Sunday Emmanuel

@sundayemmanuel · 25 Nov 2021

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What are P-N junction

A p-n junction diode is formed if there is an abrupt change from a p-doped region to an n-doped region within the same semiconductor material. The p-type region of the diode is known as the anode(A)while the n-type region is called the cathode(K). Recall that an n-type material has a large concentration of electrons and few thermally generated holes whereas, in p-type material, there is a large concentration of holes and few thermally generated electrons.

As soon as the junction is formed, carrier concentration gradients are created such that holes diffuse from the P-side to the n-side while the electrons diffuse from the n-side to the p-side. when the holes reach the n-side, they recombine with the free electrons, similarly, electrons diffusing into the p-side recombine with the free holes in the p- side. Consequently, an area is created on either side of the junction that is depleted of free charges. This region is called the depletion region.

Now as the mobile majority carriers in the p-type material (holes) migrate across the junction, they leave behind them immobile negative charges. This has the effect of making the depletion region in the p-side become negatively charged. A similar situation causes the depletion region in the n-side to become positively charged.

Two types of Biased p-n junction

Forward bias:A p-n junction is said to be forward bias when a DC voltage source with an EMF greater than the barrier potential difference is connected with its positive terminal to the anode (A) and its negative terminal to the cathode (K). This causes the majority carriers to be repelled by the source terminals towards the junction i.e. electrons which are the majority carriers in the n-side (cathode)are repelled towards, the junction by the negative terminal while the holes which are the majority careers in the p- side(anode) are repelled towards the junction by the positive terminal. The effect of this is that there is a recombination of the majority at this junction which results in to flow of current in the forward direction. A small increase in the applied voltage in the forward direction resorts to an exponential increase in the current due to the reduction of the potential barrier.

Reverse bias: under reverse bias conditions, the negative terminal of the DC voltage source is connected to the anode (A) and the positive terminal of the cathode (K). Under this condition, the majority of carriers in the p-region (holes) are attracted by the negative terminal of the voltage source while the electrons (majority carriers in n-region) are attracted by the positive terminal of the source such that the width of the depletion region widens and the barrier potential difference is reinforced. Since the majority carrier and not cross the junction, there is no flow of current in the forward direction.

Merits of p-n junction diode over the vacuum tube

MERITS

1. P-n junction diode are smaller, more rugged, and more durable
2. They are cheaper to manufacture
3. They are more power-efficient since they produce less heat than the equivalent vacuum diode
   
   

1. They are extremely sensitive to heat. They have easily destroyed only the rated value of temperature is exceeded
2. No current handling capability
3. New voltage handling capability