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.
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.
Zener diodes 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.
Schottky diodes 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.