What is a Step Recovery Diode : Working & Its Applications

Diodes are simple and very versatile devices. These are available in different types which are used based on the requirement. Each type of diode has its own characteristics so that it can be used in specific applications. So, one of the less common but very useful diodes is the SRD or Step Recovery Diode. This diode is also known as a charge-storage diode or snap-off diode or memory varactor. So, this article gives brief information on a step recovery diode or SRD.

What is a Step Recovery Diode?

The semiconductor diode which is used to generate very short pulses is known as a step recovery diode or SRD.  This diode provides maximum switching speed at low frequency so it is also used as a charge-controlled switch. This diode has the capacity to generate very sharp pulses. The step recovery diode symbol is shown below.

The working principle of a step recovery diode is similar to a normal diode at less frequency. When this diode is forward biased then they conduct and in reverse biased condition, it acts as an open circuit. However, once this diode is switched from forward to reverse bias, it instantly changes its condition. The switching process at high frequencies takes time.

Step Recovery Diode Construction

The construction of this diode can be done similarly to a normal diode but the main difference is the intensity of doping. The doping of the diode near the junction is very less.The basic structure of the SRD is based on the Shockley diode and consists of two pn-junctions connected in series with a third terminal for the input voltage. So it exhibits special dynamic switching characteristics as compared to the single PN junction diode.

This structure has a lower switching time about few nano seconds due to less number of charge carriers within the region of the junction. When there is less charge within the region, then a small amount of charge can be produced quickly once moving from forward bias to reverse bias. The following diagram explains the step recovery diode’s doping profile.

Generally, at low frequencies, this step recovery diode performs like a typical diode . This kind of diode provides extremely low impedance which is nearly 1 ohm, but its impedance starts increasing slowly because of the storage of charge at the junction.

Once the negative input cycle is given to this diode, the charge carriers will take some time to drain off totally from the junction so, a small amount of current can be observed at the starting of the negative input cycle.

Step Recovery Diode Working and Characteristics

Step recovery diodes are the most common diode used for repetitively pulsed operation. These diodes feature a very fast recovery time, which allows them to handle high peak currents and voltages. They are also called peak current diodes or avalanche diodes.

They have a junction capacitance that must be charged before they conduct. Their capacitance is so large that it takes a significant amount of time to charge up enough to conduct normal levels of current. These capacitors are formed by the depletion region between two p-n junctions.

When voltage is applied across the diode in reverse bias, the junction capacitance charges up until it reaches a critical point at which the device conducts avalanche breakdown. This breakdown occurs when electrons flow through all parts of the device without resistance, creating enormous current flow as well as excessive heat generation.

Once the stored charge vanishes then the reverse current fall to zero suddenly, which removes the reverse current tailing. This diode has a very shortfall and also good forward conductivity.

So, due to this conductivity, it will simply store a huge amount of minority charge carriers at the forward voltage. The forward bias characteristics of this diode is similar to a normal diode. Since this diode will store a huge amount of minority charge carriers throughout forwarding operation, it is called a charge storage diode.

They have a negative resistance characteristic, which means that they conduct more current when their voltage is decreased.

 To better the understand how Step Recovery Diodes work, we must first understand the concept of negative resistance. This phenomenon occurs when a device passes current through it and the voltage across it decreases, instead of increasing like in most cases. When this occurs, a negative voltage can be produced and harnessed as energy.

This behavior allows them to act as an amplifier or oscillator when used with certain circuits. At low frequency, this diode works like a normal diode which means once the diode is forward biased, it conducts. As compared to other diodes, its doping level is less.

Step Recovery Diode Circuit

An implementation of the SRD (step recovery diode) based UWB pulse generator circuit is shown below. Generally, UWB(Ultra-wideband) wireless communications as well as carrier-less UWB radar technology use sub-nanosecond or narrow nanosecond pulse as the transmission medium. So this circuit is used to implement a sub-nanosecond level pulse generator. The circuit can be built with a resistor ‘R’. SRD and load resistor ‘RL’.

The main principle of this circuit is to use the quick step recovery characteristics of SRD to form the driving pulse, cut down the rise time, and narrow the pulse width after that achieve the necessary UWB pulse. So the final measured pulse width is approximately 600 ps for 5V amplitude. So it is enough for practical applications, however, the buzzing is a little too big and the amplitude is fairly low. The input and output waveforms of this circuit are shown in the above diagram.

Different IC Packages

The step recovery diode or SRD uses various IC packages which include the following.

• DO or Diode Outline.
• SOD or Small Outline Diode.
• TO or Transistor Outline.
• SOT or Small Outline Transistor.
• DPAK or Discrete Package.
• MELF or Metal Electrode Leadless Face.
• For step-recovery diodes, MELF packages mainly include MicroMELF, QuadroMELF & MiniMELF.
• The large surface-mounted package like D2PAK includes a heat sink.
• The SC-74, SC-76 & SC-59 are plastic, surface-mounted packages including three leads.
• Powermite®3 is a high-power density and compact surface mount rectifier package that provides low thermal resistance.

Advantages

The advantages of the step recovery diode include the following.

• As compared to a conventional diode, this diode generates forward current more quickly.
• The required switching time to this diode is low so it gives a quick response.
• This diode has the capacity to generate very sharp pulses which are very important for the pulse generator.
• These diodes are highly efficient and their design is simple.
• Quick transient response.
• Inexpensive.
• Less power loss and reverse recovery time.

Disadvantages

The disadvantages of step recovery diode include the following.

• The main drawback of this diode is; that the switching speed decreases when the frequency increases.
• It is not applicable for higher frequencies.
• If the difference between input-output is high then efficiency is low.
• It may need a heat sink
• It is capable exclusively of step-down operations.

Applications

The applications of step recovery diode include the following.

• The SRDs are mainly used for high-speed pulse shaping, frequency multipliers, high-frequency harmonic generators, and generators.
• Once this diode is used as a frequency multiplier, it maintains high efficiency approximately 20 times the multiplier. Thus, this diode is an outstanding microwave frequency doubling component.
• The step recovery diode is used as a parametric amplifier or pulse generator.
• This diode can be used as a multiplier to multiply the signal frequency up to 20 times.
• This can be used as a harmonic generator.
• This diode can be used in VCOs (voltage-controlled oscillators).
• It is used within a frequency synthesizer, comb generator, and sampling phase detector.
• This diode is used as a parametric amplifier or pulse generator within microwave electronics.

Thus, this is all about an overview of step recovery diode or SRD which is a form of semiconductor diode, used to generate very sharp pulses. So this diode is used in microwave radio frequency electronics applications. Here is a question for you, what is Drift Step Recovery Diode or DSRD?