Class 12 Physics Semiconductor Electronics Forward biasing

Forward biasing

  • 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

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