What Determines the Maximum Operating Frequency of a Diode?

Ⅰ. Simple Basics

1. Junction Capacitance

The diode will have parasitic capacitance, which is mainly the junction capacitance, which is a simple diode model.

Figure. 1

 2. Reverse Recovery Time

When the voltage abruptly reverses, the diode current in actual applications does not drop to zero right away; instead, a relatively substantial reverse current remains, which must be reduced to 0.1 times the maximum value. The reverse recovery time is time.

Figure. 2

Ⅱ. Facts

Why can the reverse recovery time never be equal to the charge and discharge time of the junction capacitance?

The junction capacitance and reverse recovery time Trr characteristics for diodes are typically provided by the manufacturer. Now let's compare the characteristics of four distinct diode types: Schottky diodes, and ultra-quick recovery diodes.  rapid recovery diodes, and standard diodes, 

We ensure that the four manufacturers of the four different types of diodes   have the same withstand voltage, packing, and maximum working current in order to make the results more credible. The maximum reverse withstand voltage is 100V, all of the packages are SMA, and the maximum operating current is 1A. The manufacturer chosen here is DIODE Semiconductor.

The models are:

● Schottky diode: B1100-13-F

● Ultra-fast recovery diode: US1B-13-F

● Fast recovery diode: RS1B-13-F

● Ordinary diode: S1B-13-F

Screenshots of these diode parameters are as follows:

Figure. 3

The parameters sorted out are as follows:

Figure. 4

The Schottky diode has the greatest junction capacitance among them, as was previously demonstrated. The Schottky diodes aren't running at the highest frequency, are they? The junction capacitance is the biggest, so why?

Although Schottky diodes' reverse recovery time isn't specified in the standard document, we should all be aware that it is the least.

In a strict sense, the Schottky diode's operation differs from the PN junction diode's in that there is no reverse recovery time. Simply said, because parasitic capacitance exists, there is a maximum operating frequency.

We are aware that various sorts of diodes operate in the following sequence from greatest to lowest operating frequency:

Figure. 5

We now know that Schottky's junction capacitance, which is 80 pF, is the greatest. The reverse recovery times of the other three diodes, which range in capacitance from 10pF to 20pF, differ by an order of magnitude.

We may also determine how long it takes for the junction capacitance to fully charge if we assume that the reverse recovery time also serves as the junction capacitance's charging time.

As an illustration, consider the fast recovery diode RS1B-13F, whose junction capacitance is 15pF and reverse recovery current as depicted in the figure below (extracted from the specification). Given that the typical reverse current is around 0.5A, it is simple to calculate that it will take 1.5ns to charge a 15pF from 0V to -50V, which is substantially less time than the actual reverse recovery time of 150ns.

Figure. 6

So it is certain that the length of the reverse recovery time is not determined by the PN junction capacitance.

Ⅲ. Answers to Questions

Returning to the original query, what factor determines the diode's maximum operating frequency?

In fact, it is simple to consider: First off, a high operating frequency cannot be used if the junction capacitance is too large. Because the capacitor's impedance decreases with frequency, the signal passes directly from the capacitor and the diode's reverse cut-off capability is lost.

Second, a high operating frequency is not possible if the reverse recovery time is too long. Because voltage flips more quickly at higher frequencies. The voltage changed again after the reverse bias, the reverse current did not return, and the diode's ability to serve as a reverse cut-off was lost.

Therefore, in general, the maximum operating frequency of the diode will depend on the junction capacitance and reverse recovery time. Exactly who chooses who has more influence.

A Schottky diode's operating frequency is determined by the junction capacitance since the reverse recovery time is so quick.

The reverse recovery time determines the maximum operating frequency of PN junction diodes since it has a far bigger impact than the junction capacitance,  which is typically tens of pF.

We also know that Schottky diodes can operate at higher frequencies and have the quickest Schottky speed when compared to PN junction diodes.

In conclusion, The reverse recovery time is not the charging and discharging time of the junction capacitance.