Step Recovery Diode

Step Recovery diode is a semiconductor device with unusual doping; it is also a voltage-dependent variable capacitor. The doping density is extremely small near junction area. Thus, the density of charge carriers is also low near the junction. Due to which charge storage is negligible near the junction, and this leads to fast switching of the diode from ON state to OFF state.

It is also used as a charge controlled switch in various application due to its fast switching at low frequencies. Step recovery diode works as an ordinary diode at low frequency i.e. they conduct when forward biased and acts as an open circuit in reverse biased mode. But when it is switched from forward biasing to reverse biasing it changes state instantly.

At high frequencies the process of switching takes time. Thus, the forward current continues to flow in the reverse cycle of input voltage for a short duration. This happens because the charge carriers are stored near the junction, although their density is low but still at higher frequencies they take some time to drain away from the junction.

The concentration of charge carriers decreases as they approach the junction, due to this, the phenomenon of reverse snap- off takes place. Thus the diode is also called Snap-off Diode

Construction of Step recovery diode

This is constructed in a similar manner to that of an ordinary diode. The major difference is the doping intensity. The doping profile of the step recovery diode is explained with the help of below diagram.

It is evident from the above diagram that the doping density decreases at the junction. This lead to decrease in mobile charge carriers at the junction. This is the requisite effect for achieving fast switching in step recovery diode.

Working of Step recovery diode

We have already discussed above that it behaves as a typical diode at low frequencies. Initially, this diode offers very low impedance about 1 ohm, but gradually its impedance starts increasing due to charge storage at the junction.

Now, when the negative cycle of the input is applied, the charge carriers take time to completely drain off from the junction or snap-off from the junction thus, a small amount of current is observed at the beginning of negative input cycle.

The switching time is very small in step recovery diode i.e. about few nanoseconds. This imparts the ability to generate very sharp pulses which play a significant role in waveshaping circuits.

Advantages & Disadvantages of Step Recovery Diode

1. More Forward Current: It generates forward current more rapidly than a conventional diode.
2. Fast Switching Time: The switching time required is low which leads to rapid response.
3. Sharp Harmonics: It has the ability to generate very sharp pulses which are significant for the pulse generator.
4. Highly Efficient: These diodes possesses high efficiency.
5. Not Suitable at Higher Frequencies: The disadvantage of the diode is that the switching speed decreases with the increase in frequency.

Applications of Step recovery diode

1. Step Recovery Diode as Multiplier: They can multiply the frequency of signal up to 20 times. The fast switching time of Step recovery diode imparts them the ability to generate sharp pulses. During forward biasing they store charge and when reversed biased they use this charge to generate harmonics.

This can be understood more clearly with the help of the below diagram of frequency multiplier using Step recovery diode.

The rise time of pulses generated by the device is equal to the snap time. The filter circuit is used to extract the signal of requisite frequency.  The range of cut off frequency of step recovery diode is from 200 GHz – 300 GHz. Thus, this high range of cut off frequency enables the diode to be used as a multiplier for circuits operating at a frequency of about 10 GHz.

2. Step Recovery Diode as Harmonic Generators: The ability of step recovery diode to generate sharp pulses makes it appropriate to use as harmonic generators for generating different pulses.
3. It is used in voltage controlled oscillators.
4. It is used in a frequency synthesizer.
5. Comb Generator: It is used in comb generator for generating multiple harmonics of its input signal. The output waveforms of this devices give the resemblance of comb teeth. Thus, the name of this device is Comb Generator.
6. Sampling Phase detector: It is used in sample phase detector for detecting the phase in a communication system.

These are the significant applications of step recovery diode. In communication system, the step recovery diode is used in the stage of modulation of signal.