HOW A DIODE WORKS

Introduction to Diodes :

The Diode can be considered as the simplest and most fundamental element in electronics, which is composed of a p-n Junction. It is a two terminal device,

 shows its schematic symbol, where the "+" terminal is called Anode and is connected to the p-Region and the "-" terminal is called Cathode and is connected to the n-Region.

Symbol of Diode :

The symbol of a diode is shown below. The arrowhead points in the direction of conventional current flow.

We can create a simple P-N junction diode by doping donor impurity in one portion and accept impurity in other portion of silicon or germanium crystal block. These doping make a P-N junction at the middle part of the block beside which one portion becomes p-type (doped with trivalent or accept impurity), and another portion becomes n-type (doped with prevalent or donor impurity). We can also form a P-N junction by joining a p-type (doped with a trivalent impurity) and n-type ( doped with a prevalent impurity) together with a special fabrication technique. Hence, it is a device with two elements, the p-type forms anode, and the n-type forms the cathode.

Working Principle of Diode

Forward Biased Diode

In a P-N junction diode when the forward voltage is applied i.e. 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, the diode is said to be in forward biased condition. We know that there is a barrier potential across the junction. This barrier potential is directed in the opposite of the forward applied voltage. So a diode can only allow current to flow in the forward direction when forward applied voltage is more than barrier potential of the junction. This voltage is called forward biased voltage. For silicon diode, it is 0.7 volts. For germanium diode, it is 0.3 volts. When forward applied voltage is more than this forward biased voltage, there will be forward current in the diode, and the diode will become short circuited.

Hence, there will be no more voltage drop across the diode beyond this forward biased voltage, and forward current is only limited by the external resistance connected in series with the diode. Thus, if forward applied voltage increases from zero, the diode will start conducting only after this voltage reaches just above the barrier potential or forward biased voltage of the junction. The time, taken by this input voltage to reach that value or in other words, the time, taken by this input voltage to overcome the forward biased voltage is called recovery time.

Reverse Biased Diode

Now if the diode is reverse biased i.e. positive terminal of the source is connected to the n-type end, and the negative terminal of the source is connected to the p-type end of the diode, there will be no current through the diode except reverse saturation current. This is because at the reverse biased condition the depilation layer of the junction becomes wider with increasing reverse biased voltage. Although there is a tiny current flowing from n-type end to p-type end in the diode due to minority carriers. This tiny current is called reverse saturation current. Minority carriers are mainly thermally generated free electrons and holes in p -type and n- type respectively. Now if reverse applied voltage across the diode is continually increased, then after certain applied voltage the depletion layer will destroy which will cause a huge reverse current to flow through the diode.

 If this current is not externally limited and it reaches beyond the safe value, the diode may be permanently destroyed. This is because, as the magnitude of the reverse voltage increases, the kinetic energy of the minority charge carriers also increase. These fast moving electrons collide with the other atoms in the device to knock-off some more free electrons from them. The free electrons so released further release much more free electrons from the atoms by breaking the covalent bonds. This process is termed as carrier multiplication and leads to a considerable increase in the flow of current through the p-n junction.


Quantum tunneling relation with diode : 

is the passing of electrons through an insulating barrier which is thin compared to the de-Broglie .electron wavelength. If the “electron wave” is large compared to the barrier, there is a possibility that the wave appears on both sides of the barrier.

In classical physics, an electron must have sufficient energy to surmount a barrier. Otherwise, it recoils from the barrier. (Figure) Quantum mechanics allows for a probability of the electron being on the other side of the barrier. If treated as a wave, the electron may look quite large compared to the thickness of the barrier. Even when treated as a wave, there is only a small probability that it will be found on the other side of a thick barrier. See green portion of curve, (Figure). Thinning the barrier increases the probability that the electron is found on the other side of the barrier.

Tunnel diode: 

The unqualified term tunnel diode refers to the esaki tunnel diode, an early quantum device. A reverse biased diode forms a depletion region, an insulating region, between the conductive anode and cathode. 

This depletion region is only thin as compared to the electron wavelength when heavily doped– 1000 times the doping of a rectifier diode. With proper biasing, quantum tunneling is possible. 

Types of Diode

The types of diode are as follow :

1. Zener diode

1. P-N junction diode

1. Tunnel diode

1. varactor diode

1. Schottky diode

1. Photo diode

1. PIN diode

1. LASER diode

1. Avalanche diode

1. Light emitting diode