• A diode is the simplest semiconductor device which finds endless application in the field of electronics and communications. The applications are too many to be included all at the same place. So wondering what makes diodes so useful? In this article, we shall discuss how diodes are formed and how they work. So make sure you understand  thoroughly the concept of this wonderful device, because believe it or not, most of the time you design a circuit, you will need a diode.
  • We also discuss what is depletion region and how it formed.
  • Finally we discuss what is a potential barrier and the factors on which it potential barrier depends.

We start by taking two semiconductor materials, one n-type and the other p-type. An n-type semiconductor has electrons as the majority carriers, and a p-type semiconductor has holes as the majority carriers. You may be surprised to know that diode is formed simply by joining p-type and n-type semiconductor material, nothing more. When p-type material is joined with n-type material, something “magical” happens which makes it one of the most widely used semiconductor device. Consider the figure below in which p-type and n-type materials are joined together.

PN junction diode before the depletion region is formed

The figure shows the situation before the formation of depletion region. It shows the condition when p-type and n-type materials are just joined together. Now the question may arise- what is depletion region? To understand depletion region, consider p-type and n-type semiconductor when they are joined together. We know that holes are majority carriers in p-type, and electrons are majority carriers in n-type materials. When the two types of semiconductor materials are joined together, the electrons from the n-type material diffuse into p-type material and combines with holes. This creates a layer of negative ions near the junction in p-type material. Negative ions are formed because the trivalent impurities ( e.g. Aluminum) now has an extra electron from the n-type material. Similarly the holes from the p-type material diffuse into n-type material resulting in a layer of positive ions in the n-type material. These two layers of positive and negative ions form the depletion region. The term “depletion” refers to the fact that the region near the junction is depleted of their respective majority charge carriers. The figure below shows the condition after the formation of depletion region.

PN junction diode  after the depletion region is formed

Now comes another important question- Upto what extent the diffusion of electrons and holes takes place? The diffusion does not occur indefinitely and it stops after a quick span of time and the depletion region is said to be completely formed. Take a look at the figure below to understand how this happens.

Electron and hole flow ceases after depletion region is completely formed

Let us discuss the things explained in the above figure. As the electrons from the n-type material diffuse into p-type material, it forms negative ions near the junction. These negative ions will create an electric field in the direction from n-type to p-type. As more electrons diffuse into p-type material, the electric field strength goes on increasing. The electrons from n-type material now diffusing into p-type material will have to overcome the electric field due to negative ions. At one point the electric field becomes sufficiently strong to stop further diffusion of electrons. The same discussion also applies to holes and positive ions in n-type material. Keep in mind that the depletion region is formed very quickly and the thickness of depletion region is very less as compared to that of n and p type material.

One more important term to keep in mind while discussing about diodes is that of depletion barrier or potential barrier. So what is potential barrier? We discussed in the previous paragraphs that depletion region consists of positive and negative ions on the opposite side of the junction. We know from Coulomb’s law that electric field is established when charges are separated from each other. The electric field acts a “barrier” which prevents further diffusion of electrons and holes after equilibrium is established. So if we want to move an electron from n-type material to p-type material, energy must be supplied to the electrons to overcome the “barrier”. The external voltage required to move the electrons through the electric field is called barrier potential. Barrier potential is measured in volts. There are many factors which affects barrier potential-type of semiconductor material, temperature, doping concentration etc. If the doping concentration is less, then the electric field becomes weaker near the junction and the width of the depletion region is wider. If the doping concentration is high, then the electric field near the junction is stronger and the width of depletion region is less. Typical value of barrier potential at 25° C is 0.7 V for silicon and 0.3 V for germanium. The symbol of the diode used in circuits is shown in the figure below.

Structure of PN junction diode and its schematicAbove figure shows the schematic symbol and basic structure of PN junction diode. The schematic symbol of the diode is similar to an arrow having direction from anode to cathode. This “arrow-like” symbol indicates the direction of current flowing through the diode. The current through the diode cannot flow in both directions. Diode allows the current to flow in only one direction (anode to cathode). The reason for this is explained in the section on diode biasing and in the section on forward bias characteristics and reverse bias characteristics. So make sure you read those topics, because this property of diode to allow the flow of current in only one direction makes it one of the most widely used semiconductor device.