You would have heard this a countless number of times, “Schottky diodes are fast”. Ever stopped to wonder what is “fast” about them and why? Let’s explore that today

When a Silicon PN jn diode is forward-biased, current flows through it once the voltage is above the depletion layer potential. Now assume the case, where you suddenly remove the forward voltage and apply a negative voltage across it. We expect the diode to become reverse-biased and have no current flow. This occurs, but not as immediately as you think. There is a finite delay post the switching, where the current flowing across the junction goes negative(Or flows opposite). This occurs because when the diode is conducting, minority carriers (electrons in the P-type material and holes in the N-type material) accumulate near the junction. When the diode switches off, these minority carriers must return to their original regions or recombine. The accumulated charge in the junction decreases to zero. Now the current starts flowing in the opposite direction and this is known as the reverse recovery current. It goes up to a negative value and then slowly recombines back to a zero state when the diode becomes non-conducting. The delay time needed for it is called Reverse Recovery Time.
This delay causes power losses whenever a diode voltage switches so you always try to use diodes with low reverse recovery time. It becomes critical in switching power supplies with high switching rates. Schottky diodes don’t have this problem because it’s not a PN junction. It’s formed with a N-type material and a metal junction. Hence they have low switching loss and have a “fast” switching time with usually an order of magnitude faster times. So always check the switching times of diodes in the datasheet and use it to find the fastest edge switching rate in your application for the diode.
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