What is a Diode?
Diode is the simplest two-terminal unilateral semiconductor device.  It allows current to flow only in one direction and blocks the current that flows in the opposite direction. The two terminals of the diode are called as anode and cathode. The symbol of diode is as shown in the figure below.
Symbol of a Diode


The characteristics of a diode closely match to that of a switch. An ideal switch when open does not conduct current in either directions and in closed state conducts in both directions. The characteristic of a diode is as shown in the figure below.
Characteristics of a Diode



Ideally, in one direction that is indicated by the arrow head diode must behave short circuited and in other one that opposite to that of the direction of arrow head must be open circuited. By ideal characteristics, the diodes is designed to meet these features theoretically but are not achieved practically. So the practical diode characteristics are only close to that of the desired.
Diode characterstics


How To Connect A Diode

Let’s look at an example.

How diodes work - right direction

In the circuit above the diode is connected in the right direction. This means current can flow through it so that the LED will light up.

But what happens if we connect it the other way around?

How diodes work - wrong direction

In this second circuit the diode is connected the wrong way. This means that no current will flow in the circuit and the LED will be turned OFF.

How a Diode Works

The diode is created from a PN junction. You get a PN junction by taking negative doped and positive doped semiconductor material and putting it together.

What is a diode? Diode PN Junction, symbol and look of component

At the intersection of these two materials a “depletion region” appears. This depletion region acts as an insulator and refuses to let any current pass.

When you apply a positive voltage from the positive side to the negative side, the “depletion layer” between the two materials disappears and the current can flow from the positive to the negative side.

When you apply a voltage in the other direction, from the negative to the positive side, the depletion region expands and resists any current flowing.

The Most Standard Diodes

Rectifier Diode

Image result for rectifier diode symbol

The rectifier diode is typically used for rectifying alternating current (AC) to direct current (DC). They can usually handle big currents and voltages. Sometimes referred to as power diodes.

Signal Diode

Image result for signal diode

The signal diode works the same way as the rectifier diode. But it can only handle small currents and voltages. It is faster than the rectifier diode and is sometimes referred to as a high-speed diode.

The most typical signal diode is the 1N4148.

Zener Diode

Circuit symbols of Zener diode

The zener diode is a component that will become very conductive (meaning that it will allow lots of current) for a certain voltage. This voltage is called the zener voltage.

What this means is that the voltage drop over the zener diode will not exceed the zener voltage. The zener diode is often used as a stable voltage reference.

Other Types of Diodes

Schottky Diode

Image result for Schottky Diode

The Schottky diode is very similar to a standard small-signal diode. The difference is that the Schottky diode has a lower voltage drop over its terminals. Normal diodes have about 0.7V voltage drop, but the Schottky has only 0.3V.

It is also much faster and is therefore often used in RF circuits.

Light Emitting Diode (LED)

Image result for led

A light emitting diode or LED is a diode that emits light when forward biased. It is available in several different colours, even ultra-violet and infrared.

Read more about different LED types.


Image result for Photodiode

A photodiode is a diode that conducts when it detects light. Could be used to receive the infrared signals from a remote control.