# Diode capacitance vs diode speed

I have some difficulties to understand what are the fatest diodes. Schottky diode are known for being the fatest diode (at least according to my knowledge) nevertheless their diode capacitance is higher than a PN diode like the 1N4148. Does someone can tell me what are the different paremeters which impact the switching speed of the diodes?

1N4148

Schottky diode :

Here are two datasheets, one which is a Schottky diode and the other the 1N4148 datasheet:

Thank you very much and have a nice day!

• Comparing a 100mA diode with a 500mA diode is not really comparing like with like, for one thing. For another, it is a mistake to assume that capacitance is the only speed limitation in a junction diode : the reverse recovery time (to drain carriers from the active region) is more important, especially in higher power junction diodes (1N4007)
– user16324
Jun 30, 2020 at 12:25
• It depends on what you call "fastest." Silicon carbide has 0 reverse recovery time! Jun 30, 2020 at 12:32
• It would be hard to find a Schottky diode with the same characteristics than a "PN diode" as generally the reverse voltage of a Schottky diode is very low. So I can take a 0A1 Schottky diode but the reverse voltage would be very low compared to the "PN diode"
– Jess
Jun 30, 2020 at 12:39
• @Stiddily No, Schottky diodes have no reverse recovery time. It's just that no one makes SiC pn diodes, while SiC Schottkys are readily available. Nov 6, 2021 at 22:47

## 2 Answers

Diodes have 2 slowness mechanisms which can be harmful in switching applications

1. Junction capacitance with reverse voltage. It takes some time to charge it to the reverse voltage, so the current flows to the reverse direction. The capacitance decreases as the reverse voltage grows so it needs either advanced simulation or a test circuit to see the effect exactly. The effect exists as well in PN diodes and Schottky diodes.

2. In PN diodes the current flows well also to the reverse direction until the minority carriers have vanished by recombinating.

The recombination doesn't in all cases seem to effectively start as soon as the voltage is reversed. High current rectifier diodes conduct to the reverse direction a substantially long time before the current starts to decay visibly. The full reverse conductivity time is in datasheets "Storage Time". After the storage has elapsed the current decays with time constant "Reverse Recovery Time" towards the final leakage value.

Reverse conductivity after a forward current isn't in all cases considered as slowness. For microwave applications we have "Snap diodes" which conduct intentionally some time to the reverse direction. But the reverse recovery happens after the storage time so fast that the current contains powerful high frequency distortion components. With a proper resonator we can extract a substantial microwave power at a frequency which is a multiple of our original signal frequency. "Frequency multipliers" can be constructed with this idea.

Si diodes have minority carriers which have a lifetime and take a while to decay, Schottky diodes do not have a minority carrier.

The reverse recovery time is the time taken for the minority carriers to be cleared out of the junction.

The capacitance is across the diode and is a separate issue (And is variable with reverse voltage!), it is unrelated to the diode recovery time.

• Thank you for your answer. In my mind, the diode capacitance should have an impact as it takes a certain time to charge it or discharge it for achieving the forward voltage form the reverse voltage.
– Jess
Jun 30, 2020 at 12:35
• @Jess You might consider diode size, and compare diodes of similar area...bigger diodes have more capacitance. Jun 30, 2020 at 12:38