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    The Learning Circuit
    sudo Sergeant
    The Ben Heck Show


    Diodes are semiconductor components that allow electric current to flow in one direction but not the other. In this episode, Karen reviews p-in junctions and talks about how they differ from other types of common diodes, such as schottky diodes, zener diodes,  LEDs (light emitting diodes), laser diodes, and photodiodes.


    P-N junctions are considered your typical didoes. They have a p-n junction with a threshold voltage that has to be reached before current will flow through them. In silicon diodes, this is 0.7V. Once this is reached, the current will continue flowing. When hooked up backwards, in reverse bias, these diodes do not allow current to flow. If a diode is reverse bias, and it’s supplied with too much voltage, more than it’s breakdown voltage, it’ll “break-down” and current will flow through it in the wrong direction. Schottky diodes often look like typical diodes. But unlike p-n junction diodes, Schottky diodes have a metal-semiconductor junction. Silicon diodes require time for their depletion zone to grow and shrink when switching from allowing forward current to blocking reverse current. There’s a recovery time. Schottky junctions have no depletion zone. Because of their metal-semiconductor junction, Schottky diodes require virtually no recovery time and therefore have much faster switching speeds. This means they can handle switching current better and faster, which makes them useful in high frequency applications.They also have a lower forward voltage drop. Silicon diodes have a voltage drop of around 0.7V, but the voltage drop of Schottky diodes is between 0.15 V and 0.46 V. This means they lose less energy to heat, making them more efficient. Schottky diodes are not useful for all applications, as they can leak a small amount of current backwards. This could be problematic for certain circuits.


    While Schottky diodes can let some voltage leak through backwards, zener diodes are designed to allow current to flow in both directions. The p-n junction of zener diodes is heavily doped, only a specific voltage, the Zener voltage (Vz) can pass through without damaging the diode. In reverse bias, current will not flow through until the zener voltage is reached, but the voltage will be limited to the zener voltage.  For example, a 3.3V zener diode will not allow current to flow until the supply voltage reaches 3.3V.  If it’s supplied with 2V, no current flows. However, this diode could be supplied with 5, 6, 9, 12 Volts and it will regulate the voltage output to 3.3V. Zeners can have zener breakdown voltages of anywhere from 1.8V all the way up to 200V.



    LEDs, light emitting diodes, use energy from the particles moving through the p-n junction to create light. They can do this because they are made with gallium arsenide. Unlike silicon diodes, diodes made with gallium arsenide release energy in the form of light or photons. Like other diodes, they typically have 2 leads, though these can vary in length depending on the manufacturer. LEDs come in a wide variety of packages. Through-hole LEDs can be 3mm, 5mm, 10mm. They can have round and square lenses. Lenses can be clear or colored. 5mm round ones are the most common through-hole LEDs.  Surface mount LEDs come in a variety of sizes as well. When choosing an LED, one of the first things you’ll look for is color, or wavelength. (Chart-VO) Here’s a chart of the color spectrum. Another choice you’ll have is beam angle or viewing angle. Beam angle is the amount of degrees where the light is visible. Depending on your application, you may want a narrow beam angle, like 10deg or a wide beam angle, like 120deg. Where common light bulbs are measured in watts or lumens, the brightness of LEDs is its luminous intensity and it is measured in millicandelas or mcd.  The higher the number, the brighter the light. A standard LED could have a luminous intensity of 7mcd, whereas a super bright LED could be 120mcd. LEDs can have a range of voltage and current ratings. They have both a minimum current and voltage required to function properly, or even at all, as well as a maximum voltage and current, that if surpassed, can shorten the life of the LED or burn it out completely.




    While LEDs emit incoherent light, laser diodes emit high-intensity, coherent light. They do this by having a P-I-N, or pin junction rather than the standard p-n junction. The PIN junction has the standard P-type and N-type regions we’re familiar with. But between them there is an area of undoped, or intrinsic, semiconductor. At the junction, in that intrinsic layer is where all the magic happens. When current flows through the diode, energy is released in the form of photons or light into that intrinsic layer. But it doesn’t stop there. In a laser diode, the p and n-type regions have polished, reflective ends, which causes the photons to reflect back and forth, hundreds of times, in that intrinsic layer. The photons eventually escape in the form of a laser beam. You can usually see where a laser hits a surface, but you can’t always see the laser beam itself. If you can see the laser beam, be extra careful. A high powered laser has the potential of causing permanent damage to your eyes.


    While LEDs and lasers emit light, Photodiodes sense light. Light contains energy particles known as photons. When light strikes the PN junction of a photodiode, the energy from the photons is transferred to the diode and creates free electron and hole pairs. Remember what happens when a diode is in reverse bias mode. The charges in the junction repel the holes towards the anode and the electrons towards the cathode. This usually restricts the flow of current. Energy is being introduced by way of the photons, so a current is created in reverse bias mode. This is called the photoelectric effect and the photodiode is considered to be in photoconductive mode. Photodiodes can also be used in zero bias or photovoltaic mode. This is where current is restricted, so the electron-hole pairs, and therefore the voltage builds up.  A photodiode’s response time gets slower the larger the photodiode gets.