IV.29 - Wide-Field Step Recovery Diodes?

Wide-field step recovery diodes (WFSRDs) act just like regular step recovery diodes (SRDs), but they are designed to operate at higher voltages. (See the comparision table in the SRD section.) They achieve this by using a wide graded doping profile, rather than the narrow abrupt epitaxial structure typical of modern SRDs. The actual charge removal mechanism is also necessarily different. For the full details, read my thesis!

Here's an extract from my thesis:

The new SRD mechanism operates as follows. The diode is biased with a small constant forward DC current. This swamps the middle region of the psn diode with electrons and holes, and high injection conditions prevail. The diode acts as a low resistance. When a large reverse bias voltage is suddenly applied, the reverse bias slowly withdraws electrons and hole from the middle region. This region is still largely neutral, so the current will be almost entirely a diffusion current. This means that the slope of the carrier concentrations will change. This will lead to the removal of charge from the quasi-neutral middle region, and this region will shrink. A significant space charge region develops at the p+ p- junction first, as discussed in Chapter 2. However, as mobile charge is removed from the center, too few holes are left on the p- side of the junction to support a positive space charge. Since a positive space charge must exist to counterbalance the ionized acceptors in the p+ region, a second space charge region develops around the metallurgical junction, and the positive space charge is now provided by the ionized donors in the n+ region. This leads to envelopment of the entire middle region with space charge. After this point, the electric fields rapidly remove the free carriers, and hence the electric fields rapidly "snap" to the full reverse voltage.

(It should be mentioned that the first SRDs manufactured also had a diffused profile, and a graded junction [Moll62], [Kocs76]. The graded junction provided a built-in electric field throughout the active region, which aided in charge removal. This is an entirely different mechanism than that presented here. The carrier densities are too high, and the doping too low, for significant built-in electric fields to exist in WFSRDs.)

WFSRDs are not commercially available.