Question

Describe the charge flow in a forward-biased Schottky barrier diode

Answer 1

Schottky diode is a metal-semiconductor junction which does not store charge carriers at the junction because it has no depletion layer. It finds its application where fast switching is needed.

Normal diode:

When a P-N junction diode is forward biased, it starts conducting, but when it is reverse biased, it stops conduction. But this transition from conduction to insulation is not instant. Diode takes some time to reach a steady state of no conduction when it is reverse biased.

This happens because during forward biasing charge carriers move across the junction and when it is suddenly reverse biased, some of the charge carriers are still at the junction, but they have not recombined yet.

Thus, this phenomenon is called charge storage. Due to this charge storage diode takes time while switching. The time taken by the diode to achieve a steady state of no conduction in reverse biasing is called reverse recovery time. The effect of reverse recovery time is negligible at frequencies below 10MHz, but at high frequencies this effect is significant.

Schottky diode :

Metals have electrons as majority carrier, and N-type semiconductor also possesses electrons as majority charge carrier. Thus, when a metal-semiconductor junction is unbiased, then current doesn’t flow in a diode. This is because electrons in N-type region do not possess sufficient energy to transit from N-type junction to metal. But when the junction is forward biased then electrons acquire energy to cross the barrier.

This barrier is called Schottky Barrier. The electrons which are moving from N-type to metals are termed as hot carriers. They are called so because when the junction is forward biased electrons acquire a significant amount of energy. And they enter metal junction with this high magnitude of energy. That’s why diode is also called hot-carrier diode.

The best thing about Schottky diode is its fast switching ability. There is no role of the depletion layer in the case of Schottky diode, that’s the reason it possesses fast switching than P-N Junction diode. Metals and N-type semiconductor have electrons as majority charge carriers.Thus, the entire current is due to bulk carriers. There are no holes, so there is no depletion layer. Thus, there is no charge storage at the junction.

Due to the absence of charge storage, the Schottky diode can be easily switched from forward biased to reverse biased. A p-n junction is bipolar because it consists of both electrons as well as holes as majority carriers, but Schottky diode is unipolar because it involves only one charge carrier i.e. electron.

The barrier potential of Schottky diode lies in the range of 0.2-0.25V which is much lower than the barrier potential of Silicon 0.7 V. The reason behind this low barrier potential is, again the absence of depletion layer.

Thus, no significant current flows from metal to semiconductor when the diode is reverse biased. Thus, the current flows only in the case of forward biased of the metal-semiconductor junction.