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Wednesday, 30 October 2019

Forward Bias and Reverse Bias Diagram


When P and N-type materials are joined, a thin junction formed 8 is called P-N junction. There are two methods of forming P-N junction. In the grown junction a single crystal is obtained from a melt which at first contains impurities of either the P or N - type. In the middle of the growth process, impurities of the opposite kind are added to the melt, so that the remainder of the crystal abruptly grows into the opposite type. In the second method, junction is by fusing together the two metals to form an alloy and is known as fused P-N junction. In this method a small disc of P-type material (Indium) is placed on a somewhat larger flat plate of N-type silicon. The materials are held in a graphite holder and heated to temperature of about 600°C. The indium disc melts at about 155°C. As the temperature rises further it dissolves away some of the silicon beneath it. In the molten region the indium first neutralizes the N-type impurities in the silicon and then leaves an excess of P-type impurities. A P-N junction is thus formed between this P-region and the remainder of the N-type silicon melts (See Diagram).

Forward Bias P-N Junction :

When an external voltage is applied across the two materials a flow of current results if the positive terminal and negative terminal of the voltage source are connected to the extreme ends of P and N materials respectively. Voltage applied in this way is called forward biasing the P-N junction (See Diagram).

This arrangement permits flow of current across the P-N junction. The holes are repelled by the positive battery terminal and electrons by the negative battery terminal with the result that both holes and electrons will be driven towards the junction where they will recombine. As long as the battery voltage is applied large current flows. In other words forward biasing lowers the potential barrier across the depletion layer thereby allowing more current to flow across the junction. The volt-amp characteristics of forward biased P-N junction are shown in Figure.
Reverse Bias P-N Junction :

If the positive of the supply voltage is connected to N type and the negative of the battery to P-type it is said to be connected as Reversed biased (See Diagram).

In this case, the holes are attracted by the negative battery terminal and electrons by the positive battery terminal, so that both holes and electrons more away from the junction. Since, there is no recombination of electron hole pairs. It increases the potential barrier at the junction thereby allowing very little current to flow through the junction. The volt-amp characteristics of Reversed biased P-N junction is given in Figure.

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