Home Uncategorized DIODES – LED, Photodiode, Formation of PN junction

DIODES – LED, Photodiode, Formation of PN junction


A conductor devise allows the current to flow in both direction (forward as well as reverse direction). However, an insulter don’t allows the current to flow in the both direction. But, it allows the current to flow in forward direction but not in reverse direction. This quality of diode makes it important.


  1. What is a diode?
  2. Why Diodes are Important
  3. Invention of Diode
  4. Formation of P and N type semiconductor
    1. P-type semiconductor
    2. N-type semiconductor
  5. Formation of PN Junction Diode
  6. Types of a Diode
    1. Light Emitting Diode (LED)
    2. Photodiode
    3. Other Types of Diodes
  7. Conclusion
  8. FAQs

What is a diode?

Diode is a semiconductor device with two terminal, which allows the current to flow in only one direction. It has zero resistance in one direction and infinity resistance in the other direction, so it act like short circuit in one direction and open circuit in other direction.

But, practically it’s not possible to have zero or infinity resistance. Actually, It has negligible resistance in one direction and very high resistance in reverse direction.

Why Diodes are Important

The LED lights, which you are using in your home are also a type of diodes. You may know LED’s full form is Light Emitting Diode. if you are using solar powered calculator, that calculator also has a photo diode. Diodes are also used to make transistors. Such type of many applications of different type of diode are there and we will see it all in this article.

Invention of Diode                                           

Diode was invented by Frederick Guthrie in 1873 but still, diodes were not in practical use. However, Thomas Edison developed its concept In 1880 and named it as Edison Effect“.

There was no very big time difference in invention of Vacuum-tube and semiconductor diodes. but still, vacuum-tubes were in use more than diodes. The main reason behind it was the un-stability of semiconductor diode at that time. but, when time passed diodes got upgraded and their use increased, this is how diode replaced the vacuum-tubes.


Diodes are made of semiconductors like silicon and gallium. However, semiconductors are not completely conductor (flow the current easily) and not even completely nonconductors or insulators (don’t allow to flow current) they are intermediate. This means that they don’t allow current to flow in normal condition. if we give them the energy required to free the electrons, then they act as a conductor.

Formation of P and N type semiconductor

  • Firstly, we need to add impurities into the semiconductors. (This process is called doping) doping is required because, after doping semiconductor has more free electrons or holes. So, the flow of current and conductivity of semiconductor increases.
silicon doped by trivalent impurity boron | image: commons.wikimedia.org

P-type semiconductor

  • Secondly, If we add a trivalent impurity in a pure semiconductor which has 3 valence electrons, so a large number of holes will be created and current flow will be increased. P-type semiconductor has holes as a majority charge carrier.

Note that holes are just an absence of electron. Which wants an electron and electron has negative charge. so, we have assumed holes as a positive charge.

Here are some examples of trivalent impurities

•Boron (B), Indium (In), Gallium (G), Aluminum (Al), etc.

N-type semiconductor

  • If we add a pentavalent impurity in a pure semiconductor which has 5 valence electrons, so a large number of a free electron will be created and current flow will be increased. This N-type semiconductor has electron as a majority charge carrier.
silicon doped by trivalent impurity Phosphorus | image: commons.wikimedia.org

Here are some examples of pentavalent impurities

•Phosphorus (P), Arsenic (As), Antimony (Sb), etc.

NOTE:   P and N-type semiconductors are not positively or negatively charged, they are neutral. The N-type semiconductor just indicates that it has electrons as a majority charge carrier However, P-type has holes. Electron denotes as negative sign and holes are just an absence of electron so it denotes as a positive sign that’s why it’s called as P-type and N-type. (Here, P stands for positive and N for negative)

Formation of PN Junction Diode

In formation of PN junction diode we need both P-type and N-type material side by side. We have take a semiconductor and dope the half semiconductor with pentavalent impurity and dope another half section with trivalent impurity. However, we have P-type semiconductor one side and N-type the other side.

Formation of PN junction diode | image: hi.m.wikipedia.org

As shown in figure above, the electrons and holes are free to move. so, the Electrons at the N-type side will start diffusing to P-type and the holes at the P-type side will start diffusing the N-type side. However, The electrons at N-type side will recombine with P-type side’s holes and P-type side’s holes will recombine with N-type side’s electrons.

The electron and holes are moving so definitely there will be some current produced, and reason of that currant is diffusion. Therefore, we call that current as diffusion current. However, this diffusion and recombination will run continuously.

If electron will recombine with hole of trivalent atom (which has three valence electron) so that atom will became negatively charged. This way a layer of negative charge will developed at P-type side and similarly holes will recombine with pentavalent atom’s free electron the atom will become positively charged. And a layer of positive charge will developed at N-type side.

Depletion Layer

Depletion layer | image: sq.wikipedia.org

These layers will get wider and wider. Now if any electron try to cross the junction, the negative charge at the other side will push that electron back because of repulsion effect. Same thing applies for the holes. This layer is known as “Depletion Layer”

When electron and holes stops to cross the depletion layer and recombination with each other,   the depletion layer will also stop to get wider. Now if we want any electron to cross the depletion it needs more energy. The energy electron needs to cross the depletion layer is known as potential barrier.

We can provide that energy by battery. So, this way a junction is created between them that’s why we know it as a PN junction diode.

Types of a Diode

1. Light Emitting Diode (LED)


               You must had seen the LED lights, which is also a diode. LEDs are in use as traffic signal, car headlight, seven segment display, etc. LED can emit broad range of rays from Infrared to Ultraviolet. We can use different type of semiconductor in it to produce different color rays.

Working of LED

LED| image: en.wikipedia.org

As shown in figure LED has two terminal one positive and other negative. Inside it, it has a PN junction diode. As explained above there will a depletion layer created at the junction. We have to connect the LED with a battery, which can provide sufficient amount of energy to electrons in valance band so they can jump to the conductance band.

We have to provide it energy more than potential barrier. When electron will jump to the conductance band it will release the energy in a form of photon. And this is how a light is extracted from LED.

[  E = h · v  ]

Here;     E = energy released by photon

               h = 6.6262×10-34 J·s

               v = frequency (Hz)

LED lights | image:

Advantages of LED

  • Low energy consumption
  • Longer life
  • Smaller size
  • Environmentally friendly
  • Brighter light and available in different colors

2. Photodiode

Photodiode is a device that allows the current to flow only when it detect the light. It also has a PN junction diode. When light hits the junction of the diode, the current start flowing. Photodiode works similarly like a sensor, that’s why it has many application in practical world.

Photodiode| image: en.wikipedia.org

Working of Photodiode

Photodiode works in reverse bias. When we connect it in reverse bias the extra electron at the N-type side attracted by the positive terminal of the battery and electron start to flow though N-type to battery and reach to P-type and recombine with the holes. And this way the depletion layer will get wider and wider.

But, at one stage it will stop to get wider because the potential barrier will be equal to the voltage provided by the battery. Now, current will also stop to flow. If we want the current to start flow again, we need to make the depletion layer smaller. So, its potential barrier become less than the battery voltage.

Photodiode manufactured such as light can reach to the junction easily. When light hits the junction all the atoms will get energy from it and the atoms at the N-type side will lose the electron and the atom will be positively charged. That atom will try to attract the electron, which was recombined with the holes.

So, the holes will lose the electron and again it will exist as a hole. This is how the holes’ count at the P-side and electrons’ count at the N-type region will increase. And the depletion layer will become narrow. Now, the potential barrier will become less than battery voltage. So, current can flow easily now.

Use of Photodiode

Photodiodes are used in:

  • fire alarm
  • optical communication system
  • smoke detectors
  • remote controls
  • compact disc players

Other Types of Diodes

  1. Zener diode
  2. PIN diode
  3. Laser Diode
  4. Tunnel Diode
  5. Schottky Diode


This experiment focuses on the investigation of the basic characteristics of a diode. The forward-biased voltage of diode is its barrier potential while the reverse-biased voltage of diode is infinite over range which shows that the diode is in healthy condition.


1. What is a power diode?

power diode is a crystalline semiconductor device used mainly to convert alternating current (AC) to direct current (DC), a process known as rectification. This makes power diodes better suited for applications where larger currents and higher voltages are involved.

2. What does a Schottky diode do?

Schottky diode is one type of electronic component, which is also known as a barrier diode and It is widely used in different applications like a mixer, in radio frequency applications, and as a rectifier in power applications. It’s a low voltage diode. The power drop is lower compared to the PN junction diodes.

3. Do diodes work on DC?

Diode Works both on AC and DC but the purpose on both are different. … But in dc circuit diodes are used for different purposes like Reverse Protection (because of its Unidirection property), And voltage Regulator , modulation, Frequency Signalling ,waveshape clipping etc.

4. What is the difference between Schottky diode and normal diode?

Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction. So it is a unipolar device. … In other words the forward voltage drop (Vf) is less compared to normal PN junction type diodes.

5. Why zener diode is called zener diode?

Zener Diode. Definition: A heavily doped semiconductor diode which is designed to operate in reverse direction is known as the Zener diode. In other words, the diode which is specially designed for optimising the breakdown region is known as the Zener diode.



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