What is Diode?

Diode

What is diode?

Definition: 

A diode is defined as two terminal electronic component that only conduct current in one direction (so long as it is operated within a specified voltage level). An ideal diode will have zero resistance in one direction, and infinite resistance in the reverse direction.

 Although in the real world, diode cannot achieve zero OR infinite resistance. Instead, a diode will have negligible resistance in one direction (to allow current flow) and a very high resistance in the reverse direction (to prevent current flow). A diode is effectively like a valve for an electrical circuit.

 Semiconductor diodes are the most common types of diode. These diodes begin conducting electricity only if a certain threshold voltage is present in the forward direction (i.e. the voltage "low resistance" direction). The diode is said to be "forward biased" . When conducting current is in the direction. When the connected within a circuit in the reverse direction (i.e. the "high resistance" direction). The diode is said to be "reverse biased".

A diode only blocks current in the reverse direction ( i.e. when the it is reverse biased) while the reverse voltage is within the specified range. Above this range, the reverse barriers breaks. The voltage at which this breakdown occurs is called as the "reverse breakdown voltage". When the voltage of the circuit is higher than the reverse breakdown voltage, the diode is able to conduct electricity in the reverse direction. (i.e. the "high resistance" direction). This is why in practice we say diodes have a high resistance in the reverse direction- not an infinite resistance.

 A P-N junction is the simplest form of the semiconductor diode. In ideal condition, this P-N junction behaves as a short circuit when it is forward biased, and as an open circuit when it is in the reverse biased. The name diode is derived from "di-ode" which means a device that has two electrodes. Diodes are commonly used in many electronics projects and are included in many of the best Arduino starter kit.

Diode symbol

The symbol of a diode is shown in below figure. The arrowhead points in the direction of conventional current flow in the forward biased condition. That means the anode is connected to p-side and the cathode is connected to the n-side.

 We can create a simple P-N junction diode by doping pentavalent for donor impurity e in one portion and trivalent or acceptor impurity in other portion of the silicon or germanium crystal block. These doping make a P-N junction at the middle part of the block. We can also form a P-N junction by joining a p type and n type semiconductor together with a special fabrication technique. The terminal connected to the p type is the anode and terminal connected to the n type is cathode.

Working principle of diode

A Diodes working principle depends on the interaction of n type and p type semiconductors. An n-type semiconductors has plenty of free electrons and a very few numbers of holes. In other words, we can say that the concentration of free electron is high and that of holes is very low in an n type semiconductor. Free electrons in the n type semiconductor are referred as majority charge carriers, and holes in n type semiconductor are referred to as minority charge carriers.

A p-type semiconductor has a high concentration of holes and a low concentration of free electrons. Holes in a p-type semiconductor are majority charge carrier, and free electrons in p type semiconductor are minority charge carrier. 

Unbiased diode

Now let us see what happens when one n type region and one p-type region come in contact. Here due to concentration differences, majority carriers diffuse from one side to another. As the concentration of holes is high in p-type region and it is low in the n type region, the holes start diffusing from the p-type region to the n type region.

Again the concentration of free electrons is high in the p-type region and it is slow in the p-type region and due to this reason free electron starting diffusing from the n-type region to the p type region.

The free electrons diffusing into the p type region from the n type region would recombine with holes available there and create uncovered negative ions in the p-type region. In the same way, the holes diffusing into the n type region from the p-type region would recombine with free electrons available there and create uncovered positive ions in the n type region.

In this way, there would a layer of negative ions in the p-type side and a layer of positive ions in the n type region appear along the junction line of these two types of semiconductors. The layer of uncovered positive ions and uncovered negative ions from the region in the middle of the diode where no charge career exists since all the charge carrier get recombined here in this region. Due to the lack of charge carriers, this region is called as a "deflection region".

 After the formation of depletion region, there is no more diffusion of charge carrier from one side to the another in the diode. This is due to the electric field appeared across the depletion region will prevent further migration of charge carrier from one side to another.

The potential of the layer of uncovered positive ions in the n-type side would repeal the holes in the p type sides and the potential of the layer of uncovered negative ions in the p-type side would repeal the free electrons in the n type side. That means the potential barrier is created across the junction to prevent further diffusion of charge carriers.

Forward biased diode

Now let us see that what happens if a positive terminal of a source is connected to the p-type side and the negative terminal of the source is connected to the n-type side of the diode and if we increase the voltage of this slowly from zero.

In the beginning, there is no current flow through the diode. This is because although there is an external electrical field applied across the diode, the majority charge carriers still do not get sufficient influence of the external field to cross the depletion region. As we told that the depletion region acts as a potential barrier against the majority charge carriers.

This potential barrier is called as a forward potential barrier. The majority charge carriers start crossing the forward potential barrier only when the value of externally applied voltage across the junction is more than the potential of the forward barrier. For silicon diode, the forward barrier potential is is 0.7 volt and the German for germanium diodes it is is 0.3 volt.

When the externally applied forward voltage across the diode becomes more than the forward barrier potential, the free majority charge carriers start crossing the barrier and contribute the forward diode current. In that situation, the diode would behave as a short circuited path and the forward current gets limited by only externally connected resistors to the diode.

Reverse bias diode

Now let us see what happens if we connect the negative terminal to the voltage source to the p-type side and positive terminal of the voltage source to the n-type side of the diode. At that condition, due to electrostatic attraction of the negative potential of the source, the holes in the p-type region would be shifted more away from the junction leaving more uncovered negative ions at the junction.

In the same way, the free electrons in the n type region would be shifted more away from the junction towards the positive terminal of the voltage source leaving more uncovered positive ions in the junction. As a result of this phenomenon, the depletion region becomes wider. This condition of the diode is called as a reverse biased condition. At that condition, no majority carriers across the junction, and they instead move away from the junction. In this way, a diode blocks the flow of current when it is reverse biased.

There are always same free electrons in the p type semiconductor and some holes in the n-type semiconductor. These opposite charge carriers in a semiconductor are called as minority charge carriers. In the reverse biased condition, the holes find themselves in the n-type side would easily cross the reverse biased depletion region as the field across the depletion region does not present rather it helps minority charge carrier to cross the depletion region.

As a result, there is a tiny current flowing through the diode from positive to the negative side. The amplitude of this current is very small as the number of minority charge carrier in the diode is very small. This current is called the reverse saturation current. 

If the reverse voltage across the diode gets increased beyond the safe value, due to higher electrostatic force and due to higher kinetic energy of minority charge carriers colliding with atoms, a number of covalent bonds get broken to contribute a huge number of free electron hole pair in the diode and the process is cumulative.

The huge number of such generated charge carriers would contribute a huge reverse current in the diode. If this current is not limited by an external resistance connected to the diode circuit, the diode may permanently be destroyed.

Types of diode

1)Zener diode:

Zener diode are widely used as voltage reference and as shunt regulator to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased. A Zener diode conducts when the voltage richest the diode reverse breakdown voltage.

2)P-N junction diode:

A P-N junction diode is a two terminal or two electrodes semiconductor device. Which allow the electric current in only one direction, while block the electric current in opposite or reverse direction. If the diode is forward biased, it allow the circuit electric current flow. On the other hand, if the diode is reverse biased, it block the electric current flow. P-N junction semiconductor diode is also called as a P-N junction diode.

In n type semiconductors, free electrons are the majority charge carrier whereas the p type semiconductors, holes are the majority charge carriers. When the n-type semiconductor is joined with the p type semiconductor, a P-N junction is formed. The p-n junction which is formed when the p-type and n-type semiconductor are joined it is called P-N junction diode.

3)Tunnel diode:

A tunnel diode also known as a esaki diode is a type of semiconductor diode that has effectively negative resistance due to the quantum mechanical effect called tunneling. Tunnel diode have a very heavily doped P-N junction that is about 10 nm wide.

4)Varactor diode:

The diode whose internal capacitance varies with the variation of the reverse voltage such type of diode is known as the varactor diode. It is used for storing the charge. The varactor diode always works in reverse bias and it is a voltage dependent semiconductor device.

5)Schottky diode:

A schottky diode is a metal semiconductor diode with a low forward voltage drop and a very fast switching speed. The schottky diode is another type of semiconductor diode but have the advantage that their forward voltage drop is substantially less than that of the conventional silicon P-N junction diode.

6)Photo-diode:

A photo-diode is a semiconductor device that convert lights into an electrical current. The current is generated when photons are absorbed in the photo-diode. Photo-diodes may contain optical filter, built n lenses, and may have large or small surface areas.

7)PIN diode:

PIN diode is a diode with a wide and intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type region are typically heavily doped because they are used for ohmic contacts.

8)Laser diode:

A laser diode (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to the light emitting diode. In which the a diode performed directly with electrical current can create lasing conditions at the diodes junction. Laser diode can directly convert electrical energy into light. Driven by voltage, the p-n transition allow for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation in the form of an emitted Photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.

9)Avalanche diode: 

An avalanche diode is a special type of semiconductor device designed to operate in reverse breakdown region. Avalanche diodes are used as relief valves (a type of valve used to control the pressure in a system) to protect electrical system from excess voltages.

10)Light emitting diode:

Light emitting diode (LED) is a semiconductor light source that emits light when current flow through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The colour of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductor or a layer of light emitting phosphor on the semiconductor device.