Pure silicon has almost no free electrons, so what engineers do is dope the silicon with a small amount of other materials to change its electrical properties. So inside the diode we have the two leads, the anode and the cathode which connect to some thin plates.
Between these plates we there is a layer of P-Type doped silicon on the anode side and a layer of N-type doped silicon on the cathode side. The whole thing is enclosed in a resin to insulate and protect the materials. Each silicon atom is surrounded by 4 other silicon atoms. Each atom wants 8 electrons in its valence shell but, the silicon atoms only have 4 electrons in their valence shell, so they sneakily share an electron with their neighbouring atom to get the 8 they desire.
This is known as Covalent bonding. When we add in the N-type material such as phosphorus, it will take the position of some silicon atoms.
The phosphorus atom has 5 electrons in its valence shell. With P-type doping we add in a material such as aluminium,. There is therefore a hole created where an electron can sit and occupy.
So we now have two doped pieces of silicon, one with too many electrons and one with not enough electrons. The two materials join to form a P-N junction.
In this region, some of the excess electrons from the N-type side will move over to occupy the holes in the P-type side. This migration will form a barrier with a buildup of electrons and holes on the opposite sides.
The electrons are negatively charged and the holes are considered therefore positively charged. So the build up causes a slightly negatively charged region and a slightly positively charged region. This creates an electric field and prevents more electrons from moving across. The potential difference across this region is about 0. When we connect a voltage source across the diode, with the anode P-Type connected to the positive and the cathode N connected to the negative, this will create a forward bias and allow current to flow.
The voltage source has to be greater than the 0. When we reverse the power supply so the positive is connected to the N-type cathode and the negative is connected to the P-type anode. The holes are pulled towards the negative and the electrons are pulled towards the positive which causes the barrier to expand, and therefore the diode acts as a conductor to prevent the flow of current.
Diodes are represented in engineering drawings with a symbol like the image above. The stripe on the body is indicated with a vertical line on the symbol and the arrow points in the direction of conventional current. When we look at a diode we see these numbers and letters on the body. These identify the diode so you can find the technical details online. The diode will have an I-V diagram that looks like above. This diagram plots the current and voltage characteristics of a diode which is plotted to form a curved line.
This side is how it should perform when acting as a conductor and this side is when acting as an insulator. You can see that the diode can only act as an insulator up to a certain voltage difference across it, if you exceed this, then it will become a conductor and allow current to flow. This will destroy the diode and probably your circuit, so you need to make sure the diode is sized correctly for the application. Equally, the diode can only handle a certain voltage or current in the forward bias.
The diode requires a certain voltage level to open and allow current to flow in the forward bias. Most are around 0. If we apply a voltage less than this, it will not open to allow current to flow.
But, as we increase past that, the amount of current that can flow will rapidly increase. The diodes will also provide a voltage drop into the circuit. For example when I added this diode into a simple LED circuit mounted to a breadboard, I get a voltage drop reading of 0.
As mentioned we use diodes to control the direction of current flow in a circuit. The diode can block the current and keep our components safe. We can also use them to convert AC to DC. As you might know AC or alternating current moves electrons forwards and backwards creating a sine wave with a positive and a negative half, but DC or direct current moves electrons in just one direction which gives a flat line in the positive region.
Using diodes, different types of rectifier circuits can be created, the most basic of which are half wave, full wave centre tapped, and full bridge rectifiers. Sensitive electronic devices need to be protected from surges in voltage, and the diode is perfect for this.
When used as voltage protection devices, diodes are nonconducting, however, they immediately short any high-voltage spike by sending it to the ground where it cannot harm sensitive integrated circuits.
These can handle large power spikes over short time periods which would normally damage sensitive components.
Maschinenfabrik Reinhausen GmbH. Rogers Germany GmbH. PCIM Europe. Schunk Sonosystems GmbH. The light-emitting diodes LEDs that we often see in our daily lives are designed to emit light when electricity flows through the PN junction. Diodes are also used in various places where we cannot see them, supporting our daily lives.
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