Introduction: How-To: Diodes

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If you have embarked upon electronics projects in the past, there is a good chance you have already encountered this common component and soldered into your circuit without second thought. Diodes are valuable in electronics and serve a variety of purposes, which will be highlighted in upcoming steps.

First, what is a diode?

A diode is a semiconducting device,  that allows current to flow in one direction but not the other.  

A semiconductor is a kind of material, in this case silicon or germanium, whose electrical properties lie between those of conductors (metals) and insulators (glass, rubber). Consider conduction: its is a measure of the relative ease of which electrons move through a material. For example, electrons move easily through a piece of metal wire. You can change the behavior of a pure material, like silicon, and turn it into a semiconductor by doping. In doping, you mix a small amount of an impurity into the pure crystalline structure.

The kinds of impurities added to pure silicon can be classified as N-type or P-type.
  • N-type:  With N-type doping, phosphorus or arsenic is added, in parts per billion, to the silicon in small quantities. Phosphorus and arsenic both have five outer electrons, so they are displaced when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. Electrons have a negative charge, hence the name N-type.
  • P-type - In P-type doping, boron or gallium is  added to the pure silicon. Those elements each have three outer electrons. When mixed into the silicon structure, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. 

Diodes are made from two differently doped layers of semiconductor material that form a PN junction. The P-type material has a surplus of positive charge carriers (holes) and the N type, a surplus of electrons. Between these layers, where the P-type and N-type materials meet, holes and electrons combine, with excees electrons combining with excess holes to cancel each other out, so a thin layer is created that has neither positive nor negative charge carriers present. This is called the depletion layer.

There are no charge carriers in this depletion layer and no current can flow across it.  But when a voltage is applied across the junction however, so that the P-type anode is made positive and the N-type cathode negative, the positive holes are attracted across the depletion layer towards the negative cathode, also the negative electrons are attracted towards the positive anode and current flows.

Think of a diode as a one-way street for electricity. When the diode is in forward bias, the diode allows traffic, or current, to flow from the anode, towards the cathode leg. In a reverse bias current is blocked so there is no flow of electricity through the circuit. When current is flowing through a diode, the voltage on the positive leg is higher than on the negative leg, this is called the diode's forward voltage drop. The severity of the voltage drop is a function of  the semiconductor material that the diode is made from. When the voltage across the diode is positive, a lot of current can flow once the voltage becomes large enough. When the voltage across the diode is negative, virtually no current flows.

Step 1: Different Uses for Different Diodes.

There are many different kinds of diodes, and each one serves a different purpose as an electronic component.

A Light Emitting Diode or LED is probably the most well known and most easily identified. The LED emits visible light when electrons jump across the PN junction.The resulting light is referred to as electroluminescence.

Photodiodes conduct only when they are exposed to light. These can be useful in making projects with a light activated switch, so that a circuit in only active in the presence of light.

Zenerdiodes are designed to conduct in the reverse direction, only when something called the breakdown voltage is reached will the circuit conduct. These are dialed to precise tolerances, see the section on Zener Diodes in step 3.

Rectifier diodes are designed to stop electricity from flowing in the wrong direction. Diodes are sometimes known as rectifiers for their use to rectify alternating current electricity into direct current, by removing the negative portion of the current.

Schottkydiodes are designed to turn on and off very rapidly when the breakdown voltage is reached, responding quickly in digital circuits. When current flows through a diode there is a very small voltage drop across the terminals.  Silicon diodes have a voltage drop, or loss; a Schottky diode voltage drop is significantly less. This lower voltage drop enables higher switching speed and better system efficiency.

Diodes can be used in a number of ways, like to protect a current-sensitive circuit. A device that uses batteries will likely contain a diode that protects it when battery is inserted improperly. The diode will stop the reversed current from traveling from the battery to the rest of the circuit-- thus, the diode protects the sensitive electronics inside the your circuit.

In the next few steps, you will find information about some of the most commonly used kinds of diodes.

Step 2: Light Emitting Diode

A light-emitting diode or LED lights up when electrically biased in the forward direction. This effect is a form of electroluminescence. 

A LED is a special type of semiconductor diode. Charge-carriers are created by an electric current passing through the pn-junction, and release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, is dictated by the materials forming the pn junction, which elements doped the pure material. A normal diode, emits invisible far-infrared light, but the materials used for a LED have bandgap energies corresponding to near-infrared, visible or near-ultraviolet light.

Unlike incandescent bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward biased. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted.

The semiconducting diode is encased in a solid plastic lens. Sometimes the plastic is colored, and you can find LEDs in almost every hue. Aside from the current rating on your LED, the size and shape of the plastic enclosure will dictate how, and how much, light the LED is able to throw. 

Step 3: Zener Diodes

Zener diodes are doped with a higher concentration of impurities to give them a very thin depletion layer. In use they are reverse biased. This means that current cannot move across a zener diode until the breakdown voltage is reached. In any diode, there comes a point where, if sufficient reverse voltage is applied, reverse current will flow from cathode to anode. The tightly bound electrons in the depletion layer are torn away from their atoms and there is an abrupt increase in current. If this current is allowed to build up to too high a value, damage can occur. However, if the reverse current is limited to a safe value, the diode will not be harmed and once the reverse voltage is reduced the diode stops conducting again.

Choose a zener diode if you need to have a voltage sensitive switch in your circuit. The available voltage breakdown ranges from about 2 volts to 200 volts. 

Step 4: Schottky Diodes

Unlike a PN-junction diode, a Schottky Diode has a metal–semiconductor (M–S) junction is a type of junction in which a metal comes in close contact with a semiconductor material. They are semiconductor diodes with a low forward voltage drop and a very fast switching action.

For the junction, molybdenum, platinum, chromium or tungsten are used; and a semiconductive an N-type silicon. The metal side acts as the anode and N-type semiconductor acts as the cathode. This is called the Schottky barrier. There are advantages in speed because Schottky diodes do not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. These kinds of diodes, by design, have a very precise breakdown voltage, and are able to respond, or switch, rapidly due to having a partially metal junction.

When current flows through a diode there is a very small voltage drop across the terminals.  This lower voltage drop is conducive of faster switching speed and better system efficiency. It reduces the power losses normally incurred in the rectifier and other diodes used within the power supply. With standard silicon diodes offering the main alternative, their turn on voltage is around 0.6 to 0.7 volts. With Schottky diode rectifiers having a turn on voltage of around 0.2 to 0.3 volts, there is a significant power saving to be gained.

Step 5: Rectifier Circuit

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

The most popular application of the diode is used for current rectification. This involves a device that only allows one-way flow of electrons. This is exactly what a semiconductor diode does.

There is a design called a called a full-wave bridge rectifier, it is built around a four-diode bridge configuration. (see image) Alternating current is fed to the bottom and top of the bridge rectifier, which the diodes filter into direct current by directing the current to the correct positive and negative points.

This circuit produces a DC output from an AC input, as well as reverse polarity protection. That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the wires from a DC power source have been reversed, and protects your circuit from damage caused by reverse polarity.

Step 6: Make an LED Grid!

A really simple way to get some experience with diodes is via LED circuits. To make an LED matrix, I used a 9V battery, a breadboard, 3V LEDs, and some 1K resistors.

I wired them with the positive on the right, moving to ground on the left. I created six distinct rows, and two columns of LEDs. Wiring in series, it goes from V(+) to the positive lead of the LED, and then another LED, then a 1K resistor to ground. Take a look at the schematic in this step.

Current moves from the anode to the cathode of each LED, and if any of the LEDs terminals are reversed - it will not illuminate.