The diode which is mainly used in harmonic generator circuits as frequency multiplier is

Step Recovery Diode:

It is a semiconductor device with unusual doping. 

• The doping density is very small near the junction area due to which the density of charge carriers is also small near the junction area.
• As the charge storage is negligible near the junction, it helps fast switch the diode from on to off state.
• It works as a normal diode under forward and reverse bias condition but it changes its state instantly when it is switched from forward to reverse bias.
• Step Recovery Diode works as a Multiplier as it can multiply the frequency of signal up to 20 times.
• The fast switching time of this diode imparts the ability to generate sharp pulses.
• During forward biasing they store charge and when reversed biased they use this charge to generate harmonics