# Step recovery diode

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.

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

#"Intensity of the forward anode current" $I_A$ flowing in the device during its steady state.
#"Minority carrier lifetime" "$au$", 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 "$au$", the stored charge has the following approximate expression

:$Q_Scong I_Acdot au$

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-catode voltage $V_\left\{AK\right\}$ has nearly the same forward conduction value"). Anode current does not ceases 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

:$t_Scongfrac\left\{Q_S\right\}\left\{I_R\right\}$

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

* Minority carrier.
* P-n junction.
* Pulse generator
* Semiconductor diode.

References

*Harvrefcol
Surname1 = Boff
Given1 = A.F.
Surname2 = Moll
Given2 = J.
Surname3 = Shen
Given3 = R.
Year = 1960
Chapter = [http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1157249 A new high speed effect in solid state diodes]
Title = [http://ieeexplore.ieee.org/xpl/RecentCon.jsp?punumber=8304 International Solid State Circuit Conference. Digest of Technical Papers.]
Publisher = IEEE Press
Page = 50-51
Place = New York
. The first paper dealing with SRDs: interesting but "restricted access".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).
* Harvrefcol
Surname = Nosov
Given = Yurii Romanovich
Title = Switching in semiconductor diodes
Publisher = Plenum Press
Place = New York
Year = 1969
.
* Harvrefcol
Surname = Tkhorik
Given = Yurii Aleksandrovich
Title = Transients in pulsed semiconductor diodes
Publisher = Israel Program for Scientific Translations
Place = Jerusalem
Year = 1968
.

*Harvrefcol
Surname1 = Tan
Given1 = Michael R.
Surname2 = Wang
Given2 = S.Y.
Surname3 = Mars
Given3 = D.E.;
Surname4 = Moll
Given4 = J.L.
Title = [http://www.hpl.hp.com/techreports/91/HPL-91-187.html Tech Report HPL-91-187 - A 12 psec GaAs Double Heterostructure Step Recovery Diode]
Publisher = Hewlett-Packard
Year = 1991, December31
. An interesting paper describing the construction and reporting the measured performance of an extremely fast heterojunction SRD.
*" [http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an918.pdf Application note AN 918 - Pulse and Waveform Generation with Step Recovery Diodes] ", Hewlett-Packard [http://www.hp.woodshot.com/hprfhelp/hprfhelp.htm HPRFhelp] .
* [http://www.medphys.ucl.ac.uk/research/borl/homepages/davek/phd/phd.html A Picosecond Optoelectronic Cross Correlator using a Gain Modulated Avalanche Photodiode for Measuring the Impulse Response of Tissue] is a PhD thesis in which an SRD is a key element. [http://www.medphys.ucl.ac.uk/research/borl/homepages/davek/phd/chapter5.pdf Chapter 5] is particularly relevant.

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