Difference between Forward Bias and Reverse Bias

Biasing or applying bias means applying a steady dc voltage. Usually, when we apply a voltage across components like resistors, inductors, capacitors, etc. we do not need to consider the polarity of the voltage applied. But in the case of semiconductor devices such as a diode, BJT, FET, etc., the polarity of the voltage is an important factor as the behavior of these devices changes with the change in polarity of applied voltage.

Depending on the polarity of applied dc voltage or bias, there are two modes of biasing for semiconductor devices – forward biasing and reverse biasing. In this article, we are going to discuss the difference between forward bias and reverse bias with respect to the PN junction diode.

Difference between Forward Bias and Reverse Bias in tabular form

In the following table, we have pointed out a few key differences between forward and reverse bias.

Forward Bias Reverse Bias
The p-type semiconductor is kept at a higher potential than the n-type semiconductor. The n-type semiconductor is kept at a higher potential than the p-type semiconductor.
The width of the depletion region reduces. The width of the depletion region increases.
The barrier potential reduces. The barrier potential increases.
The PN junction diode acts as a conductor when a forward bias is applied. The PN junction does not let current flow from anode to cathode but a small amount of current called reverse bias current flows from cathode to anode.
The PN junction allows the current to flow through it. The PN junction does not let current flow from anode to cathode but a small amount of current called reverse bias current flows from cathode to anode.
The amount of current flowing through the device is dependent upon the applied voltage. The reverse bias current almost remains constant up to breakdown voltage.

What is Forward Bias?

When the applied voltage to the p-type material is higher than the applied voltage of the n-type material then the mode of biasing is called forward bias. In this case, the direction of the applied electric field is from p-type semiconductor to n-type semiconductor. Ideally, the PN diode acts as a short circuit even if a very small forward bias is applied and there will be a large amount of current flowing through the junction.

But in practice, when the applied bias exceeds the potential of the depletion region, then the charge carriers will have enough energy to move through the junction, and a current will flow through the junction. The amplitude of the current depends on the amplitude of the bias.

What is Reverse Bias?

When the applied voltage to n-type material is higher than the applied voltage to the p-type material then the mode of biasing is called reverse biasing. In this case, the direction of the applied electric field is from the n-type to the p-type semiconductor. In such conditions, more immobile charge carriers gather at the depletion region thus increasing the width of the depletion region.

When a reverse bias is applied, ideally, the PN diode acts as an open circuit and no current flows through it. But in practice, a very small amount of reverse bias current flows through the junction, and the amplitude of the current remains almost constant up to a certain value of reverse bias. After that, the junction breaks down and a large amount of current flows through it from cathode to anode. This voltage is called the breakdown voltage.

Conclusion

We have considered the effect of forward and reverse bias only on the PN diode. But components like Zener diode, BJT, FET, TRIAC, etc. are also made using highly, moderately, or lightly doped p-type and n-type semiconductors. So, the effect of forward and reverse bias applied across any PN junction of any semiconductor device is the same as that of a PN junction diode. But due to the difference in doping concentration, there are some differences as well.

Author
Subhrajyoti Choudhury

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