What is a Diode?

A diode is a semiconductor that allows current to flow in one direction but blocks it in the opposite direction. You’ll find them in all sorts of applications like rectification, power conversion, microwaves, and optics.

Unlike most electronic components, the diode’s voltage drop is fairly stable for large increases in current, as long as it is correctly biased.

The Basics

Diodes are specialized electronic components that act as one-way switches. They allow current to run in one direction (called the forward direction) while blocking it in the opposite direction, acting similar to a check valve. digital to analog This unidirectional behavior is what makes a diode work as a rectifier, turning AC signals into DC signal for use in microelectronics and other household electronics.

An external voltage potential applied across the PN junction will cancel out this barrier and permit current to flow freely with anode to cathode connection. This is called forward bias. It is for this reason that the arrow in the diode symbol points against the conventional current flow in diagrams, indicating that it is not “conventional.”

When the diode is in reverse bias, it blocks current, except for a very small amount of current that can flow through it due to the charge stored on the PN junction’s “n” side. This reverse saturation current is not a problem, and is actually a critical feature for many applications such as photodiodes (light-sensitive diodes that produce current when light falls on them) and LEDs (light-emitting diodes).

The datasheet for any given diode will often quote a typical value of the peak reverse voltage, which describes how much of this reverse saturation current a diode can sustain before it is destroyed. The smaller the diode’s PN junction, the less this reverse recovery charge can store, and thus the lower the peak reverse voltage will be.

The Materials

Diodes are made from semiconductor materials like silicon and germanium. They can be made to conduct current in one direction (forward bias) or restrict current in the other (reverse bias). There are many different types of diodes, but they all have similar characteristics.

For example, the voltage dropped across a conducting diode varies very little with the amount of current flowing through it. This property makes diodes very useful in circuits that require on-off control.

However, there are some exceptions to this rule. When the reverse current in a diode reaches a specific value, the resistance of the p-n junction suddenly drops to zero, allowing current to flow in the opposite direction. This value is called the breakdown voltage and it varies depending on the semiconductor material.

Other exceptions include photo-diodes that emit coherent light when current flows through them. These diodes are used in CD drives and laser devices. There are also avalanche diodes that break down by the avalanche effect when operated in reverse bias.

The most common semiconductor basic materials have four valence electrons in their outermost orbital that they share with neighboring silicon atoms to form full orbitals of eight electrons. Because of this, they are very good insulators. To make them conduct, they must have additional impurity atoms added to them. Adding amounts of arsenic, phosphorous, antimony or bismuth creates N-type semiconductor material with extra electrons. Adding boron, gallium or indium creates P-type semiconductor material with holes that accept electrons.

The PN Junction

The PN junction is the interface or boundary between p-type and n-type semiconductor materials. It is important because it determines how current flows through the diode. The p-type semiconductor material has an excess of positively charged holes, and the n-type semiconductor has an excess of negatively charged electrons. When the two semiconductors are joined together, the free electrons from the n-type material wander into the p-type and combine with the holes, cancelling each other out. The result is that the region near the p-n junction becomes depleted of mobile charge carriers and is called the space charge region or depletion layer (see figure A).

If no external voltage is applied the potential barrier at the junction is high, preventing current flow. This mode of operation is known as zero bias.

When an external voltage is applied in the forward direction (indicated by a positive symbol) an electric field is developed which helps electrons overcome the coulomb barrier and pass across the junction. laser driver As they move across the junction, the negative space charge regions on either side develop positive electric fields which repel them. Eventually, this leads to a condition where the width of the depletion region is so small that current can flow. This mode of operation is called forward bias. A similar process can occur when the voltage is applied in the reverse direction.

The Current Flow

A diode can conduct current only if an external voltage is applied that is greater than and opposite to its built-in potential. When this happens, recombination can proceed and a substantial electric current can flow through the p-n junction. This condition is referred to as forward biasing the diode.

As current flows across the PN junction in the forward direction, free electrons and holes are created in the n region. They immediately recombine to form electron-hole pairs and increase the barrier that other charges have to cross the p region in the center of the junction. As a result, the current in a forward-biased diode increases until it reaches a value known as the threshold voltage, at which point the current stops increasing.

If the diode is subjected to a negative voltage (reverse bias), the free electrons and holes are drawn away from the PN junction, decreasing the width of the depletion layer. As a result, the diode’s resistance to current flowing through it decreases for a short time, known as the reverse recovery period.

This behavior makes a diode a perfect electrical component for use in circuits that convert AC power to DC power. As such, a diode is often used as part of half-wave and full-wave rectifier circuits. It’s also often found in radio receivers, which use diodes to remove the positive and negative peaks of the amplitude modulated (AM) radio signal that contain information about audio signals.