Step Recovery Diode, SRD
- basics or tutorial of the step recovery diode or SRD and how it may be used for signal generation, pulse and waveform shaping and parametric amplification.
Step recovery diode, SRD, includes:
See also: Other types of diodes
The step recovery diode or SRD is a form of semiconductor diode that can be used as a charge controlled switch and it has the ability to generate very sharp pulses. In view of its method of operation, it is also called the "Snap-off" diode, "charge storage" diode or "memory varactor".
The device finds a number of applications in microwave radio frequency electronics as pulse generator or parametric amplifier. It finds uses in a number of different roles including very short pulse generation, ultra-fast waveform generation, comb generation, and high order frequency multiplication. The diode is also capable of working at moderate power levels, and this gives it a distinct advantage over some other radio frequency technologies that are available.
The SRD is not as common as many other forms of semiconductor diode, but it can be very useful in many microwave radio frequency applications.
Step recovery diode structure
The device is fabricated with the doping level gradually decreasing as the junction is approached or as a direct PIN structure. This reduces the switching time because there are fewer charge carriers in the region of the junction and hence less charge is stored in this region. This allows the charge stored in this region of the step recovery diode to be released more rapidly when changing from forward to reverse bias. A further advantage is that the forward current can also be established more rapidly than in the ordinary junction diode.
Step recovery diode operation
The step recovery diode is used as what is termed a charge controlled switch. When the diode is forward biased and charge enters, it 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 SRD 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 enables them to be used as comb generators where a comb of harmonically related frequencies are generated.
The step recovery diode or SRD is able to be used as a microwave radio frequency generator and pulse sharpener. Although used in more specialist applications, the SRD is nevertheless a very useful component that is capable of some very high levels of performance. As such it is a very useful tool in the armoury of the RF design engineer to be used when the occasion requires.
By Ian Poole
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