Step recovery diode

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Step recovery diode

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 step recovery diode 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 step recovery 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 step recovery diode, 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 step recovery diode 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 step recovery diode is forward biased and charge enters it, the diode 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 step recovery diode 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 from step recovery diode circuits enables them to be used as comb generators where a comb of harmonically related frequencies are generated.

Operating the SRD

[edit]Physical principles

The main phenomenon used in SRDs is the storage of electric charge during forward conduction, which is present in all semiconductor junction diodes and is due to finite lifetime of minority carriers in semiconductors. Assume that the SRD is forward biased and in steady state i.e. the anode bias current does not change with time: since charge transport in a junction diode is mainly due to diffusion, i.e. to a non constant spatial charge carrier density caused by bias voltage, a charge Q_s is stored in the device. This stored charge depends on

1. Intensity of the forward anode current I_A flowing in the device during its steady state.
2. Minority carrier lifetime τ, i.e. the mean time a free charge carrier moves inside a semiconductor region before recombining.

Quantitatively, if the steady state of forward conduction lasts for a time much greater than τ, the stored charge has the following approximate expression

Now suppose that the voltage bias abruptly changes, switching from its stationary positive value to a higher magnitude constant negative value: then, since a certain amount of charge has been stored during forward conduction, diode resistance is still low (i.e. the anode-to-cathode voltage V_AK has nearly the same forward conduction value). Anode current does not cease but reverses its polarity (i.e. the direction of its flow) and stored charge Q_s starts to flow out of the device at an almost constant rate I_R. All the stored charge is thus removed in a certain amount of time: this time is the storage time t_S and its approximate expression is

When all stored charge has been removed, diode resistance suddenly changes, rising to its cut-off value at reverse bias within a timet_Tr, the transition time: this behavior can be used to produce pulses with rise time equal to this time.

[edit]Operation of the Drift Step Recovery Diode (DSRD)

Drift Step Recovery Diode (DSRD)has been discovered by Russian scientists in 1981 (Grekhov et al., 1981).
The Principle of the DSRD operation is similar to the SRD. However there is an essential difference - the forward pumping current should be pulsed, not continuous, because drift diodes function with slow carriers.
The principle of DSRD operation can be explained as follows: Short pulse of current is applied in the forward direction of the DSRD effectively "pumping" the P-N junction, or in other words, “charging” the P-N junction capacitively. When the current direction reverses, the accumulated charges are removed from the base region. As soon as the accumulated charge decreases to zero, the diode opens rapidly. A high voltage spike can appear due to the self-induction of the diode circuit. The larger the commutation current and the shorter the transition from forward to reverse conduction, the higher the pulse amplitude and efficiency of the pulse generator (Kardo-Sysoev et al., 1997).

SRD summary

the step recovery diode or SRD is able to be sued as a microwave radio frequency generator and pulse sharpener. Although used in more specialist applications, the step recovery diode, SRD is nevertheless a very useful component that is capable of some very high levels of performance. As such the SRD is a very useful tool in the armoury of the RF design engineer to be used when the occasion requires.