Step recovery diode

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In electronics, a Step recovery diode (SRD) is a semiconductor junction diode having the ability to recover extremely quickly from a strong forward conduction state to a cut off state. The SRD is basically employed to generate extremely fast voltage pulses when switched quickly from full conduction to cut-off. 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.

1 Physical principles
2 See also
3 References
4 External links

[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

Intensity of the forward anode current $I A$ 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 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

$Q_S\cong I_A\cdot\tau$

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-catode voltage $V A K$ has nearly the same forward conduction value). Anode current does not ceases but reverses his 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 starts to rise quickly, reaching its cut-off value at reverse bias within a time $t T r$ , the tansition time, which basically is the time needed to discharge diode junction capacitance. Since the junction capacitance of semiconductor junction diodes can be made very small by using proper constructive technologies, transition time can be lowered well under the nanosecond. It is therefore possible to switch extremely quickly a semiconductor junction diode between two very different conduction states (forward conduction and cut-off), producing step-like voltage waveforms.

[edit] See also

[edit] References

Boff, A.F.; J. Moll & R. Shen (1960), "A new high speed effect in solid state diodes", International Solid State Circuit Conference. Digest of Technical Papers., New York: IEEE Press. 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).

Nosov, Yurii Romanovich (1969), Switching in semiconductor diodes, New York: Plenum Press.
Tkhorik, Yurii Aleksandrovich (1968), Transients in pulsed semiconductor diodes, Jerusalem: Israel Program for Scientific Translations.

[edit] External links

Tan, Michael R.; Wang, S. Y.; Mars, D. E.; Moll, J. L. "A 12 psec GaAs Double Heterostructure Step Recovery Diode", Hewlett-Packard Tech Report HPL-91-187, December 31, 1991. An interesting paper describing the construction and reporting the measured performance of an extremely fast heterojunction SRD.
"Pulse and Waveform Generation with Step Recovery Diodes" Hewlett-Packard Application note AN 918, HPRFhelp.