What is a diode?
a diode is an electronics device which has following things
- p-type and n-type regions
- a PN junction
- a depletion region
- a barrier potential
P-type region is connected to one terminal of the battery and n-type region is connected to the other terminal of the battery in order to conduct current. (See fig-1a)
Fig-1: PN Junction diode
The symbol of diode is given below
Fig-1b: Diode Symbol
Without knowing the basic idea of p-type and n-type materials, pn junction, depletion region and barrier potential, we are unable to understand the working of a diode as well as a transistor and many other electronic devices.
How does pn junction form?
Suppose we have a piece of silicon (Si), we add some trivalent impurity to one part of it and some pentavalent impurity to other part of it. So the one side will be changed to p-type semiconductor and other side into n-type semiconductor. (See fig-2)
Fig-2: Process of doping to make a diode
The blue circles in fig-2 shows silicon atoms. Whereas orange and red circles represent arsenic and indium atoms respectively. Indium atoms produce more holes on the left side of the piece of silicon and convert it into p-type semiconductor. On the other side the arsenic atoms produce more electrons and convert it into n-types semiconductor.(See Fig-3)
Fig-3: Electrons and Holes on both sides of PN junction
This kind of doping on the same block of silicon creates a boundary between p-type and n type areas. This boundary is called PN junction. (See fig-4)
Fig-4: PN junction
How does depletion region form?
As we know the p-type area has large number of holes due to trivalent impurity and n-type area has huge number of electrons due to pentavalent impurity. After PN junction is formed the n-type area loses some free electrons because they move towards p-type area and combined with some of the holes near junction (See fig-5). As electrons continue to move towards p-type area a time comes when no more electrons can move towards p-type area because the layer of previous electrons repels the coming electrons.
Fig-5: Electrons moving to p-type area leaving holes behind them
Depletion means reduction in numbers or quantity of something. You can observe the n-type area loses some electrons as well as the p-type area loses some holes. Just like a layer of electrons is created at the boundary of p-type area,a layer of holes is also created on at the boundary of n-type area (See fig-6). These two layers of electrons and holes create depletion region. This depletion region is formed very quickly and is very small as compared to p-type and n-type areas.
Fig-6: Layer of electron on p-side
What is barrier potential?
Refer to the figure-6 above there is a force of attraction between the electrons in the electron layer and the holes in the holes layer. This force creates an electric field between the positive and negative charges of depletion region (See fig-7). Now due to this electric field free electrons of the n-type area need energy to move towards the p-type area. Electron does not have this energy itself we need to provide energy to electron externally.
Fig-7: Electric field causes barrier potential
The potential difference between the two layers (i.e. electron and hole) of the depletion region is called the barrier potential and we express this in terms of volts.
Now if you want to move electron to the p-type area we must provide energy to it equal to the barrier potential. The value of the barrier potential depends upon mainly three things
- Type of material
- Amount of doping
Mostly electronic material silicon is used which has 0.7 volt barrier potential and Germanium has 0.3 volt.