Step Recovery Diode, SRD

The step recovery diode is also called the snap-off diode, charge storage diode and the memory varactor and it is often used as a pulse generator or parametric amplifier.

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Step Recovery Diode    

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The step recovery diode or SRD is a form of semiconductor diode that can be used as a charge controlled switch and it has the ability to generate very sharp pulses.

The step recovery diode, SRD is a rather specialist device that finds a number of applications in microwave radio frequency electronics. It can be used as pulse generator or parametric amplifier.

The step recovery diode finds uses in a number of different roles including very short pulse generation, ultra-fast waveform generation, comb generation, and high order frequency multiplication. The diode is also capable of working at moderate power levels, and this gives it a distinct advantage over some other radio frequency technologies that are available.

Step recovery diode structure

The device is fabricated with the doping level gradually decreasing as the junction is approached or as a direct PIN structure.

The PIN structure of the SRD reduces the switching time because there are fewer charge carriers in the region of the junction and hence less charge is stored. As there is less charge in the area, the little charge can be released far more rapidly when moving from forward to reverse bias than if higher levels were stored.

A further advantage is that the forward current can also be established more rapidly than in the ordinary junction diode.

Step recovery diode operation

The step recovery diode, SRD is used as what is termed a charge controlled switch. When the diode is forward biased and charge enters, it appears as a normal diode and it behaves in much the same way.

When SRD switches from forward conduction to reverse cut-off, a reverse current flows briefly as stored charge is removed. When all the charge is removed it suddenly turns off or snaps off. It is the abruptness with which the reverse current ceases that enables the SRD to be used for the generation of microwave pulses and also for waveform shaping.

To explain this in more detail, under normal forward bias conditions the diode will conduct normally. Then if it is quickly reverse biased it will initially appear as a low impedance, typically less than an ohm. Once the charge that is stored in the device is depleted, the impedance will very abruptly increase to its normal reverse impedance which will be very high. This transition occurs very quickly, typically well under a nanosecond.

This property allows the step recovery diode to be used in pulse shaping (sharpening) and in pulse generator circuits. The high harmonic content of the signal produced by any repetitive waveforms enables them to be used as comb generators where a comb of harmonically related frequencies are generated.

The step recovery diode, SRD tends to be a specialist form of electronic component that is used for some niche microwave applications. Although it is not as widely used as some devices, the step recovery diode, SRD is able to fulfil some critical requirements when needed.

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