Step recovery diode

In electronics, a step recovery diode (SRD) is a semiconductor junction diode having the ability to generate extremely short pulses. It is also called snap-off diode or charge-storage diode or memory varactor, and has a variety of uses in microwave electronics as pulse generator or parametric amplifier.

When diodes switch from forward conduction to reverse cut-off, a reverse current flows briefly as stored charge is removed. It is the abruptness with which this reverse current ceases which characterises the step recovery diode.

Contents

Historical note

The first published paper on the SRD is Boff, Moll & Shen (1960): the authors start the brief survey stating that "the recovery characteristics of certain types of pn-junction diodes exhibit a discontinuity which may be used to advantage for the generation of harmonics or for the production of millimicrosecond pulses". They also refer that they first observed this phenomenon in February, 1959.

Operating the SRD

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 \tau, 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 \tau, the stored charge has the following approximate expression

Q_S\cong I_A\cdot\tau

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

t_S\cong\frac{Q_S}{I_R}

When all stored charge has been removed, diode resistance suddenly changes, rising to its cut-off value at reverse bias within a time t_{Tr}, the transition time. This can be exploited to produce a pulse with a rise time of t_{Tr}.

See also

References

The following two books contain a comprehensive analysis of the theory of non-equilibrium charge transport in semiconductor diodes, and give also an overview of applications (at least up to the end of the seventies).

The following application notes deals extensively with practical circuits and applications using SRDs.

External links