / voltmeter circuit, provided that the voltage source is of high enough value to force the diode into its breakdown region.

COMMON DIODE TYPES BASICS AND TUTORIALS

WHAT ARE THE COMMON DIODE TYPES?

Depending on their applications, diodes can be segregated into the following major divisions:

Small Signal Diode.

These are the semiconductor devices used most often in a wide variety of applications. In general purpose applications, they are used as a switch in rectifiers, limiters, capacitors, and in wave shaping.

The common diode parameters a designer needs to know include forward voltage, reverse breakdown voltage, reverse leakage current, and recovery time.

Silicon Rectifier Diode.

These are the diodes that have high forward-current carrying capability, typically up to several hundred amperes. They usually have a forward resistance of only a fraction of an ohm while their reverse resistance is in the megaohm range.

Their primary application is in power conversion, such as for power supplies, UPS, rectifiers=inverinverters etc. In case of current exceeding the rated value, their case temperature will rise. For stud mounted diodes, their thermal resistance is between 0.1 to 1# C=W.

Zener Diode.

Its primary applications are in the voltage reference or regulation. However, its ability to maintain a certain voltage depends on its temperature coefficient and impedance.

The voltage reference or regulation application of Zener diodes are based on their avalanche properties. In the reverse-biased mode, at a certain voltage the resistance of these devices may suddenly drop. This occurs at the Zener voltage VX , a parameter the designer knows beforehand.

Figure 2.4 shows a circuit in which a Zener diode is used to control the reference voltage of a linear power supply.

Under normal operating conditions, the transistor will transmit power to the load (output) circuit. The output power level will depend on the transistor base current.

A very high base current will impose a large voltage across the Zener and it may attain Zener voltage VX , at which point it will crush and limit the power supply to the load.

Photodiode.

When a semiconductor junction is exposed to light, photons generate hole-electron pairs. When these charges diffuse across the junction, they produce photo current. Hence this device acts as a source of current that increases with the intensity of light.

Light-Emitting Diode (LED).

Power diodes used in PE circuits are high-power versions of the commonly used devices employed in analog and digital circuits. They are manufactured in many varieties and ranges. The current rating can be from a few amperes to several hundreds while the voltage rating varies from tens of volts to several thousand volts.