What Is a Diode?
A diode allows current to flow in one direction but blocks it in the opposite direction, acting like an electronic check valve. They are essential for converting alternating current to direct current and for protecting electronic equipment from voltage spikes.
When an external voltage that is greater than and opposite to the built-in potential is applied, electron holes recombine at the p-n junction, allowing current to flow. This is known as forward bias.
The Basics
Diodes are semiconductor devices that regulate the direction of current flow within a circuit. They do this by allowing current to pass in one direction from the anode to the cathode (with a little help from the p-n junction), while staunchly blocking current trying to pass in the opposite direction.
This is why the diode symbol has an arrow pointed against it. Engineers conceived of the symbol to show that current flows through the device in the opposite direction than it does conventionally.
When a diode is subjected to a positive voltage, it turns “on” and conducts current from the anode to the cathode. The voltage needed to turn a diode on is called the forward bias voltage, or VF.
If a negative voltage is applied to the diode it turns off and conducts no current. This is because electrons from the P-type material fill holes from the N-type materials along the p-n junction, creating a potential barrier that blocks electricity. This state is known as the depletion region. This condition is indicated by a VBR value on the diode datasheet.
The Anode
The anode is where oxidation takes place, and is the positive ac to dc converter electrode in a galvanic cell or electrolytic cell. If a zinc metal anode is dipped in ZnSO4 solution, for example, it will undergo oxidation, giving off two electrons into the external circuit. In an electric frying pan, an anode made of tin will be subject to corrosion, causing a thin layer of rust to form on the underlying steel.
A diode is designed to allow current to flow from its anode to its cathode — but only if a certain amount of voltage is applied. This voltage, known as the forward voltage drop (or VF) is a function of the semiconductor material that the diode is made out of; light-emitting diodes, for example, typically have lower VFs than standard silicon-based diodes.
Despite their relatively humble origins, diodes play a huge role in today’s electronic devices. They regulate the direction of current flow, helping batteries charge properly and protecting circuits from damage caused by voltage spikes and surges. These days, you can find them in your car’s battery, your water heater, and more.
The Cathode
The cathode is the electrode through which current flows out of a polarized electrical device. It is positively charged and receives electrons from the anode. It is sometimes referred to as the plate or the terminal.
Early diodes were vacuum tubes containing two electrodes, the anode and the cathode. The cathode was heated to emit electrons which flowed to the anode. The resulting voltage was used to shape and control the flow of current in electronic devices such as oscilloscopes, cathode ray tubes, and electrolytic cells for hydrogen production.
The modern semiconductor diode is based on the principle of recombination, a process that allows electrons and holes to recombine at the p-n junction. Normally, the recombination process is halted by the built-in voltage between the n and p materials. However, an external voltage greater than and opposite to the built-in voltage can overcome the threshold and allow current to flow through the diode. The diode is then said to be forward biased. The amount of current a diode can carry depends on its maximum forward voltage and power dissipation. If the diode is subjected to more voltage and current than it can handle, expect it to heat up and eventually blow.
The P-N Junction
Electrons in the p-type region of the semiconductor material have more energy than holes in the n-type region. When the two regions are in thermal equilibrium electrons will diffuse from the p-type side to the n-type side and combine with the holes, creating a space charge region (depletion layer). The width of this depletion region is determined by how heavily each side of the junction is doped, the distance travelled by the electrons and their energy levels (as shown on the left hand part of the diagram above).
After this diffusion has occurred, the majority carrier density of the n-type and p-type material becomes smaller, reducing the built-in electric potential that exists across the junction due to the different concentrations of impurity ions on each Delay Lines side. This reduction in carrier density causes the depletion region to become wider, making it more difficult for free charges to pass across the PN junction.
However, if we apply a negative voltage to the diode the depletion region will become narrower as the electric field from the negative voltage pushes or repels electrons away from the junction. This makes it easier for holes to cross over and combine with each other, which allows current to flow through the diode.
The Depletion Region
In a diode, current can only flow in one direction, from the anode to the cathode. This unidirectional current is vital for the proper function of countless electronic components. Diodes prevent erratic current flows that could damage or destroy these sensitive components, and they are often used to guard against voltage reversals in electrical circuits.
When the p-type semiconductor and n-type semiconductor meet at the junction, there is a region between them that does not contain any free charge carriers (holes or electrons). This region is called the depletion region, depletion zone, or space charge region.
Over time, the majority of charge carrier diffusion across the p-n junction has depleted this area. The resulting gap is filled with immobile positive and negative space charges (ions) that cannot move.
An electric field develops due to the Coulomb force between these ions. This field creates a tendency for electrons from the n-side to migrate towards the p-side, and holes from the p-side to migrate toward the n-side. Eventually, these oppositely charged electrons and holes recombine. This recombination is slow and the depletion region can not grow indefinitely large. At some point, the depletion regions come together near the drain end and a condition known as pinchoff is reached. This point is where the drain current saturates and the resistance of the constricted channel increases significantly.
