Monday, January 15, 2018

Step Recovery Diodes

       The diode recovery is the switching of the diode from forward state to reverse state. The step recovery diode has a step type recovery characteristics. When a diode is switched from ON  to OFF state then due to stored charges, continues to a carry reverse current for a while  and then suddenly drops to zero. Due to the sudden drop, it gives type effect. Practically the carrier concentration in such diodes decreases near the junction. The doping profile of such diode is shown in the Fig.1. This unusual distribution of charge carries is responsible to reduce the current suddenly to zero. This phenomenon is called reverse snap-off. Hence the diode is also called a snap diode.
Fig. 1 Doping profile of step recovery diode

    The Fig. 2 shows the symbol of a step recovery diode. The main application of such diodes is frequency multipliers where output frequency is a multiplier of the input frequency. This is because, a sudden step in current contains harmonics and such harmonics can be filtered to select the required frequency component. 
Fig. 2 Symbol of step recovery diode

       The Fig. 3 shows the use of step recovery diode in rectification and corresponding waveforms.
Fig. 3 Use of step recovery diode

      The diode works like conventional diode in positive half cycle. In negative half cycle, a current pulse is generated as current continues to flow in negative half cycle for sometime and then suddenly becomes zero. Output filtering can be used to select the desired harmonic.
       The step recovery diodes are most useful for high order low power multipliers. They are very useful for high frequency ranges of the order of megahertz.

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