72.208.80.113: /* Advantages and drawbacks */

2014-08-14T21:53:08Z

Advantages and drawbacks

en>GoingBatty: Disambiguated: notation system → Mathematical notation

2012-08-19T19:39:56Z

Disambiguated: notation system → Mathematical notation

New page

In [[electronics]], a '''step recovery diode''' ('''SRD''') is a [[Diode#Semiconductor_diodes|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.

== Historical note ==
The first published paper on the SRD is {{Harv|Boff|Moll|Shen|1960}}: the authors start the brief survey stating that "the recovery characteristics of certain types of [[Diode#pn-junction diode|pn-junction diodes]] exhibit a discontinuity which may be used to advantage for the generation of harmonics or for the production of [[nanosecond|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 [[Electrical conduction|conduction]], which is present in all [[Diode#Semiconductor_diodes|semiconductor junction diodes]] and is due to finite lifetime of [[minority carrier]]s in [[semiconductor]]s. Assume that the SRD is forward [[Voltage bias|biased]] and in ''[[steady state]]'' i.e. the [[anode]] [[current bias|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 ''Qs'' is stored in the device. This ''stored charge'' depends on

#''Intensity of the forward anode current'' '''''IA''''' flowing in the device during its steady state.
#''Minority carrier lifetime'' '''''τ''''', i.e. the mean time a free [[charge carrier]] moves inside a semiconductor region before [[Carrier generation and recombination|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

:<math>Q_S\cong I_A\cdot\tau</math>

Now suppose that the voltage bias abruptly changes, switching from its stationary positive value to a higher [[Magnitude (mathematics)|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 '''VAK''' 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 ''Qs'' starts to flow out of the device at an almost constant rate '''''IR'''''. All the stored charge is thus removed in a certain amount of time: this time is the ''storage time '''tS''''' and its approximate expression is

:<math>t_S\cong\frac{Q_S}{I_R}</math>

When all stored charge has been removed, diode resistance suddenly changes, rising to its [[Cut-off (electronics)|cut-off]] value at [[P-n_junction#Reverse bias|reverse bias]] within a time '''''tTr''''', the ''transition time'': this behavior can be used to produce pulses with rise time equal to this time.

== Operation of the Drift Step Recovery Diode (DSRD) ==
Drift Step Recovery Diode (DSRD) was discovered by Russian scientists in 1981 (Grekhov et al., 1981). The Principle of the DSRD operation is similar to the SRD, with one n 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).

== See also ==
* [[Minority carrier]].
* [[P-n junction]].
* [[Pulse generator]]
* [[Diode#Semiconductor_diodes|Semiconductor diode]].

== References ==
*{{Citation
| first = A. F.
| last = Boff
| author-link =
| first2 = J.
| last2 = Moll
| author2-link =
| first3 = R.
| last3 = Shen
| contribution = A new high speed effect in solid state diodes
| contribution-url = http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1157249
| series = IRE International Solid-State Circuits Conference
| title = 1960 IEEE International Solid-State Circuits Conference. Digest of Technical Papers.
| volume = III
|date=February 1960
| pages = 50–51
| place = New York
| publisher = [[IEEE]] Press
| url = http://ieeexplore.ieee.org/xpl/RecentCon.jsp?punumber=8304}}. 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 [[diode]]s, and give also an overview of applications (at least up to the end of the seventies).
*{{Citation
| last = Nosov
| first = Yurii Romanovich
| author-link =
| title = Switching in semiconductor diodes
| place = [[New York]]
| publisher = [[Plenum Press]]
| year = 1969
| series = Monographs in Semiconductor Physics
| volume = 4
| doi =
| id =
| isbn = }}.
*{{Citation
| last = Tkhorik
| first = Yurii Aleksandrovich
| author-link =
| title = Transients in pulsed semiconductor diodes
| place = [[Jerusalem]]
| publisher = [[Keter Publishing House#History|Israel Program for Scientific Translations, Ltd.]]
| year = 1968
| doi =
| id =
| isbn = }}.
The following application notes deals extensively with practical circuits and applications using SRDs.
*{{Citation
| contribution = Pulse and Waveform Generation with Step Recovery Diodes
| contribution-url = http://www.hp.woodshot.com/hprfhelp/5_downld/lit/diodelit/an918.pdf
| series = Application note AN 918
|date=October 1984
| place = [[Palo Alto]]
| publisher = [[Hewlett-Packard]]
| url =
| doi =
| id = }}. Available at Hewlett-Packard [http://www.hp.woodshot.com/hprfhelp/hprfhelp.htm HPRFhelp].

== External links ==
{{Commons cat|Step-recovery diodes}}
*{{Citation
| first = Michael R.
| last = Tan
| author-link =
| first2 = S. Y.
| last2 = Wang
| author2-link =
| first3 = D. E.
| last3 = Mars
| author3-link =
| first4 = J. L.
| last4 = Moll
| author4-link =
| title = A 12 psec GaAs Double Heterostructure Step Recovery Diode
| series = [http://www.hpl.hp.com/techreports/ HP Labs Technical Reports]
| volume = HPL-91-187
| date = 31 December 1991
| place = [[Palo Alto]]
| publisher = [[Hewlett-Packard]]
| url = http://www.hpl.hp.com/techreports/91/HPL-91-187.html
| doi =
| id = }}. An interesting paper describing the construction and reporting the measured performance of an extremely fast [[heterojunction]] SRD.
*{{Citation
| last = Kirkby
| first = David
| author-link =
| title = A Picosecond Optoelectronic Cross Correlator using a Gain Modulated Avalanche Photodiode for Measuring the Impulse Response of Tissue
| chapter = Chapter 5 - Pulse Generators
| chapterurl = http://www.medphys.ucl.ac.uk/research/borl/homepages/davek/phd/chapter5.pdf
|date=April 1999
| url = http://www.medphys.ucl.ac.uk/research/borl/homepages/davek/phd/phd.html
| doi =
| id = }}. It is a PhD thesis in which an SRD is a key element. Chapter 5 is particularly relevant.

[[Category:Diodes]]

Cutler's bar notation - Revision history

72.208.80.113: /* Advantages and drawbacks */

en>GoingBatty: Disambiguated: notation system → Mathematical notation