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I-V CurveIV.4 - Step Recovery Diodes (SRDs)?

When diodes are switched from forward bias to reverse bias, the diode still conducts for a short time, since some charge is left in the device. Normal diodes remove this leftover charge very slowly, and the conductance of the diode also changes slowly. SRDs are optimized so that the charge removal period ends very abruptly, and consequently the reverse conduction stops very abruptly. This abrupt change can be used to create very fast switching pulses, or to generate harmonics of the switching signal.

Here's an extract from my thesis:

Step recovery diodes (SRDs) have remained extremely useful in wave-shaping applications in the three and a half decades since they were first presented [Boff60], [Moll62]. No other device rivals their combination of fast switching speed and ease of use. Figure 1.1 shows the typical circuit configurations for SRD pulse sharpening. In both circuits, the SRD is initially biased with a constant forward bias current IBIAS, which stores charge in the SRD. When the voltage source VIN rises, reverse biasing the SRD, the SRD conducts for a short period of time, removing the stored charge. This keeps the voltage across the diode very low. Then the stored charge is abruptly exhausted, and the SRD switches to a high-impedance, high-voltage state, resulting in a sharpening of the output voltage waveform.

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Figure 1.1 - Standard step recovery diode pulse sharpening circuits.

Table 1.1 - Commercially Available SRDs. Note the rapid increase in switching time with breakdown voltage. (The data for the last four diodes are measured values, the other data was obtained from the manufacturers' data books.)

Manufacturer [Ref] Part No. Diode Type VOP

(V)

 tR

(ns)

 VOP/tR (V/ns) τEFF

(ns)
Hewlett Packard [HP90] 5082-0020 SRD 25 0.06 417 20
Hewlett Packard [HP90] 5082-0021 SRD 40 0.1 400 100
Hewlett Packard [HP90] 5082-0017 SRD 75 0.3 250 300
Alpha [Alpha92] DVB6104-06 SRD 75 0.4 187 100
M/A-COM [MACO88] MA44753 SRD 100 1.0 100 150
M/A-COM [MACO88] MA44750 SRD 180 3.0 60 500
M/A-COM [MACO88] MA44952 multiple series SRD 300 1.0 300 195
experimental [Chapter 7] A8.6 WFSRD 300 0.9 333 4500
experimental [Chapter 7] A8.PT.850.1 WFSRD 300 0.6 500 950
experimental [Foci96] "Type II" DSRD 1700 5.0 340 250

Beware that not all SRDs are the same: some, with longer lifetimes, are more suited to pulse sharpening. Others, with shorter lifetimes, are more suited to frequency multiplication. (Some SRDs may indeed be sold as "frequency multiplication varactors", or something similar.)

Some regular run-of-the-mill diodes can act as step recovery diodes, but this is increasingly rare. Manufacturers go to great lengths to suppress the step recovery action in modern diodes, because most applications require a "soft" recovery transient.

 

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Note: this FAQ is a personal endeavor of Dr. Michael J. Chudobiak.
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