JEE : P-n Junction Diode (Forward & Reverse Bias) Class 12 Notes | EduRev
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p-n Junction Diode
- A semiconductor diode is a p-n junction with metallic contacts provided at the ends for the application of an external voltage
- Thus p-n junction diode is a two terminal device represented as
- The equilibrium potential barrier can be altered by applying an external voltage V across the diode
There are two methods of biasing a p-n unction – Forward bias and reverse bias
- If the positive terminal of the external battery is connected to the p-side and the negative terminal of the external battery is connected to the n-side, then the p-n junction is said to be forward biased
- The direction of the applied voltage V is in a direction opposite to that of the potential barrier setup at the junction
- As a result of this, the depletion layer width decreases and the barrier height is reduced. The effective barrier height under forward bias is VB – V
- If the applied voltage V is small, the barrier potential will be reduced only slightly below the equilibrium value. Hence, only small number carriers will possess energy to cross the junction. Thus, the current is small
- If the applied voltage V is large, the barrier potential will be reduced significantly. Hence, the current is significant
- Due to the applied voltage, the electrons from the n-side cross the depletion region and reach the p-side. Similarly, the holes from the p-side reach the n-side
- As electrons reach the p-side and electrons are minority carriers in p-region, the forward bias is also known as minority carrier injection
- At the junction, the minority carrier concentration increases significantly
- Due to concentration gradient, the injected electrons on p-side diffuse from the junction edge of p-side to the other end of the p-side
- Similarly, the injected holes on the n-side diffuse from the junction edge of n-side to the other end of n-side
- The motion of charged carriers on either side gives rise to current and is usually measured in mA
- The total diode forward current is sum of hole diffusion current and conventional current due to electron diffusion
- If the positive terminal of the external battery is connected to the n-side and the negative terminal of the external battery is connected to the p-side, then the p-n junction is said to be reverse biased
- The direction of the applied voltage is same as that of the barrier potential.
- As a result, the barrier height increases and the depletion region widens due to change in electric field.
- The effective barrier height is VB + V.
- This suppresses the flow of electrons from n region to p region and holes from the p region to n region. Hence, diffusion current decreases.
- The electric field direction of the junction is such that if electrons on p-side or holes on n-side in their random motion come close to the junction, they will be swept to its majority zone. This gives rise to drift current of order of few µA.
- The diode reverse current is not much dependent on the applied voltage. Even a small voltage is sufficient to sweep the minority carriers from one side of the junction to the other side of the junction.
- The current under reverse bias is essentially voltage independent up to a critical reverse bias voltage, known as breakdown voltage VBE.
- When V = VBE, the diode reverse current increases sharply. If the current is not limited, the p-n junction will get destroyed.