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The step recovery diode is used as what is termed a charge controlled switch. When the step recovery diode is forward biased and charge enters it, the diode appears as a normal diode and it behaves in much the same way. When diodes switch 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 step recovery diode 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 from step recovery diode circuits enables them to be used as comb generators where a comb of harmonically related frequencies are generated.
