Mar 1, 2015

Forward and Reverse biased PN Junction

The potential difference across a PN junction can be applied in two ways. They are
  1. Forward Biasing
  2. Reverse Biasing

Forward Biased PN Junction

When an external voltage applied to the PN junctions is in such a direction that it stops the potential barrier, thus allowing current flow, is called forward biasing.
Forward Biased PN JunctionForward Biasing

  • To put on forward bias concept, connect positive terminal of the battery to P-type and negative terminal to N-type as represented in figure.
  • The applied forward potential creates an electric field which acts beside the field due to potential barrier.
The current flow may be explained in the following two ways:
  • As soon as battery connection is made, holes are repelled by the positive battery terminal and electrons are repelled by the negative terminal with the result that both the electrons and the holes are determined towards the junction where they recombine.
This enmasse movement of electrons to the left and that od holes to the right of the joint constitutes a large current drift through the semiconductor.

  • One more way to explain current flows is in the forward direction to say that due to the applied external voltage, the barrier potential is reduced which now allows more current to flow across the junction as shown in fig.
Incidentally, it may be noted that forward bias reduces the thickness of the depletion layer as shown in fig.
Forward biasing PN Junction circuit
                                                   W' < W

With forward bias to PN junction, the following points are worth noting:
  1. The potential barrier is decreases and at some forward voltage (0.1 V to 0.3V), it is eliminated also there.
  2. The function offers low resistance called forward resistance, Rf to current flow.
  3. Current runs in the circuit due to the formation of low resistance path. The magnitude of current depends upon the applied forward voltage.

Reverse Biased PN Junction

When an external voltage applied to the junction is in such a direction that potential barrier is enlarged, it is called reverse biasing.
When battery connections to the semiconductor are made as shown in the fig, the junction is said to be a reverse biased.
Reverse Biased Pn junctionReverse biasing

  • In this case, holes are attached by the negative battery terminal and electrons by the positive terminal. So that both holes and electrons moves away from the junction and away from one other. Since there is no electrons and holes combination, no current flows and the junction offer high resistance.
  • Other way of considering at the method is that, in this event, the applied voltage risess the barrier potential there by blocking the movement of majority carriers as shown in the figure.
  • Incidentally it may be noted that under reverse bias condition, width of depletion layer is increased because majority charge carriers are pulled away from the junction. It also increases the potential barrier as shown in the fig.
Bias circuit

Reverse biasingReverse Bias

With reverse bias to PN Junction, the following points are worth noting:
  1. The potential barrier is increased.
  2. The junction offers very high resistance known as reverse resistance, Rf to current flow.
  3. No current drifts in the circuit due to the creation of high resistance path.


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